HIGH-FIELD NMR-STUDY OF THE BINDING OF LEAD(II) TO CYSTEINE AND GLUTATHIONE

High field proton and carbon-13 NMR spectroscopy has been used to study the interaction of lead(II) with both cysteine and the tripeptide glutathione in D2O over a wide pD range. No binding of lead(II) to either biological ligand was observed in acid solution. In alkaline solution, PbL and/or PbL2 complexes were formed with cysteine, depending on the [Pb2+]:[cysteine] ratio. Chemical shifts experienced by the cysteine protons in the 1:1 complex, together with calculated rotamer populations, indicate a mixture of terdentate (NH2, COO-, S-) and bidentate coordination with binding through the sulfur and carboxylate groups favoured in the bidentate case. For the PbL, complex, only limited chemical shift vs pD data could be obtained due to precipitation problems between pD 3.4 and 9.8. However, in alkaline solution, coordination of Pb2+ through the S- donor was again confirmed. With glutathione, both PbL and PbL2 complexes were also formed in alkaline solution. Proton chemical shift data are inconsistent with a previously proposed tetradentate binding mode of glutathione. For the PbL2 complex no change in the partial rotamer populations of the cysteinyl residue occurs upon complexation, indicating monodentate coordination only through the S- group. Chemical shift data also support monodentate coordination through the S- group and monodentate coordination through the S-group of glutathione in the 1:1 PbL complex.