ZINC-COMPLEXES OF AMINO-ACIDS AND PEPTIDES .2. COORDINATION OF SIMPLE HISTIDINE DERIVATIVES TO ZINC

The bis(L-histidine)ZnCl2 complex 1 is likely to have tetrahedral ZnN2Cl2 coordination. For the compound (L-histidine)-ZnCl2 . HCl (2) a structure determination has revealed a tetrahedral ZnOCl3 coordination. Of the C-protected histidine derivatives His-OMe and His-NH2 and,zinc salts of noncoordinating anions the complexes (His-OMe)(2)ZnX(2) (3) and (His-NH2)(2)Zn(ClO4)(2) (4) were obtained. A structure determination of (His-OMe)(2)Zn(BPh(4))(2) . H2O has revealed a ZnN4 coordination with chelating histidine units. Of the N-protected histidine derivative N alpha-acetylhistidine the complexes (AcHis)ZnX . H2O (5, X = ClO4, BF4) have been isolated which are coordination polymers in the solid state. From pH-dependent C-13-NMR studies it is conduced that in solution the imidazole N and carboxyl O are coordinated to zinc in (AcHis)(2)n and (Ac-His)(2)Zn units. The doubly protected histidine derivative Bz-His-OMe acts as a monodentate ligand forming the tetrahedral complexes (Bz-His-OMe)(4)zn(ClO4)(2) (6) and [(Bz-His-OMe)(2)Zn(2,9-dimethyl-o-phenanthroline)](ClO4)(2) (7).