INTERACTION OF MG(II), CA(II) AND MN(II) IONS WITH THE LIGHT-HARVESTING PROTEINS OF CHLOROPLAST THYLAKOID MEMBRANES STUDIED BY FT-IR DIFFERENCE SPECTROSCOPY

The interaction of Mg(II), Ca(II) and Mn(II) ions with the light-harvesting (LHC-II) proteins of chloroplast thylakoid membranes was investigated in aqueous solution at different metal ion concentrations (0.01-20 mM), using Fourier transform-infrared (FT-IR) difference spectroscopy. The infrared difference spectroscopic results for the amide I and amide II regions (1800-1500 cm-1) have shown a strong metal-protein interaction at high metal ion concentrations (5-20 mM), whereas at very low concentrations (0.01-1 mM) the metal cation binding is negligibie. The metal ion binding is mainly via the protein carbonyl group at low cation concentration, whereas metal ion coordination to the protein C=O and C-N groups were observed at higher cation concentrations. The Mn-tyrosine binding was also observed at high metal ion concentrations. Major conformational changes from alpha-helix (48% in uncomplexed protein) to beta-sheet and turn structures were observed in the presence of these metal cations at high concentrations (10-20 mM).