COMPLEXATION OF COPPER AND ZINC IONS WITH PROTEINS OF A LIGHT-HARVESTING COMPLEX (LHC-II) OF CHLOROPLAST THYLAKOID MEMBRANES STUDIED BY FT-IR SPECTROSCOPY

The interaction of Zn(II) and Cu(II) ions with the light-harvesting proteins (LHC-II) of chloroplast thylakoid membranes was studied in aqueous solution with metal ion concentrations of 0.01 to 20 mM, using Fourier transform-infrared (FT-IR) spectroscopy. Analyses of the metal ion binding mode and protein conformational variations were carried out and correlations between spectral changes and metal protein complexation were established. Infrared difference spectroscopic results revealed the presence of a strong metal protein interaction at high metal ion concentrations, while at low concentrations complexation was negligible. The binding of Zn and Cu ions was found to be with the protein carbonyl groups at low metal ion concentrations, whereas C=O and C-N groups were the main coordination sites at higher concentrations. A major conformational variation from alpha-helix to beta-sheet and turn structures was observed in the presence of a concentrated metal ion solution.