METHOD OF PRODUCING A HOLLOW FIBER MEMBRANE

Abstract

A method of producing a hollow fiber membrane having an outside coating with a thin film composite (TFC) layer with a transmembrane protein, the method including the steps of: preparing an aqueous solution including a self-assembled nanostructure including polyalkyleneimine (PAI) and a detergent solubilized transmembrane protein and a di- or triamine, preparing an apolar solution including a di- or triacyl halide in an apolar organic solvent, contacting a hollow fiber membrane with the either the solution according to step a) or the solution according to step b), removing excess solution if any, contacting the hollow fiber membrane with the other solution, allowing an interfacial polymerization reaction to take place, and rinsing the hollow fiber membrane with an aqueous solvent. The hollow fiber is applicable e.g. for extracting water from the product solution.

Claims

1. A method of producing a hollow fiber membrane having an outside coating with a thin film composite (TFC) layer comprising a transmembrane protein, said method comprising the steps of: a) preparing an aqueous solution comprising a self-assembled nanostructure comprising polyalkyleneimine (PAI) and a detergent solubilized transmembrane protein and a di- or triamine, b) preparing an apolar solution comprising a di- or triacyl halide in an apolar organic solvent, c) contacting a hollow fiber membrane with either the aqueous solution or the apolar solution, d) allowing the hollow fiber membrane to absorb the solution and, optionally, removing excess solution, e) contacting the hollow fiber membrane with the other solution not used in step c), f) allowing an interfacial polymerization reaction to take place, and g) rinsing the hollow fiber membrane with an aqueous solvent.

2. The method according to claim 1, wherein the PAI is polyethyleneimine (PEI).

3. The method according to claim 2, wherein the PEI is a substantially linear polymer having an average molecular weight of between about 2,000 Da to about 10,000 Da, such as between about 3,000 Da to about 5,000 Da.

4. The method according to claim 1, wherein the transmembrane protein is an aquaporin water channel.

5. The method according to claim 4, wherein the aquaporin water channels are AqpZ channels or SoPIP2;1 water channels.

6. The method according to claim 1, wherein the detergent is selected from the group consisting of N,N-dimethyldodecylamine N-oxide (LDAO), octyl-glucoside (OG), n-dodecyl -D-maltoside (DDM), or a combination thereof.

7. The method according to claim 1, wherein the aqueous solution of step a) further comprises a buffer.

8. The method according to claim 1, wherein the self-assembled nanostructure of step a) is prepared by mixing a solution of polyalkyleneimine with a detergent solubilized transmembrane protein.

9. The method according to claim 1, wherein the di- or triamine is m-phenylenediamine (MPD).

10. The method according to claim 1, wherein the hollow fiber membrane material is polyethersulfone (PES) or polysulfone (PS).

11. The method according to claim 1, wherein the concentration of the di- or triamine in the aqueous solution is about 1% to about 5% (w/w).

12. The method according to claim 1, wherein the di- or triacyl halide is benzene-1,3,5-tricarbonyl chloride (TMC).

13. The method according to claim 1, wherein the concentration of the di- or triacyl halide in the apolar solution is 0.05% to about 1% (w/v).

14. The method according to claim 1, wherein the apolar organic solvent is hexane, heptane, octane, or a mixture thereof.

15. The method according to claim 1, wherein the hollow fiber membrane is an endless single hollow fiber.

16. (canceled)

17. The method according to claim 1, wherein the hollow fiber membrane is a bundle of hollow fibers, the bundle of hollow fibers being arranged in a protective shell having an inlet and an outlet for liquid present outside the fibers and an inlet and an outlet for liquids present in the lumen of the hollow fibers, thereby forming a hollow fiber (HF) module.

18. The method according to claim 17, wherein the protective shell has an elongate form and the bundle of fibers is longitudinally arranged inside the protective shell.

19. A hollow fiber membrane obtainable by the method according to claim 1.

20. (canceled)

21. A method for the concentration of a product solution by osmosis, said method comprising utilizing a hollow fiber membrane according claim 19 to extract water from a product solution.

22. A method for the production of salinity power using pressure retarded osmosis, said method comprising utilizing a hollow fiber membrane according to claim 19 to increase hydrostatic pressure, and using the increase in hydrostatic pressure as a source of salinity power.

Description

DETAILED DESCRIPTION

[0027] More specifically, the aspects of the disclosed embodiments relate to a method of producing a hollow fiber membrane having an outside coating with a thin film composite (TFC) layer comprising a transmembrane protein in a self-assembled nanostructure as disclosed herein, which nanostructure is formed by self-assembly of the positively charged PEI in presence of the protein when being negatively charged.

[0028] Examples of transmembrane proteins are integral membrane proteins including aquaporin water channels. i.e. aquaporins and aquaglyceroporins, such as those listed below. At pH 7.5 the protein is negatively charged as proved when reconstituted inside proteoliposomes that will exhibit a negative value of their zeta potential value (Wang S et al., Membranes, 5(3): 369-384, 2015). This is due to the presence of negatively charged amino acid residues (aspartic acid and glutamic acid) with pKa at 3.9 for aspartic acid and 4.2 for glutamic acid (Kong S et al. RSC Adv., 4: 37592-37599, 2014). In addition, these proteins have a cysteine residue situated at the exterior surface when properly folded, said cysteine being negatively charged at pH 7.5 inducing also a negative value of the zeta potential as has been shown for 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) proteoliposomes containing AqpZ (Li et al., Fusion behaviour of aquaporin Z incorporated proteoliposomes investigated by quartz crystal microbalance with dissipation (QCM-D); Colloids and Surfaces B-biointerfaces, 111: 446-452, 2013).

[0029] The aspects of the disclosed embodiments include a liquid composition in a method step, comprising a self-assembled nanostructure as disclosed herein in which the transmembrane protein is detergent solubilized, for example using a suitable detergent such as LDAO, OG, DDM or the like, optionally in combination with a pH buffer to maintain negatively charged amino acid residues of said transmembrane protein, e.g. for aquaporins buffering above pH 4, using e.g. a phosphate buffer, corresponding to the pKa of negatively charged amino acid residues. Other optional components of the liquid composition are biological buffer, e.g. HEPES sodium salt, cf. 54466 HEPES sodium salt from Fluka, (Industriestrasse 25, CH-9471 Buchs), Tris buffer, TES, MES, and MOPS buffers.

[0030] In addition, the aspects of the disclosed embodiments involve a method step of making the liquid composition as disclosed, where, as an example, a solution of a PAI, such as polyethyleneimine (PEI), is mixed with a suspension of negatively charged protein having a negatively charged amino acid residue. In this way structures are formed when the PEI solution complexes the charged protein as has been shown for PEI DNA and PEI RNA complexes (Sun et al., Molecular Dynamics Simulations of DNA/PEI Complexes: Effect of PEI Branching and Protonation State, Biophysical Journal Volume 100 June 2011 2754-2763; Mansoor M Amiji Polymeric Gene Delivery: Principles and Applications. Examples of PEIs are linear and branched polymers as outlined below.

[0031] Moreover, the aspects of the disclosed embodiments relates to a hollow fiber membrane having an outside coating with a thin film composite (TFC) layer, such as in the form of a semipermeable or selectively permeable membrane, comprising self-assembled nanostructures as disclosed herein. The separation membrane may be in the form of a hollow fiber membrane comprising a porous substrate or support membrane, e.g. a nanoporous or microporous membrane having a separation layer or an active layer in which the self-assembled nanostructures are immobilized.

[0032] The TFC skin layer is typically formed by interfacial polymerization to form a thin film or TFC layer, e.g. a crosslinked aromatic polyamide layer. In an alternative embodiment, the active/selective layer may be formed by depositing nanosized alternating positive and negatively charged polyelectrolyte layers, i.e. a layer-by-layer film (LbL) (see Wang et al., Membranes, 5(3): 369-384, 2015).

[0033] The filtration membrane according to the present disclosure may be prepared by adding a liquid composition comprising said self-assembled nanostructures, e.g. with aquaporin water channel proteins as the transmembrane protein needed to form structures, during the membrane fabrication process, such as adding the liquid composition to the di- or triamine solution when forming a TFC layer, or by addition to a liquid positively charged polyelectrolyte when forming an LbL skin.

[0034] In another aspect of the method of the disclosed embodiments, the transmembrane protein may be an anion channel protein, such as a voltage-dependent anion channel, which is useful in preparation of ion exchange membranes for reverse electrodialysis, cf. Dlugolecki et al. (Journal of Membrane Science, 319 214-222, 2008).

Definitions and Terms

[0035] The term transmembrane protein (TP) as used herein is a type of membrane protein spanning the entirety of the biological membrane to which it is permanently attached in nature. That is, in nature, transmembrane proteins span from one side of a membrane through to the other side of the membrane. Examples of transmembrane proteins are ammonia transporters, urea transporters, chloride channels, and aquaporin water channels.

[0036] The term aquaporin water channel as used herein includes a functional natural or synthetic aquaporin or aquaglyceroporin water channel, such as aquaporin Z (AqpZ), GIPf, SoPIP2;1, aquaporin 1 and/or aquaporin 2. Aquaporin water channels include bacterial aquaporins and eukaryotic aquaporins, such as yeast aquaporins, plant aquaporins and mammalian aquaporins, such as aquaporins 9 and 12 as well as related channel proteins, such as aquaglyceroporins. Examples of aquaporins and aquaglyceroporins include: prokaryotic aquaporins such as AqpZ; mammalian aquaporins, such as Aqp1 and Aqp2; plant aquaporins, such as plasma intrinsic proteins (PIP), tonoplast intrinsic proteins (TIP), nodulin intrinsic proteins (NIP) and small intrinsic proteins (SIP), e.g. SoPIP2;1, PttPIP2;5 and PtPIP2;2; yeast aquaporins, such as AQY1 and AQY2; and aquaglyceroporins, such as GlpF and Yfl054. Aquaporin water channel proteins may be prepared according to the methods described herein or as set out in Karlsson et al. (FEBS Letters 537: 68-72, 2003) or as described in Jensen et al. US 2012/0080377 A1 (e.g. see Example 6).

[0037] The term separation membrane as used herein includes membranes of tubular or hollow fiber structures useful for separating water and, optionally, certain small size solutes including anions and cations, from other solutes, particles, colloids and macromolecules. Examples of separation membranes are filtration membranes such as nanofiltration (NF) membranes, forward osmosis (FO) membranes and reverse osmosis (RO) membranes. One type of filtration membranes is a thin film composite (or TFC) membrane, often classified as nanofiltration and reverse osmosis membrane. TFC hollow fiber membranes are made by forming a polyamide layer through interfacial polymerization on the outside of a polyethersulfone (PES) or polysulfone (PS) porous layer. The polyamide rejection layer is formed through interfacial polymerization of an aqueous solution of an amine with a solution of an acid chloride in an organic solvent. Another type of filtration membranes is those formed by the layer-by-layer (LbL) deposition method, such as described in Gribova et al. (Chem. Mater., 24: 854-869, 2012) and Wang et al. (Membranes, 5(3): 369-384, 2015).

[0038] HF modules are known in the art and generally are based on polyethersulfone (PES) fibers or fibers of other suitable porous support material, such as polysulfone (PS), polyphenylene sulfone, polyether imide, polyvinylpyrrolidone and polyacrylonitrile including blends and mixtures thereof. In addition, various doping materials may be used when manufacturing the hollow fiber support materials. The present invention uses the known HF modules or the single fibers included in the HF modules as a supporting structure for the TFC layer. Such known HF modules are commonly used in food and beverage applications such as filtering beer and wine, in some water and wastewater applications including wastewater reuse and pool water recycling and for hemodialysis. For instance, the German company Membrana supplies a hollow fiber module containing several thousands of fibers with an overall surface area of 75 square meters per module. Smaller modules with typically 1-2 square meters and around 8,000 to 20,000 fibers are commonly used in medical dialysis applications (Fresenius Medical Care, Gambro). HF modules for use in the method of the invention are typically microfiltration modules or nanofiltration modules having a molecular weight cut-off range of from 20 to 50 kDa. or such as from 30 to 40 kDa. In principle, all these commercial products can be coated through interfacial polymerization with the self assembled nanostructures or compositions of the present invention, for example resulting in a thin film composite layer wherein aquaporin water channels are incorporated. The housing material of the HF modules of the invention can be any suitable material commonly used for HF modules, such as polycarbonate, polysulfone, POM (all of which are transparent), or polypropylene, polyethylene, PVDF and stainless steel can be used. The fibers may be sealed into the HF module housing using commonly known polyurethanes or epoxy adhesive materials and the like. Tubular modules may also be coated in the same way, such as modules described in http://synderfiltration.com/learning-center/articles/module-configurations-process/tubular-membranes/

[0039] Additional examples of HF modules that may be TFC modified according to the invention are found on websites of membrane producers, such as: [0040] http://www.membranafiltration.com/filtration-modules/documentation.cfm [0041] http://www.kochmembrane.com/PDFs/KMS_Puron_Hollow_Fiber_PSH300_PSH600_PSH1800_Modul.aspx [0042] http://www.kochmembrane.com/Membrane-Products/Hollow-Fiber/Ultrafiltration/PURON-Series.aspx [0043] http://www.daicen.co.jp/english/membrane/kogata.html [0044] http://www.spectrumlabs.com/filtration/hfmods.html [0045] http://www.microdyn-nadircom/en/Products/

[0046] Thin-film-composite or TFC membranes as used herein may be prepared using an amine reactant, preferably an aromatic amine, such as a diamine or triamine, e.g., 1,3-diaminobenzene (m-Phenylenediamineabbreviated MPD, generally having a purity of >99%, e.g. as purchased from Sigma-Aldrich) in an aqueous solution, and an acyl halide reactant also known herein as an amine-reactive molecule, such as a di- or triacid chloride, preferably an aromatic acyl halide, e.g. benzene-1,3,5-tricarbonyl chloride (CAS No. 84270-84-8, trimesoyl chloride (TMC), having a purity of around 98%, e.g. as purchased from Sigma-Aldrich) dissolved in an organic solvent where said reactants combine in an interfacial polymerization reaction, cf. U.S. Pat. No. 4,277,344 which describes in detail the formation of a composite membrane comprising a polyamide laminated to a porous membrane support, at the surface of the support membrane, e.g. a polyethersulfone membrane. In general the concentration of the di- or triamine in the aqueous solution is about 1% to about 5% (w/w). Benzene-1,3,5-tricarbonyl chloride is dissolved in a solvent, such as a C.sub.6-C.sub.12 hydrocarbon including hexane (>99.9%, Fisher Chemicals), heptane, octane, nonane, decane etc. (straight chain or branched hydrocarbons) or other low aromatic hydrocarbon solvent, e.g. Isopar G Fluid which is produced from petroleum-based raw materials treated with hydrogen in the presence of a catalyst to produce a low odour fluid the major components of which include isoalkanes. Isopar G Fluid: Chemical Name: Hydrocarbons, C10-C12, isoalkanes, <2% aromatics; CAS No: 64742-48-9, chemical name: Naphtha (petroleum), hydrotreated heavy (from ExxonMobil Chemical). Generally, the concentration of the di- or triacyl halide in the apolar solution is 0.05% to about 1% (w/v). Alternatives to the reactant 1,3-diaminobenzene include diamines such as hexamethylenediamine etc., and alternatives to the reactant benzene-1,3,5-tricarbonyl chloride include a diacyl chloride, adipoyl chloride etc. as known in the art. Interestingly, the amine-reactive molecule, such as TMC, may form condensation products or polymers with amine groups in the PAI polymers in an interfacial polymerization reaction, e.g. as described by X. Feng et al. (Journal of Membrane Science, 472: 141-153, 2014) who have shown how successive layers of aromatic polyamide are formed on a PES support in an interfacial polymerization reaction between PEI and TMC to create a nanofiltration membrane.

[0047] The term polyalkylimine (PAI) includes any oligomer or polymer, or mixture thereof, which is made by polymerization of an alkylimine monomer having at least one imine group (N(H)) incorporated therein. In one embodiment, the polyalkylimine is an amine containing polymer comprising alkyl- or alkenyl-derived units (such as C.sub.2-C.sub.8 alkyl- or alkenyl-derived units) in the backbone. Preferably, the PAI comprises imine-derived units (amine groups) and alkyl-derived units, and most preferably, the PAI consists of amine groups and alkyl-derived units. Preferably, the PAI is selected from the group consisting of poly(ethyleneimine), poly(propyleneimine), poly(propyl-co-ethyleneimine), poly(allylamine), and mixtures thereof. In any case, the PAI preferably has a weight average molecular weight (M.sub.w) of about 500 or about 1,000 or higher, such as about 2,000 or 3,000 or 5,000 or 10,000 or 20,000 or 40,000 Da.

[0048] The condensation polymers described herein are typically produced in an aqueous solution by combining the amine-reactive molecules with a PAI. Amine-reactive molecules are molecules that include at least one moiety that will react with an amine/imine to form a covalent or ionic chemical bond, preferably covalent. Desirably, there are from 0.1 or 0.2 to 0.6 or 0.8 or 1.0 or 1.1 or 1.2 or 2.0 or 2.5 or 3.0 amine-reactive equivalents (ARE) of the amine-reactive molecules that are combined with the PAI. Desirably, the product can be isolated from an aqueous diluent in solution or substantially in solution at a pH of at least 8. The product of the condensation reaction between the PAI and amine-reactive molecule is the condensation polymer as described herein, but in certain embodiments it is not necessary to specifically isolate the condensation polymer from the reaction medium, hence, in certain embodiments, the usefulness of the condensation polymer is as the entire mixture or condensation product.

[0049] The term polyethyleneimine (PEI) as used herein includes a polymer with repeating units composed of NHCH.sub.2CH.sub.2 (an amine group and two methylene groups):

##STR00001##

[0050] Linear polyethyleneimines contain secondary amines, whereas branched polyethyleneimines may contain primary, secondary and tertiary amino groups. Various linear PEI with molecular weight ranging from 1,000 Da or 1,500 Da or 2,000 Da 2500 Da to 4,000 Da or 5000 Da or 8,000 Da or 10,000 Da may be used as well as branched PEI with molecular weights ranging from 800 Da to 2,000 Da. In general, an increase in the branching and molecular weight of the PEI, with lead to an increase in the possible loading of the protein in the structures formed between the PEI and the protein, typically accompanied by size increase of the formed structures. Having in mind that for some applications, such as the production of filtration membranes, the final size of the formed structures should preferably not exceed a maximum of 200 nm the entire library of available PEIs with various molecular weights, linear and branched can be considered. For the PEI-protein complexes a narrow size distribution of around 200 nm (250 to 150 nm) is presently considered the most promising for incorporation into the active layer of filtration.

[0051] All functional polymers have positive charges concentration dependent in accordance with the literature and observations. The more negative the self-assembled structures the larger the structures as measured by DLS. LDAO is very dilute and has negative charge as measured by zeta potential. A colloidal system where structures are sustained in suspension.

[0052] The abbreviation M.sub.n means number average molecular weight. It means the total weight of polymer divided by the number of polymer molecules. Thus, M.sub.n is the molecular weight weighted according to number fractions. The abbreviation M.sub.w means weight average molecular weight. The molecular weight weighted according to weight fractions. Molecular mass may be measured by gel permeation chromatography (GPC) in tetrahydrofuran. Polydispersity index defined as M.sub.n/M.sub.w will be determined from the elution curves obtained in GPC.

[0053] Size of the nanostructures: Preferably, the nanostructures of the present invention have a particle size of between about 10 nm diameter up to 200 nm to about 250 nm diameter depending on the precise components of the nanostructures and the conditions used for their formation. It will be clear to those skilled in the art that a particle size refers to a range of sizes and the number quoted herein refers to the average diameter, most commonly mean hydrodynamic diameter of that range of particles. The nanostructure compositions of the present invention comprise nanoparticles having mean diameters of from about 250 nm to about 200 nm or less, in some cases mean diameters that are less than 200 nm such as less than about 180 nm or less than about 150 nm.

[0054] Examples of molar ratios of transmembrane protein to PAI (such as PEI) is dependent on the transmembrane protein used, the type of PAI used, the degree or amount of branched PAI as opposed to linear PAI, and the desired size of the nanostructure. As an example, for nanostructures of PEI and aquaporin water channels, the molar ratio of transmembrane protein to PEI may be between 1:200 to 1:2000, such as 1:400 to 1:1500, such as 1:600 to 1:1000.

[0055] Examples of linear polyethyleneimines that may be used in accordance with the present invention include: Polyethylenimine, linear, 764582 ALDRICH, (average Mn 5,000, PDI<1.2); Polyethylenimine, linear, 764604 ALDRICH, (average Mn 2,500, PDI<1.2); and Polyethylenimine, linear, 765090 ALDRICH, (average Mn 10,000, PDI 1.2). Examples of branched polyethyleneimine are: Polyethylenimine, branched, 407819 ALDRICH (average Mw 800 by LS, average Mn 600 by GPC), Polyethylenimine, branched, 408727 ALDRICH (average Mw25,000 by LS, average Mn10,000 by GPC). Polysciences, Inc is also provider of some linear polyethyleneimines, polyallylamine M.sub.w17,000, 479136 ALDRICH, CAS Number 30551-89-4, Linear Formula [CH2CH(CH2NH2)]n.

[0056] BASF Lupasol, cf http://product-finder.basf.com/group/corporate/product-finder/de/brand/LUPASOL (accessed on Feb. 8, 2016). Examples of dendritic PEI are: Polyethylene imine dendritic 5000 Da and 25000 Da (Nanopartica) and Polyethylene imine dendritic 5000 Da alkane functionalized (Nanopartica).

[0057] The term self-assembled as used herein refers to the process by which nanostructures formed from polyalkyleneimine, such as polyethyleneimine, components and transmembrane protein components form organized, ordered structures as a result of the ionic or charged interactions between the components, without external direction. In the present invention, self-assembled is synonymous with the term molecular self-assembly. The general properties of self-assembled systems are described at https://en.wikipedia.org/wiki/Self-assembly#Interactions [accessed 8 Feb. 2016], In the present invention, the self-assembled nanostructures formed between polyethyleneimine components and (negatively) charged protein components are believed to be driven by the ionic interactions between the cationically charged polyethyleneimine molecules and anionically charged amino acid residues of the protein component.

[0058] The term nanostructures as used herein refers to particles having a nanomeric scale, and is not intended to convey any specific shape limitation. In particular, nanostructure encompasses nanospheres, nanotubes, nanoboxes, nanoclusters, nanorods, and the like. In certain embodiments, the nanostructures may be nanoparticles and/or nanoparticle cores having a generally polyhedral or spherical geometry.

[0059] The term size as used herein refers to a hydrodynamic diameter of the self-assembled nanostructures.

EXAMPLES

[0060] The aspects of the disclosed embodiments are further illustrated with reference to the following non-limiting examples

Experimental Section

Equipment:

[0061] KTA Start FPLC connected with Laptop, using Unicorn operating system.
Vacuum stream.
Sterile 0.45 M vacuum filter cup.
15 mL PP tubes.

Abbreviations

[0062] CV: column volume.
AQP: Aquaporin Z from E. coli.

LDAO: N,N-Dimethyldodecylamine N-oxide (#40234, Sigma).

[0063] PAGE: Polyacrylamide gel electrophoresis.

Materials and Chemicals:

[0064] HisTrap Gel filtration material (Ni Sepharose 6 Fast Flow #17-5318-03, GE Healthcare) packed into a XK16/20 column (GE Healthcare) at known volume or prepacked 1 ml, 5 ml HisTrap column.
AQP Binding buffer: 20 mM sodium phosphate, 300 mM NaCl, 20 mM imidazole, 10% glycerol, 0.2% LDAO, pH8.0.
LDAO-free AQP Binding buffer: 20 mM sodium phosphate, 300 mM NaCl, 20 mM imidazole, 10% glycerol, pH8.0.
Imidazole-free AQP Binding buffer: 20 mM sodium phosphate, 300 mM NaCl, 10% glycerol, 0.2% LDAO pH8.0.
AQP Elution buffer: 20 mM sodium phosphate, 300 mM NaCl, 200 mM imidazole, 10% glycerol, 0.2% LDAO, pH8.0, ddH.sub.2O.

[0065] General Purification of Aquaporin and Preparation of Aquaporin Stock Solution

Recombinant Production of Aquaporin Z

[0066] All types and variants of aquaporin water channel proteins, including aquaglyceroporins, may be used in the manufacture of membranes and compositions according to this invention, cf. methods described in WO2010/146365. Representative examples include the spinach aquaporin SoPIP2;1 protein and the bacterial aquaporin-Z from E. coli.

[0067] Functional aquaporin-Z was overproduced in E. coli strain BL21(DE3) bacterial cultures as His-tagged protein with a tobacco etch virus cleavage site. The fusion protein has 264 amino acid and a Mw of 27234 Da. Genomic DNA from E. coli DH5 was used as a source for amplifying the AqpZ gene. The AqpZ gene was amplified using gene specific primers with the addition of a tobacco etch virus cleavage site (TEV); ENLYFQSN at the N-terminus of AqpZ. The amplified AqpZ was digested with the enzyme NdeI and BamHI and then ligated to the similarly digested 6-His tagged expression pET28b vector DNA. The positive clones were verified by PCR-screening. The authenticity of the constructs was then confirmed by DNA sequencing.

[0068] The E. coli strain BL21(DE3) was used for expression of the protein. Luria Broth cultures containing 50 g/ml kanamycin were incubated for 13-16 hours at 37 C, diluted 100-fold into fresh LB broth and propagated to a density of about 1.2-1.5 (OD at 600 nm). Expression of recombinant protein was induced by addition of 1 mM IPTG for 3 hour at 35 C. before centrifugation. Harvested cells were resuspended in ice-cold binding buffer (20 mM Tris pH 8.0, 50 mM NaCl, 2 mM -mercaptoethanol, 10% glycerol) in the presence of 0.4 mg/ml lysozyme, 50 units Bensonase and 3% n-octyl -D-Glucopyranoside. The sample was subjected to five times lysis cycles in a microfluidizer at 12,000 Pa. Insoluble material was pelleted by 30 minutes centrifugation at 40,000g. The supernatant was passed through a Q-Sepharose fast flow column (Amersham Pharmacia), and the flow through was 10 collected. The flow though fraction was topped up with NaCl to 300 mM before loaded onto a pre-equilibrated Ni-NTA column. The column was washed with 100 column volumes of a wash buffer (20 mM Tris pH 8.0, 300 mM NaCl, 25 mM imidazole, 2 mM -mercaptoethanol, 10% glycerol) to remove non-specifically bound material. Ni-NTA agarose bound material was eluted with five bed volumes of elution buffer (20 mM Tris pH 8.0, 300 mM NaCl, 300 mM imidazole, 2 mM -mercaptoethanol, 10% 15 glycerol, containing 30 mM n-octyl -D-Glucopyranoside). AqpZ was further purified with anion exchange chromatography; monoQ column (GE healthcare). The sample mixture was diluted and concentrated to bring the salt and imidazole concentration to approximately 10 mM with Amicon concentrator, membrane cut off 10,000 Da before loading to MonoQ column. The buffer used during anion exchange chromatography were (A) 20 mM Tris pH 8.0, 30 mM OG, 10% glycerol and (B) 20 mM 20 Tris pH 8.0, 1 M NaCl, 30 mM OG, 10% glycerol. The eluted peak fractions containing AqpZ from the ion exchange column was pooled. The purified AqpZ extract was kept frozen at 80 C.

Procedure for Purification of Aquaporin Protein

[0069] A batch of frozen extract of aquaporin protein, such as aquaporin Z, AQPZ, e.g. from an E. coli fermentation, was obtained and treated as follows for use in the experiments to produce and characterize membranes comprising protein-PAI nanostructures of the present invention.

[0070] One day before the purification experiment, the AQP extract (stored at 80 C. freezer) was thawed on ice or in a 4 C. refrigerator. Portions of the buffers and ddH.sub.2O were readied at 4 C. The AQP extract was stirred in an adequate chilled beaker on ice bath by a magnetic stick to dissolve any precipitate. 1.5 volumes of pre-chilled LDAO-free AQP binding buffer was gradually added into 1 volume of the solubilized extract (using a further 0.5 volume buffer for rinsing the extract tubes and filtration cup), mixed well and filtered through a sterile 0.45 M vacuum filter cup. Vacuum was applied to the filter cup to avoid excess foaming and the filtrate was placed on ice to use within 2 hours.

[0071] A Histrap column was equilibrated with sterile water followed by AQP Binding buffer at RT. The flow rate was set at 1 ml/min (for 1 mL prepacked column) or 2.5 ml/min (for 5 ml prepacked column and self-packed column). The 3 times diluted extract (on ice water bath) was loaded onto the Histrap column using KTA program. The flow rate was set at 1 ml/min (for 1 mL prepacked column) or 2.5 ml/min (for 5 mL prepacked column and self-packed column). The loading volume was less than 30 ml/ml resin. The extract flow-through on ice-water bath was collected and stored at 4 C. for further use. The column was washed with 10 CV (column volume) ice cold AQP binding buffer. The flow rate was set at 2.5 ml/min (for 5 ml prepacked column and self-packed column) or set at 1 ml/min for 1 ml prepacked column. The AQP protein was eluted with ice cold AQP elution buffer (10 column volume) at flow rate 2.5 ml/min using KTA program. The fraction volume was set to 10 ml and collection started in 15 mL PP tubes after 0.5-1 CV.

[0072] Eluted fractions were capped and stored on ice or 4 C. The AQP purity and conformation was examined by denaturing and native PAGE analysis respectively. Protein concentration was measured by Nanodrop. The extract flow-through may be processed a second and a third time as needed to produce an AQP composition of suitable quality.

[0073] When AQP quality analyses are passed, the protein concentration may be adjusted to 5 mg/ml by adding ice cold imidazole-free AQP binding buffer containing 2% LDAO. Finally the AQP was sterilized by filtration through 0.45 M sterilized cup and stored at 4 C. in refrigerator for use within a month or else stored at 80 C. in a freezer.

Example 1. Preparation and Stopped Flow Testing of PEI-Aquaporin-Z Nanoparticles

[0074] Poly(ethyleneimine) with MW of 4000 Da (linear) (PEI) was purchased from Sigma Aldrich and was used as received without any other purification. 10 mM phosphate saline solution (PBS) (pH 7.2, 136 mM NaCl, 2.6 mM KCl) was prepared by dissolving 8 g NaCl, 0.2 g KCl, 1.44 g Na.sub.2HPO.sub.4 and 0.24 g of KH.sub.2PO.sub.4 in 800 mL MilliQ purified H.sub.2O, adjusting the pH to 7.2 with HCl and completing the volume to 1 L. N,N-Dimethyldodecylamine N-oxide BioXtra (Lauryldimethylamine N-oxide) (99% purity), LDAO was purchased from Sigma Aldrich.

[0075] PEI based self-assembled structures were prepared by direct dissolution method. For that first 100 ml 5 mg/mL PEI in phosphate saline buffer with pH of 7.2 solution was prepared by dissolving 500 mg PEI powder in 100 mL PBS with pH 7.2. Further, 100 mL 5 mg/mL PEI were directly mixed with 1.5 mg as 0.3 mL from 5 mg/mL Aquaporin Z (AQPZ) purified stock solution in 2% LDAO, corresponding to a 1/800 AQPZ/PEI molar ratio of the two components mixture. The PEI-AQPZ mixture was stirred overnight at 170 rotations per minute, overnight not more than 20 hours (but not less than 12 h).

[0076] After stirring, next day the mixture was transferred in 100 mL glass bottle and kept at room temperature. The mixture (the liquid formulationor compositionof PEI-AQPZ based self-assembled structures) can be kept for more than two months. After transfer to the storage glass bottle, the size and the permeability of the PEI-AQPZ self-assembled structures were determined by dynamic light scattering using a ZetaSizer NanoZs from Malvern and stopped-flow using a Bio-Logic SFM 300.

[0077] The size of PEI-AQPZ based self-assembled structures was determined as 11420 nm (80%) of the population, while 20% had a size of around 14715 nm. In plus the zeta potential was determined for the PEI-AQPZ self-assembled structures as 18.4 mV, indicating the positive charge of the structures.

[0078] The permeability data obtained from stopped-flow measurements in 0.3 M NaCl as the osmolyte lead to a fast diffusion coefficient K.sub.i of 1605 s.sup.1, corresponding to an osmotic permeability Pf of 22 m/sec and a permeability of 11 LMH/bar (L/m.sup.2/h)/bar).

[0079] The temperature stability of the PEI based self-assembled structures was determined by warming up 5 mL for 10 min at various temperatures ranging from 30 to 100 C. and their size and water permeability was further determined by dynamic light scattering and stopped-flow measurements.

[0080] The size of the self-assembled structures after the thermal treatment are presented in Table 1.

TABLE-US-00001 TABLE 1 Size distribution of nanoparticles in aqueous composition with PEI self-assembled polymer structures after thermal treatment. Population Temperature/ Population 1 1 Size/ Population 2 Population 2 C. distribution/% nm distribution/% Size/nm RT 80 111 18 20 147 25 30 81 117 20 19 178 28 40 69 118 15 31 164 14 50 93 112 22 7 156 23 60 85 108 23 15 148 32 70 94 92 15 6 132 24 80 81 108 12 19 137 18 90 84 110 14 16 136 22 100 76 115 23 24 142 16

[0081] As seen from Table 1, the size of the formed structures is not affected by temperature exposure even though a slight decrease with around 10 nm is observed from 60 C. degrees up. This decrease does not affect the functionality of the nanostructures nor the aquaporin water channels complexed in them when incorporated in the active layer of filtration membranes.

[0082] The permeability in the formed nanoparticles after thermal treatment in the presence of AQPZ was determined from the stopped-flow measurements.

TABLE-US-00002 TABLE 2 Stopped flow results of composition PEI-AQPZ in 0.3M NaCl. Temperature/ C. Ki Value/s.sup.1 Pf/m/sec Water flux lmh/bar RT 1589 21.7 10.4 30 1601 23.1 11.1 40 1679 24.4 11.7 50 1611 22.2 10.7 60 1707 22.7 10.9 70 1713 20.1 9.9 80 1589 21.2 10.1 90 1540 20.9 10.03 100 1533 21.7 10.4

[0083] From a water permeability point of view in this particular experiment, no significant changes can be observed up to 100 C., a slight fluctuation of both permeability and water flux is correlated with the size change.

Example 2. Preparation and Testing of PEI Nanostructures with AQP9 and AQP12

[0084] In a similar manner, PEI based self-assembled structures with human aquaporins, more specifically aquaporin-9 and aquaporin-12, were prepared by direct dissolution method. For that first 100 ml 5 mg/mL PEI in phosphate saline buffer with pH of 7.2 solution was prepared by dissolving 500 mg PEI powder in 100 mL PBS with pH 7.2. Further, 100 mL 5 mg/mL PEI were directly mixed with 1.5 mg as 0.15 mL from 10 mg/mL Aquaporin-9 and 0.17 mL from 9 mg/mL Aquaporin-12 purified stock solution in 2% LDAO, corresponding to a 1/800 AQP/PEI molar ratio of the two components mixture. The PEI AQP mixture was stirred overnight at 170 rotations per minute, overnight not more than 20 hours (but not less than 12 h).

[0085] After stirring next day the mixture was transferred in 100 mL glass bottle, kept at room temperature and measured in the following day. The size and the permeability of the PEI-AQPZ self-assembled structures were determined by dynamic light scattering using a ZetaSizer NanoZs from Malvern and stopped-flow using a Bio-Logic SFM 300.

[0086] The size of PEI-AQP9 based self-assembled structures was determined as 9115 nm (85%) of the population, while 15% had a size of around 32532 nm. In plus the zeta potential was determined for the PEI-AQP self-assembled structures as 17.9 mV, indicating the positive charge of the structures.

[0087] The permeability data obtained from stopped-flow measurements in 0.3 M NaCl as the osmolyte lead to a fast diffusion coefficient K.sub.i of 1326 s.sup.1.

[0088] The size of PEI-AQP12 based self-assembled structures was determined as 24436 nm (92%) of the population, while 8% had a size of around 32 nm. In plus the zeta potential was determined for the PEI-AQP self-assembled structures as 17.3 mV, indicating the positive charge of the structures.

[0089] The permeability data obtained from stopped-flow measurements in 0.3 M NaCl as the osmolyte lead to a fast diffusion coefficient K.sub.i of 1432 s.sup.1.

Example 3. Preparation and Testing of Nanostructures with PEI 800 b and 1000 b

[0090] In a similar manner as for PEI 4000 Da self-assembled structures with AQPZ, and PEI 800 branched and 10000 branched, were prepared by direct dissolution method. For that first 100 ml 5 mg/mL PEI 800 branched or 10000 branched, in phosphate saline buffer with pH of 7.2 solution was prepared by dissolving 500 mg PEI 800 branched or 10000 branched powder in 100 mL PBS with pH 7.2. Further, 100 mL 5 mg/mL PEI were directly mixed with 1.5 mg as 0.3 mL from 5 mg/mL AqpZ purified stock solution in 2% LDAO, corresponding to a 1/800 AQP/PEI molar ratio of the two components mixture. The PEI AQP mixtures were stirred overnight at 170 rotations per minute, overnight not more than 20 hours (but not less than 12 h). After stirring next day the mixture was transferred in 100 mL glass bottle, kept at room temperature and measured in the following day. The size and the permeability of the PEI-AQPZ self-assembled structures were determined by dynamic light scattering using a ZetaSizer NanoZs from Malvern and stopped-flow using a Bio-Logic SFM 300.

[0091] The size of PEI 800 bAQPZ based self-assembled structures was determined as 18023 nm (100%) of the population. In plus the zeta potential was determined for the PEI-AQP self-assembled structures as 14 mV, indicating the positive charge of the structures.

[0092] The permeability data obtained from stopped-flow measurements in 0.3 M NaCl as the osmolyte lead to a fast diffusion coefficient K.sub.i of 747 s.sup.1.

[0093] The size of PEI 10000 bAQPZ based self-assembled structures was determined as 22037 nm (100%) of the population. In plus the zeta potential was determined for the PEI-AQP self-assembled structures as 4 mV, indicating the positive charge of the structures.

[0094] The permeability data obtained from stopped-flow measurements in 0.3 M NaCl as the osmolyte lead to a fast diffusion coefficient K.sub.i of 1516 s.sup.1.

[0095] The temperature stability of the PEI based self-assembled structures was determined by warming up 5 mL for 10 min at various temperatures ranging from 30 to 100 C. and their size and water permeability was further determined by dynamic light scattering and stopped-flow measurements. For the PEI 600 b based AQPZ self-assembled nanostructures the size does not change up to 50 C., and is increasing with more than 200 nm from 50 C. up, while the Ki values varies with less than 100 s.sup.1. For the PEI 10000 b based AQPZ self-assembled nanostructures the size does not change up to 40 C., and is increasing with more than 150 nm from 40 C. up, while the Ki values varies with less than 100 s.sup.1.

Example 4. Fluorescence Correlation Spectroscopy

[0096] The fluorescent labelling of the transmembrane protein AQPZ was performed by N-hydroxysuccinimide ester (NHS-ester) coupling reaction to primary amines on the proteins. The N terminus of the AQPZ protein contains only hydrophilic amino acid residues. Therefore, it is available for the labelling reaction in the aqueous phase and it is not buried in the hydrophobic part of the membrane protein. In this case, the fluorescent dye Oregon Green 488 (OG488) was used.

[0097] For labeling (cf. Itel et al., Nano. Lett., 15 (6): 3871-3878, 2015), 1 mg AQPZ stock solution was mixed with Oregon Green 488 succinimidyl ester (OG488, 10 mg/ml in DMSO) at a 10-fold molar excess and incubated on ice with agitation for 3 hours in the dark. After the non-reacted dye was removed by centrifugation through 2000 Da filters the labelled AQPZ was further used for reconstitution in linear PEI 4000 Da self-assembled nanostructures. From the molecular brightness the number of AQPZ tetramers per PEI nanostructure was determined as 81. No free protein was detected. For PEI 600 b and 10000 b the number of AQPZ tetramers per PEI nanostructure was determined as 41 (PEI 600 b) and 52 (PEI 1000 b)

Example 5. Characterization by Circular Dichroism

[0098] The protein conformation of the formed self-assembled nanostructures may be characterized by circular dichroism (CD). CD is a suitable method for rapid determination of the secondary structure (-helix and the sheet) and folding properties of proteins that determine its functionality. CD is a spectroscopic technique where the CD spectra of molecules is measured over a range of wavelengths (Greenfield N.J., Nat Protoc. 2006; 1(6): 2876-2890). For that the liquid formulation of PEI-AQPZ based self-assembled structures as prepared according to Example 1 was placed in the measuring cuvette and the CD spectra was recorded and analysed in order to assess the secondary structure of incorporated AQPZ, respectively its functionality.

[0099] The secondary structure of AQPZ reconstituted into PEI self-assembled nanostructures showed a negative ellipicity band at 222 nm compared to 208 nm (222/208 nm ratio equal to 1.15), similar as reported for spinach aquaporin reconstituted in E. coli total lipids, indicating that the protein in not unfolded. (Hansen et al., Biochimica et Biophysica Acta 1808: 2600-2607, 2011).

Example 6. Pilot Machine for Preparing FO Filtration Hollow Fiber Membranes

[0100] A TFC layer was formed on a PES hollow fiber membrane using a pilot coating machine.

a) An MPD/water solution was made by dissolving MPD in MilliQ water to get a 2.5% (W/W) concentration.
b) An Aquaporin/MPD/water solution was made by dissolving 6.25 g MPD, 10 mL Composition as prepared in example 1 in 240 mL Di water.
c) TMC was dissolved in Isopar to a final concentration of 0.15% W/V;
d) A roll of hollow fiber support membrane (UltraPES fibers from Membrana GmbH) was installed on the unwinding unit of the machine,
e) The membrane was threaded through the coating rolls
f) The washing bath was filled with Di water
g) The coating process was run (at 0.6 m/min): [0101] membrane was unrolled from unwinder; [0102] then soaked in MPD/water solution in foulard bath; [0103] surface water was removed by air knife (0.5 bar air); [0104] Aquaporin/MPD/water solution was applied via slot die at pump rate of 1.2 mL/min; [0105] surface water was removed via air knife to ensure a droplet free surface before interfacial polymerization (0.75 bar); [0106] TMC/Isopar solution was applied via slot die at 4.2 mL/min to start interfacial polymerization; [0107] Isopar dried off the surface of the membrane at ambient air; [0108] leftover chemicals were removed in wash bath; [0109] coated membrane was rolled up.
h) The coated membrane was run through a final drying step.

[0110] The coated hollow fiber membranes were bundled and surrounded with a cylindrical casing and polymer plugs in each end to form a hollow fiber module. The module was tested for forward osmosis performance. It was possible to obtain a forward osmosis effect, which showed that there was a coating on the membrane.

Example 7. Preparation of a Hollow Fiber Module Wherein the Outside Surface of the Fibres has been Functionalised with Immobilised AqpZ

[0111] A hollow fiber module having polyethersulfone membranes, such as a custom-made module having 9 fibers corresponding to about 10 cm.sup.2 is used. Alternatively, a membrane area of up to 0.5 m.sup.2 which may correspond to several hundred fibers depending on module length can be used. A thin film composite layer is prepared on the outside fiber surface through interfacial polymerization involving the following steps:

1) Obtain 4 mL PEI-AQPZ mixture as prepared in example 1 above.
2) Dissolve 1,3-diaminobenzene (m-phenylene diamineMPD) in MilliQ water to obtain a solution of 2.5% (w/w) concentration.
3) benzene-1,3,5-tricarbonyl chloride (Trimesoyl ChlorideTMC) is dissolved in hexane to obtain a final concentration of 0.15% (w/v).
4) A 1,3-diaminobenzene/AqpZ mixture is prepared by dissolving/mixing 4 mL of the PEI-AQPZ mixture from step 1 with 6 mL of the solution from step 2.
5) fill solution from step 4) into the module on the outside of the fibers using side inlets of the module; flowrate: 5 mL/min.
6) stop flow after 1 min and leave fibers soaking for 1 min.
7) empty the module and purge with air to get leftover MPD solution out.
8) use air flow to remove surface water from the fibers (air flow rate 25 L/min).
9) pump solution from step 3) into the module using a flow rate of 15 mL/min.
10) after module is filled continue pumping for 30 s.
11) disconnect module from syringe and turn upside down to get last solution out; connect to air and purge at 10 L/min for 5-10 s.
12) fill module with MiliQ by sucking it in from a glass.

[0112] Forward osmosis performance of the formed HF TFC membrane were determined by the characteristics of the polyamide layer formed in the interfacial polymerization process. A standard membrane test is carried out with 1 M NaCl as a draw solution, and before use conductivity of such prepared solution should be about 92.51.5 mS/cm. RO water having a standardized conductivity of <10 S/cm is used as a feed solution without further processing.

[0113] Feed solution was introduced into the module with 4 cm/s on the outside of the fibers through the inlet, whereas draw solution was introduced in the inside of the fibers with 1 cm/s. Draw and the feed were connected in counter-current flow. The pressure of the feed solution and the draw solution from all the inlets were monitored. Tests were carried out at average TMP of 0.2 bar from the feed side. Tests were conducted for 1 hour. The feed solution (here: RO water) and draw solution (1 M NaCl) were not recirculated. Instead, concentrate of feed solution was collected on the balance.

[0114] It was possible to obtain a forward osmosis effect, which showed that there was a coating on the membrane. The water flux, Jw, was measured in 5 experiments to be between 2.11 and 3.40 L/m.sup.2 h. The reverse salt flux, Js, was measured to be in the range of 114 to 254 g/m.sup.2 h.