N-P-AMINO-PHENYLSULFONYL AND N-P-NITRO-PHENYLSULFONYL DERIVATIVES OF DIPEPTIDES, A NEW FAMILY OF LIGANDS FOR COPPER(II) - POTENTIOMETRIC AND SPECTROSCOPIC STUDIES

The co-ordination ability of four dipeptide analogues substituted on t he N-terminal amino group with p-nitrophenylsulfonyl (nps-Ala-Ala and nps-Ala-His) and p-aminophenylsulfonyl (aps-Ala-Ala and aps-Ala-His) g roups was studied by potentiometric and spectroscopic (UV/VIS absorpti on, CD and EPR) techniques. The N-terminal sulfonyl substituent drasti cally changes the acidity of the sulfonamide proton making nitrogen ve ry efficient in binding to Cu-II The sulfonamide nitrogen having pK be tween 9 and 11 does not need any anchoring binding group to-form compl exes with Cu-II. The para substituent on the phenyl ring (amino or nit ro) influences very strongly the acidity of the sulfonamide proton. Th e nps or aps moieties change the co-ordination equilibria considerably when compared to the parent dipeptide Ala-His. Both groups enforce th e formation of dimeric complexes, whereas in the case of the parent di peptide the major species are only monomeric.