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 of the light-induced reversible changes in the circular dichroism (Delta CD) of chloroplast thylakoid membranes. Earlier, these changes, which originate from structural changes affecting the chiral macroorganization 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 activity of thylakoids, and Delta CD has been observed in lamellar aggregates of the light harvesting chlorophyll a/b complex (LHC II) of Photosystem 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 degrees 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 about 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 noticeably affecting the electron and proton transport, and the chiral macroorganization of the chromophores in the dark. We conclude that Delta CD is conditioned but not driven by the photochemical activity of the membranes, and the structural changes are given rise by a physical mechanism previously unrecognized in thylakoids, thermooptic effect described for liquid crystals. We discuss the possible link between the deactivation(s) of the excess excitation energy and Delta CD, the light-induced changes in the chiral macroorganization of the chromophores of the photophysical apparatus in thylakoids.