INTERACTION OF TOXIC METAL-IONS CD(2+), HG(2+), AND PB(2+) WITH LIGHT-HARVESTING PROTEINS OF CHLOROPLAST THYLAKOID MEMBRANES - AN FTIR SPECTROSCOPIC STUDY

The interaction of divalent metal ions Cd, Hg, and Pb with the light-harvesting proteins (LHC-II) of chloroplast thylakoid membranes was investigated in aqueous. solution with metal ion concentrations of 0.01 to 20 mM, using Fourier Transform infrared (FTIR) difference spectroscopy. Correlations between the metal ion binding mode, protein conformational transitions, and structural variations are established. Infrared difference spectroscopic evidence has shown strong Hg ion binding with different protein subunits at very low metal ion concentration with drastic structural modifications of interacted proteins. The Cd ion binding was observed at higher metal ion concentration with major protein conformational changes, while Pb ion interaction was less effective on protein conformation. The major metal ion binding sites were those of the protein carbonyl group or the nitrogen atom and/or both, while the sulphur donor sites were also the target of Hg-protein complexation.