A COMPARATIVE-STUDY OF FE(II) AND FE(III) IONS COMPLEXATION WITH PROTEINS OF THE LIGHT-HARVESTING COMPLEX OF CHLOROPLAST THYLAKOID MEMBRANES

The interactions of Fe(II) and Fe(III) ions with proteins of the light -harvesting complex (LHC-II) of chloroplast thylakoid membranes were s tudied in aqueous solution with metal ion concentrations of 0.01 to 20 mM, using Fourier Transform infrared (FTIR) difference spectroscopic technique. Correlations between spectral changes and metal ion binding mode, protein conformational transitions, and structural variations w ere established. FTIR difference spectroscopic results have shown the presence of a strong iron-protein complexation at high metal ion conce ntrations (5-20 mM), while at low cation concentrations (0.01-1 mM) me tal-protein binding is less significant. Fe(III) ion binding was mainl y through the protein carbonyl groups, whereas Fe(II) coordination was via both C=O and C-N donor atoms. A comparison between the Fe-protein and Hg-protein complexes of the LHC-II systems indicated the presence of a strong metal-sulfur binding for the Hg(II) ion with no significa nt Fe-sulfur interaction. Major conformational changes from that of th e alpha-helix to beta-sheet and turn structures were observed for the protein in the presence of both Fe(II) and Fe(III) ions at high cation concentrations (5-20 mM).