COMPLEXATION OF HEAVY-METAL CATIONS HG, CD, AND PB WITH PROTEINS OF PSII - EVIDENCE FOR METAL-SULFUR BINDING AND PROTEIN CONFORMATIONAL TRANSITION BY FTIR SPECTROSCOPY

We report the interaction of divalent Cd, Hg, and Pb cations with prot eins of photosystem II (PSII) enriched membranes, in dried film with m etal ion concentrations of 0.01, 0.1, 1, 5, 10, and 20 mM. Fourier tra nsform infrared (FTIR) difference spectroscopy with self-deconvolution and second-derivative resolution enhancement measurements were used t o determine the nature of the metal-protein interaction, the protein c onformational transition, and the structural variations, upon metal co mplex formation. The infrared difference spectroscopic results reveale d the presence of protein aggregation at low metal ion concentration ( 0.01 mM) with no major metal-protein interaction. At higher cation con centrations (1 to 20 mM), metal-protein binding was observed through p eptide C=O and C-N groups with the participation of tyrosine residues. In addition, a major metal-sulfur binding was observed for the Hg ion but not for the Cd or Pb cation. Major conformational transitions fro m those of the mainly alpha-helix (64%) and beta-sheet (10%), in the u ncomplexed proteins to alpha-helix (55-40%) and beta-sheet (10-20%), w ere observed in the presence of high cation concentration. (C) 1996 Ac ademic Press, Inc.