STRUCTURE OF THE PHOSPHATIDYLGLYCEROL-PHOTOSYSTEM-II COMPLEX STUDIED BY FT-IR SPECTROSCOPY - MG(II) EFFECT ON THE POLAR HEAD GROUP OF PHOSPHATIDYLGLYCEROL

MgCl2 enhances the oxygen-evolving activity in a complex formed betwee n photosystem II (PSII) and vesicles constituted of phosphatidylglycer ol (PG), an anionic lipid of the thylakoid membrane of plant chloropla sts, i.e. PSII-PGV [M. Fragata, K. Strzalka and E.K. Nenonene, J. Phot ochem. Photobiol. B: Biol., 11 (1991) 329]. In the present work we use d Fourier transform infrared (FT-IR) spectroscopy to identify the mole cular sites in PG and PSII that determine the formation of the PSII-PG V complex on the one hand, and the sites that are affected by Mg(II) o n the other hand. New infrared spectroscopic evidence is given for the specific association of phosphatidylglycerol and Mg(II) leading to st ructural and functional optimization within PSII. At first, the analys is of the the amide I region between 1700 and 1600 cm(-1) and the amid e II region between 1600 and 1500 cm(-1), showed that the binding of P G to PSII induces changes in the polypeptide skeleton of the PSII: pro teins. To investigate the Mg(II) effect on the conformation of PSII an d the polar head group of phosphatidylglycerol in PGV, PSII and PSII-P GV, the following spectral regions were examined: (i) the ester C=O re gion between 1750 and 1700 cm(-1) and the vibrational modes of the pho sphate group in the region from 1300 to 1000 cm(-1), i.e. the asymmetr ic and symmetric stretching modes of the PO2- group (nu(a)PO(2)(-), nu (s)PO(2)(-)), and the stretching mode of the P-O ester bond (nu C-O-P- O-C), and (ii) the amide I and amide II regions. The results show that MgCl2 gives origin to frequency shifts in the infrared spectra of the phosphate group which are interpreted as Mg(II)-mediated structural c hanges in the phosphate group region of the PG head group. The present work discloses the important new notion that the presence of Mg(II), or some kind of Mg(II)-containing ion-pairs, in the PSII proteins stru ctures or in the lipid-protein interfaces will favor the optimal PSII function. (C) 1997 Elsevier Science B.V.