Compound of the general formula (Ia), (Ib) and (Ic) R.sub.50 and R.sub.60 are different form each other and are selected from the group consisting of R.sub.70, ZR.sub.70, ZOH, ZNH.sub.2, ZSH, ZOC(O)R.sub.70, OC(O)R.sub.70, COOH and its corresponding salts, C(O)NH.sub.2, C(O)NHR.sub.70, C(O)N(R.sub.70).sub.2, COOR.sub.70, ZCOOH and its corresponding salts, ZC(O)NHR.sub.70, ZC(O)NH.sub.2, ZC(O)N(R.sub.70).sub.2, ZCOOR.sub.70, CH(COOH).sub.2 and its corresponding salts, CH(COOR.sub.70).sub.2, and ZSO.sub.3 wherein R.sub.70 is selected from the group consisting of a linear or branched C.sub.1 to C.sub.20 alkyl, (C.sub.1 to C.sub.10)-alkyloxy-(C.sub.1 to C.sub.10)-alkyl, C.sub.2 to C.sub.10 alkenyl, C.sub.6 to C.sub.12 aryl, C.sub.3 to C.sub.10 cycloalkyl, cycloalkylalkyl and cycloalkylalkenyl, wherein Z is a linear or branched C.sub.1 to C.sub.10 alkyl, linear or branched C.sub.3 to C.sub.10 cycloalkyl, a linear or branched C.sub.6 to C.sub.10 aryl or a (C.sub.1 to C.sub.10)-alkyloxy-(C.sub.1 to C.sub.10)-alkyl, cycloalkylalkyl and cycloalkylalkenyl.

##STR00001##