Solution behaviour and zinc complexation of di- and tripeptides with two cysteine units

The dipeptides HCysCysOH, HCysCysOEt, AcCysCysOH, AcCysCysOEt, AclCysdCysOEt and the tripeptides AcCysCysGlyOEt, BzCysGlyCysOEt and BzCysPheCysOEt were prepared analytically pure in gram quantities. Potentiometric titrations in the presence of zinc nitrate have shown that most of them form ZnL and ZnL2 complexes in solution, representing ZnS2 and ZnS4 coordinations, which also occur as structural motifs in zinc enzymes. The fully protected CysCys donors AcCysCysOEt and AcCysCysGlyOEt also form Zn2L4 and Zn3L4 species in solution, indicating the beginning of aggregation. Accordingly the isolated complexes with a 1:1 Zn:L composition are insoluble polymers, and monomers exist only in dilute solutions or in the presence of the N,N coligand neocuproin. The stability of the purely thiolate-coordinated complexes increases for L=lCyslCys