F-19 NMR INVESTIGATIONS OF COBALT(II) COMPLEXES WITH CYSTEINE-CONTAINING PEPTIDE LIGANDS

Isotropically shifted F-19 NMR signals were observed for novel mononuc lear Co(II) complexes, (Et(4)N)(2)[Co(Z-Cys-Pro-Leu-Cys-Gly-X)(2)] (Z = benzyloxycarbonyl, X = NHC6H4-p = F, NHC6H4-m-F and NHCH2CH2C6H4-p-F ). Such isotropically shifted signals show very short F-19 spin-lattic e relaxation times (T-1) in the range 20-150 ms compared with the corr esponding SH-free peptide ligands, Z-Cys(SH)-Pro-Leu-Cys(SH)-Gly-X, wh ich revealed signals with F-19 T-1 values in the range 1600-3200 ms. T he observed F-19 NMR isotropic shifts and short F-19 T-1 values in the Co(II)-cysteine-peptide complexes are ascribed to the formation of NH -S hydrogen bonds and the presence of interactions of aromatic groups with the sulphur atom of the coordinated cysteine residue.