N-p-Amino- and N-p-nitro-phenylsulfonyl derivatives of dipeptides, a new family of ligands for copper(II). Potentiometric and spectroscopic studies

Abstract

The co-ordination ability of four dipeptide analogues substituted on the N-terminal amino group with p-nitrophenylsulfonyl (nps-Ala-Ala and nps-Ala-His) and p-aminophenylsulfonyl (aps-Ala-Ala and aps-Ala-His) groups was studied by potentiometric and spectroscopic (UV/VIS absorption, CD and EPR) techniques. The N-terminal sulfonyl substituent drastically changes the acidity of the sulfonamide proton making nitrogen very efficient in binding to Cu^II. The sulfonamide nitrogen having pK between 9 and 11 does not need any anchoring binding group to form complexes with Cu^II. The para substituent on the phenyl ring (amino or nitro) influences very strongly the acidity of the sulfonamide proton. The nps or aps moieties change the co-ordination equilibria considerably when compared to the parent dipeptide Ala-His. Both groups enforce the formation of dimeric complexes, whereas in the case of the parent dipeptide the major species are only monomeric.