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Copper(II) complexes of the pentapeptides Ac-HisAlaHisValHis-NH2, Ac-HisValHisAlaHis-NH2, Ac-HisProHisAlaHis-NH2, Ac-HisAlaHisProHis-NH2, Ac-HisGlyHisValHis-NH2 and Ac-HisValHisGlyHis-NH2 have been studied by potentiometric, UV-Vis, CD and EPR spectroscopic methods. It has been found that the pentapeptides are efficient ligands for the complexation with copper(II) and exhibit an outstanding versatility in the co-ordination geometry of complexes. The presence of three histidyl residues provides a high possibility for the formation of macrochelates via the exclusive binding of imidazole-N donor atoms. The macrochelation suppresses, but cannot preclude the deprotonation and metal ion co-ordination of amide functions and the species [CuH(-2)L] and [Cu2H(-4)L] predominate at physiological pH in equimolar solutions and in the presence of excess metal ions, respectively. It is also clear from the data that both C-terminal and internal histidyl residues can work as the anchoring sites for metal binding and subsequent amide deprotonation resulting in the formation of co-ordination isomers and dinuclear species in equimolar solutions and in the presence of excess metal ions, respectively. In more alkaline solutions (pH approximately 10) a third amide function can be deprotonated and co-ordinated in the species [CuH(-3)L]- with (N-,N-,N-,N(im)) co-ordination. The dinuclear species [Cu2H(-5)L]- and [Cu2H(-6)L](2-) containing hydroxide ions and/or imidazolato bridges are formed at high pH in the presence of excess of metal ions. The insertion of one proline into the sequence preceding histidyl residues hinders the deprotonation of amide functions at that site and the formation of only mononuclear complexes was observed with these peptides.

The cis isomer of the pendant arm macrocyclic hexaamine 6,13-dimethyl-1,4,8,11 -tetraazacyclotetradecane-6,13-diamine ( L2) has been isolated for the first time. The ligand has been complexed with cadmium(ii) and characterised structurally. The complex [CdL2] [CIO,], crystallises in the monoclinic space group P2,/n, a = 10.238(1), b = 16.445(2), c = 13.106(2) A, p = 103.33(1)", and Z = 4. The Cd-N bond lengths are not unusual by comparison with those exhibited by typical CdN,*+ complexes, compared with the generally short M-N bond lengths that have been observed in sexidentate co-ordinated complexes of the trans isomer L'. Reaction of L' with cadmium(l1) does not yield a hexaamine complex, instead the pendant primary amines are not co-ordinated and the four secondary amines encircle the metal centre.