PROTEIN-STRUCTURE OF PHOTOSYSTEM-II STUDIED BY FTIR SPECTROSCOPY - EFFECT OF DIGALACTOSYLDIACYLGLYCEROL ON THE TYROSINE SIDE-CHAIN RESIDUES

Digalactosyldiacylglycerol (DGDG), a non-ionic galactolipid of the thy lakoid membrane, enhances the activity of the oxygen-evolving complex in photosystem II (PSII). To elucidate the structural aspects of the D GDG effect, a Fourier transform infrared (FT-IR) spectroscopic study o f PSII and the PSII-DGDG complex was performed in the 1650-1500 cm(-1) region. Analysis of the [PSII-DGDG] -PSII difference spectrum indicat es that DGDG induces significant changes in the intensities and positi ons of the tyrosine bands in the PSII spectra. The v(8) intensity in t he 1630-1580 cm(-1) region of tyrosine increases concomitantly with a loss in v(19a) intensity from 1520 to 1500 cm(-1). These spectral inve rsions have their origin in the phenol ring configurations of the tyro syl residues in the PSII proteins. The results are interpreted as the variation of: (i) the dihedral angle between a H-bond acceptor in DGDG and the phenol ring in tyrosine caused by the displacement of the H-a tom in the OH group out of the plane of the phenol ring; and (ii) the arrangement of the PSII proteins in the vicinity of the phenol ring of the tyrosine residues into a more compact molecular structure. It was also observed that the H-bond region from 3500 to 3100 cm(-1) is affe cted as well upon DGDG interaction with the PSII proteins. Thus, the D GDG effect on the PSII complex most likely originates in the galactoli pid capacity to participate in hydrogen-bonding formation either as a donor or as an acceptor. These interactions are facilitated by the thr ee-dimensional characteristics of the DGDG molecule which determine a wide range of distributions of H-bond groups in space. In this perspec tive, the galactolipid effect is interpreted on the basis of the digal actosyl head group ability: (i) to participate in hydrogen-bonding int eractions; and (ii) to favor the docking of the DGDG molecules with th e aromatic side chains in the PSII proteins. (C) 1998 Elsevier Science B.V.