CHARACTERIZATION OF THE LIGHT-INDUCED REVERSIBLE CHANGES IN THE CHIRAL MACROORGANIZATION OF THE CHROMOPHORES IN CHLOROPLAST THYLAKOID MEMBRANES - TEMPERATURE-DEPENDENCE AND EFFECT OF INHIBITORS

We investigated the temperature dependence and inhibitor sensitivity o f the light-induced reversible changes in the circular dichroism (Delt a CD) of chloroplast thylakoid membranes. Earlier, these changes, whic h originate from structural changes affecting the chiral macroorganiza tion of the chromophores, were thought to be driven by photochemically generated proton and/or ion gradients in the thylakoids [Garab et al. (1988) Biochemistry 27: 2430]. However, more recently, these changes have been shown to be largely independent of the photochemical activit y of thylakoids, and Delta CD has been observed in lamellar aggregates of the light harvesting chlorophyll a/b complex (LHC II) of Photosyst em II [Barzda et al. (1996) Biochemistry 35: 8981]. Here, we show that in thylakoids (i) Delta CD is gradually and substantially decelerated upon gradually decreasing the temperature from 33 degrees C to 2 degr ees C, and abruptly disappears above 35-37 degrees C; (ii) Delta CD is inhibited with nigericin with I-50 approximate to 1 mu M, which is ab out 10 times higher than the I-50 for the transmembrane Delta pH; (iii ) Delta CD can be inhibited with dicyclohexylcarbodiimide that blocks proton binding at the lumenal side of LHC II; (iv) quinone antagonists , such as antimycin-A and myxothiazol, inhibit Delta CD without notice ably affecting the electron and proton transport, and the chiral macro organization of the chromophores in the dark. We conclude that Delta C D is conditioned but not driven by the photochemical activity of the m embranes, and the structural changes are given rise by a physical mech anism previously unrecognized in thylakoids, thermooptic effect descri bed for liquid crystals. We discuss the possible link between the deac tivation(s) of the excess excitation energy and Delta CD, the light-in duced changes in the chiral macroorganization of the chromophores of t he photophysical apparatus in thylakoids.