Method for NMR measurements on quadrupolar nuclei

Abstract

A method is offered which permits NMR measurements of integer spin nuclei to be performed at higher sensitivity than heretofore. In particular, the method enables high-resolution multidimensional correlation NMR measurements on integer spin nucleus S having integer spin S and nucleus I of other spin species. The method starts with applying an RF magnetic field having a frequency that is n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the spin S. Magnetization transfer is effected between the nucleus I and the integer spin nucleus S.

Claims

1. A method of performing high-resolution multidimensional correlation NMR measurements on integer spin nucleus S having integer spin S and other nucleus I having a spin species I different from the integer spin S, wherein said method comprises the steps of: (1) exciting magnetization of the spin I within a sample by applying, with an NMR spectrometer, a first 90-degree RF pulse having a frequency that is n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S and thereby effecting a magnetization transfer between the nucleus I of the different spin species and the integer spin S; (2) converting the excited magnetization into heteronuclear coherence between the spins I and S during a period of T by utilizing at least one of the following (a) through (c) interactions, or a combination thereof: (a) heteronuclear J coupling between I and S; (b) heteronuclear residual dipolar splitting between I and S; and (c) heteronuclear dipolar coupling between I and S, and wherein an NMR signal is derived from the sample, with the sample being either stationary within a static magnetic field, or spun at high speed about one axis; (3) applying with the NMR spectrometer, an overtone RF pulse having a frequency that is n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S after the period of T in order to start a temporal evolution of the heteronuclear coherence while reflecting the environment of the integer spin S; (4) canceling the temporal evolution reflecting the environment of the spin I during a period t.sub.1 and between two periods L by the use of an applied 180-degree RF pulse also provided by the NMR spectrometer; (5) applying with the NMR spectrometer, another overtone RF pulse having a frequency that is n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S after the period of t.sub.1, thus terminating the temporal evolution of the heteronuclear coherence which reflects the environment of the integer spin S; (6) converting the heteronuclear coherence into magnetization of the spin I during the second period of T by again using of at least one of the interactions, or the combination of interactions, from step (2); and (7) detecting, recording, and providing high-resolution multidimensional correlation NMR measurements, of the spin I reflecting the environment of the integer spin S, with the NMR spectrometer, during a period of t2.

2. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 1, wherein an NMR signal from the integer spin nucleus S is indirectly obtained through an NMR signal originated from the spin I by heteronuclear multiple quantum coherence (HMQC).

3. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 1, wherein an NMR signal from the integer spin nucleus S is indirectly obtained through an NMR signal originated from the spin I by heteronuclear single quantum coherence (HSQC).

4. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 2, combined with double quantum coherence NMR spectroscopy.

5. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 3, combined with double quantum coherence NMR spectroscopy.

6. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 1 wherein said integer spin nucleus S is a quadrupolar nucleus.

7. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 2, wherein said integer spin nucleus S is a quadrupolar nucleus.

8. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 3, wherein said integer spin nucleus S is a quadrupolar nucleus.

9. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 4 wherein said integer spin nucleus S is a quadrupolar nucleus.

10. A method of performing high-resolution multidimensional correlation NMR measurements on integer spin nucleus S having integer spin S and other nucleus I having a spin species I different from the integer spin S, wherein said method comprises the steps of: (1) exciting magnetization of the spin I within a sample by applying, with an NMR spectrometer, a first 90-degree RF pulse having a frequency that is n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S and thereby effecting a magnetization transfer between the nucleus I of the different spin species and the integer spin S; (2) converting the excited magnetization into heteronuclear coherence between the spins I and S during a period of T by utilizing at least one of the following (a) through (c) interactions, or a combination thereof: (a) heteronuclear J coupling between I and S; (b) heteronuclear residual dipolar splitting between I and S; and (c) heteronuclear dipolar coupling between I and S, and wherein an NMR signal is derived from the sample, with the sample being either stationary within a static magnetic field, or spun at high speed about one axis; (3) starting temporal evolution of the heteronuclear coherence which reflects the environment of the spin S by applying with the NMR spectrometer, additional RF pulses having a frequency n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S after the period of T and converting a lateral magnetization signal from the spin I into vertical magnetization by applying with the NMR spectrometer a 90-degree RF pulse to the spin I at the same time as the overtone RF pulse; (4) terminating the temporal evolution of the heteronuclear coherence which reflects the environment of the spin S by applying with the NMR spectrometer, additional RF pulses having a frequency n times (where n is an integer equal to or greater than 2) the Larmor frequency of the integer spin nucleus S to the integer spin S after a period of t1 and applying with the NMR spectrometer a 90-degree RF pulse to the spin I at the same time as the overtone RF pulse in order to convert the vertical magnetization signal from the spin I back into lateral magnetization; (5) converting the heteronuclear coherence into magnetization of the spin I during the second period of T by repeating the use of at least one of the interactions or the combination of interactions from step 2; and (6) detecting, recording, and providing high-resolution multidimensional correlation NMR measurements, of the spin I reflecting the environment of the integer spin S, with the NMR spectrometer, during a period of t2.

11. A method of performing high-resolution multidimensional correlation NMR measurements as set forth in claim 10 wherein said integer spin nucleus S is a quadrupolar nucleus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a conventional method for NMR measurements on integer spin nuclei.

(2) FIG. 2 further illustrates the conventional method illustrated in FIG. 1.

(3) FIG. 3 further illustrates the conventional method illustrated in FIG. 1.

(4) FIG. 4 illustrates another conventional method for NMR measurements on integer spin nuclei.

(5) FIG. 5 further illustrates the conventional method illustrated in FIG. 4.

(6) FIG. 6 illustrates a further conventional method for NMR measurements on integer spin nuclei.

(7) FIG. 7 illustrates a still other conventional method for NMR measurements on integer spin nuclei.

(8) FIG. 8 illustrates one method for NMR measurements on integer spin nuclei, the method being according to the present invention.

(9) FIG. 9 shows NMR data obtained by a method for NMR measurements on integer spin nuclei, the method being according to the invention.

(10) FIG. 10 illustrates another method for NMR measurements on integer spin nuclei, the method being according to the present invention.

(11) FIG. 11 illustrates a further method for NMR measurements on integer spin nuclei, the method being according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(12) The preferred embodiments of the present invention are hereinafter described with reference to the drawings. In the present invention, at least one overtone excitation is applied to integer spin S to excite two quantum transitions. The excited transitions are observed. A multidimensional NMR measurement of correlation with other different nuclear species I is performed. The excitation of the two quantum transitions using the overtone excitation suppresses first-order quadrupolar powder pattern. The NMR detection sensitivity is improved by (i) using spin I for initial magnetization, (ii) observing the spin I, or (iii) using both simultaneously. Generally, integer spin nuclei S described below are all quadrupolar nuclei.

Embodiment 1

(13) This is a normal heteronuclear multiple quantum coherence (HMQC) measurement in which irradiation of spin S is replaced by overtone irradiation.

(14) First, heteronuclear multiple quantum coherence (HMQC) is described briefly. Where there are two different nuclear species I and S, HMQC is a technique for indirectly measuring an NMR signal of the nucleus S through the nucleus I. In most cases, the signal of the nucleus S of lower sensitivity is observed via the nucleus I of higher sensitivity. Consequently, the NMR detection sensitivity is improved.

(15) The principle of measurement is as follows. First, a magnetization signal of the nucleus I is shifted to the nucleus S. An NMR signal of the nucleus S is recorded. Then, the magnetization signal is shifted again to the nucleus I and an NMR spectrum is recorded. During recording of the NMR signal of the nucleus S, the magnetization signal of the nucleus I is lateral magnetization.

(16) In the present embodiment, such HMQC is used. Overtone excitation of .sup.14N nucleus is performed, and an observation is made. A pulse sequence for this purpose is illustrated in FIG. 8. The pulse sequence is described below.

(17) (1) A first 90-degree pulse excites magnetization of spin I.

(18) (2) The magnetization is converted into coherence between different nuclear species, i.e., spins I and S, during the first period τ. For this conversion, one of J coupling between the spins I and S, residual dipolar splitting (RDS), and dipolar coupling or any combination of them is used. For this purpose, no pulse needs to be applied during the period τ. However, the conversion can be carried out more positively by application of a pulse. For example, the conversion can be carried out more efficiently by applying a pulse for restoring the dipolar interaction between the spins I and S.

(19) (3) An overtone pulse is applied to the spin S after the first period τ. This starts the coherence between the different nuclear species evolving temporally while reflecting the environment of the spin S.

(20) (4) A 180-degree pulse cancels the temporal evolution reflecting the environment of the spin I during a period of t.sub.1 and between two periods T.

(21) (5) An overtone pulse of the spin S is applied after the period t.sub.1. This terminates the temporal evolution of the coherence between the different nuclear species, the evolution reflecting the environment of the spin S.

(22) (6) During the second period τ, the heteronuclear coherence is converted to magnetization of the spin I.

(23) (7) During a period of t.sub.2, the magnetization of the spin I reflecting the environment of the spin S is observed. A signal of the spin S is indirectly detected via the spin I.

(24) The use of this pulse sequence permits improvement of the sensitivity because the spins I of higher sensitivity than the spin S are initially magnetized and because the spin I of higher sensitivity than the spin S is observed.

(25) Furthermore, the use of double quantum coherence making use of overtone excitation can suppress first-order quadrupolar powder pattern. When the vertical magnetization relaxation time of the spin I is shorter than the vertical magnetization relaxation time of the spin S, a decrease in the observation time can be accomplished. The operation of the present embodiment is described below.

(26) (1) Single quantum coherence (I.sup.(±1)) of the spin I is excited, where the superscript indicates the order of the coherence.

(27) (2) During the first period τ, magnetization (I.sup.(±1) S.sup.(0)) between the spins I and S is generated using one or any combination of heteronuclear J coupling, heteronuclear residual dipolar splitting, and heteronuclear dipolar coupling.

(28) (3) Overtone excitation is applied to the spin S to convert the magnetization of the spin S into double quantum coherence (I.sup.(±1) S.sup.(±2)).

(29) (4) During the period t.sub.1, the double quantum coherence (I.sup.(±1) S.sup.(±2)) is made to evolve temporally. The temporal evolution of the single quantum coherence (I.sup.(±1)) is canceled by a 180-degree pulse applied to the spin I. Only the temporal evolution of S.sup.(±2) is recorded.

(30) (5) The double quantum coherence I.sup.(±1) S.sup.(±2) that has been evolved temporally is converted into I.sup.(±1) S.sup.(0) by overtone irradiation of the spin S.

(31) (6) During the second period τ, the double quantum coherence is converted into single quantum coherence (I.sup.(±1)) of the spin I using one or any combination of heteronuclear J coupling, heteronuclear residual dipolar splitting, and heteronuclear dipolar coupling. A signal of the spin I is observed at the time t.sub.2. The results of a measurement performed according to the present embodiment are given below.

(32) Overtone heteronuclear multiple quantum coherence (HMQC) spectroscopy was applied to L-histidine, and a .sup.1H/.sup.14N measurement was performed. This is illustrated in FIG. 9. At this time, I=.sup.1H and S=.sup.14N. .sup.14N nucleus was irradiated with an overtone frequency that was double the Larmor frequency of .sup.14N nucleus, and then HMQC was measured. Correlation between the double quantum coherence of .sup.14N nucleus excited by the overtone irradiation and .sup.1H nucleus was obtained.

Embodiment 2

(33) This is a heteronuclear single quantum coherence (HSQC) measurement in which irradiation of the spin S has been replaced by overtone irradiation. First, HSCQ is described briefly. HSQC is a technique for indirectly observing a magnetization signal of the nucleus S via the nucleus I in the same way as for HMQC of embodiment 1. However, the difference is that the magnetization signal from the nucleus I is converted into vertical magnetization while an NMR signal from the nucleus S is being recorded. It can be expected that the NMR signal from the nucleus S will be detected at higher resolution than in the case of HMQC.

(34) In the present embodiment, such HSQC is used. .sup.14N nucleus is subjected to overtone irradiation and observed. A pulse sequence for this purpose is illustrated in FIG. 10. The pulse sequence is described now.

(35) (1) A first 90-degree pulse is applied to excite magnetization of spin I.

(36) (2) The magnetization is converted into coherence between different nuclear species, i.e., spins I and S, during the first period τ. For the conversion, any one of J coupling between the spins I and S, residual dipolar splitting (RDS), and dipolar coupling or any combination of them is used. For this conversion, no pulse needs to be applied during the first period τ. However, if a pulse is applied, the conversion can be performed more positively. For instance, the conversion can be performed more efficiently by applying a pulse to restore the dipolar interaction between the spins I and S.

(37) (3) An overtone pulse is applied to the spin S after the first period τ to start temporal evolution of heteronuclear coherence that reflects the environment of the spin S. A magnetization signal from the spin I is converted into vertical magnetization by applying a 90-degree pulse to the spin I at the same timing as the overtone pulse.

(38) (4) A 180-degree pulse cancels the temporal evolution during the period t.sub.1 and between two periods τ, the evolution reflecting the environment of the spin I. This 180-degree pulse is not essential.

(39) (5) An overtone pulse of the spin S applied after the period t.sub.1 ceases the temporal evolution of the heteronuclear coherence reflecting the environment of the spin S. A 90-degree pulse is applied to the spin I at the same timing as the overtone pulse to return the magnetization signal from the spin I to lateral magnetization.

(40) (6) During the second period τ, the heteronuclear coherence is converted into magnetization of the spin I.

(41) (7) During a period of t.sub.2, magnetization of the spin I reflecting the environment of the spin S is observed, and a signal from the spin S is indirectly detected via the spin I.

(42) The use of this pulse sequence permits improvement of the sensitivity because the spin I of higher sensitivity than the spin S is initially magnetized and because the spin I of higher sensitivity than the spin S is observed.

(43) Furthermore, first-order quadrupolar powder pattern can be suppressed by using double quantum coherence utilizing overtone irradiation. In addition, where the vertical magnetization relaxation time of the spin I is shorter than the vertical magnetization relaxation time of the spin S, a decrease in the NMR observation time can be accomplished. The operation of the present embodiment is described below.

(44) (1) Single quantum coherence (I.sup.(±1) of the spin I is excited, where the superscript indicates the order of the coherence.

(45) (2) During the first period τ, magnetization (I.sup.(±1) S.sup.(0)) between the spins I and S is generated using one or any combination of heteronuclear J coupling, heteronuclear residual dipolar splitting, and heteronuclear dipolar coupling.

(46) (3) Overtone excitation is applied to the spin S and excitation is applied to the spin I to convert the magnetization of the spin S into double quantum coherence (I.sup.(±1) S.sup.(±2)).

(47) (4) During the period t.sub.1, the double quantum coherence (I.sup.(0) S.sup.(±2)) is evolved temporally. The temporal evolution of the single quantum coherence (I.sup.(±1)) is canceled by a 180-degree pulse applied to the spin I. Since I.sup.(0) does not evolve in time, only the temporal evolution of S.sup.(±2) is recorded. In order to reduce the effects of the interaction between the spins I and S (i.e., to decouple the interaction), a 180-degree pulse can be applied to the spin I.

(48) (5) The double quantum coherence I.sup.(0) S.sup.(±2) that has been evolved temporally is converted into I.sup.(±1) S.sup.(0) by overtone irradiation of the spin S and irradiation of the spin I.

(49) (6) During the second period τ, the double quantum coherence is converted into single quantum coherence I.sup.(±1) of the spin I using one or any combination of heteronuclear J coupling, heteronuclear residual dipolar splitting, and heteronuclear dipolar coupling. A signal of the spin I is observed at the time t.sub.2.

Embodiment 3

(50) The present embodiment is an example of combination of heteronuclear multiple quantum coherence (HMQC) and other multidimensional NMR method. It is easy to combine embodiment 1 and other multidimensional NMR method. As one example, an example of combination of HMQC and a measurement of exchange of magnetization with spin I is shown in FIG. 11. Note that such combinations are infinite in number and that the following example is merely one example. The following six advantageous effects can be obtained from the embodiments described so far.

(51) (1) Double quantum coherence is excited and observed by applying overtone excitation to the integer spin S and, therefore, odd orders (first order, third order, and so on) of quadrupolar powder pattern can be removed from a spectrum of the spin S. This leads to an improvement of the resolution.

(52) (2) The sensitivity is improved by using magnetization of the spin I of higher sensitivity than the spin S for initial magnetization.

(53) (3) The sensitivity is improved by observing the spin I of higher sensitivity than the spin S.

(54) (4) The process becomes more tolerant to deviation of the setting of the magic angle by employing double quantum coherence of integer spin S.

(55) (5) The period t.sub.1 in which an observation is made can be set at will by utilizing double quantum coherence of the integer spin S.

(56) (6) Where the spin I is shorter in vertical magnetization relaxation time than the spin S, the number of measurements per unit time can be increased. This results in an improvement of sensitivity per unit time.

(57) The present invention can be widely applied to high-sensitivity NMR measurements of integer spin nuclei.

(58) Having thus described our invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.