PROTEIN SECONDARY STRUCTURE OF THE PHOTOSYSTEM-II COMPLEX STUDIED BY FT-IR SPECTROSCOPY - CHANGES IN THE AMIDE-I REGION MEDIATED BY DIGALACTOSYLDIACYLGLYCEROL AND DIVALENT-CATIONS

To investigate the molecular interaction between the photosystem II (P SII) complex and digalactosyldiacylglycerol (DGDG) in the presence and absence of CaCl2 and MgCl2, we performed a Fourier transform-infrared (FT-IR) spectroscopic study of the amide I region (1700-1600 cm-1). F rom the spectral simulation of the amide I band envelope using second derivative spectra and gaussian curve-fitting, we showed that the PSII complex is predominantly composed of 31% alpha-helix, 18% beta-sheet and 27% beta-turn. Upon interaction with DGDG, the protein secondary s tructure is significantly altered. One observes a loss of alpha-helica l content (about 7%) which is compensated by an increase of random str uctures. We also found that the proportions of the components in the a mide I band vary upon MgCl2 addition, that is, an increase in the beta -sheet content (about 6%) and a decrease of the beta-turn content (abo ut 5%). The results indicate that the active sites of Mg2+ might be lo cated in the beta-sheets.