Mechanism of action of anions on the electron transport chain in thylakoid membranes of higher plants.

With an aim to improve our understanding of the mechanisms behind specific anion effects in biological membranes, we have studied the effects of sodium salts of anions of varying valency in thylakoid membranes. Rates of electron transport of PS II and PS I, 77K fluorescence emission and excitation spectra, cyclic electron flow around PS I and circular dichroism (CD) spectra were measured in thylakoid membranes in order to elucidate a general mechanism of action of inorganic anions on photosynthetic electron transport chain. Re-distribution of absorbed excitation energy has been observed as a signature effect of inorganic anions. In the presence of anions, such as nitrite, sulphate and phosphate, distribution of absorbed excitation energy was found to be more in favor of Photosystem I (PS I). The amount of energy distributed towards PS I depended on the valency of the anion. In this paper, we propose for the first time that energy re-distribution and its valence dependence may not be the effect of anions per se. The entry of negative charge (anion) is accompanied by influx of positive charge (protons) to maintain a balance of charge across the thylakoid membranes. As reflected by the CD spectra, the observed energy re-distribution could be a result of structural rearrangements of the protein complexes of PS II caused by changes in the ionic environment of the thylakoid lumen.

Evidence that pH can drive state transitions in isolated thylakoid membranes from spinach.

Our observation that the F735/F685 ratio at 77 K increased when the lumenal pH decreased led us to investigate the role of pH in explaining the mechanism of state transitions in spinach (Spinacea oleracea L.) thylakoid membranes. As the lumenal pH was changed from pH 7.5 to 5.5, the quantum yield of PS II decreased, while that of PS I increased. In the presence of an uncoupler, NH(4)Cl, which sequesters protons, a reversal of the effects observed at pH 5.5 were noticed. The thylakoid membranes treated with NaF at pH 5.5, when suspended in a buffer of pH 7.5, showed enhanced PS II fluorescence and a decreased PS I fluorescence, suggesting migration of LHC II back to PS II from PS I. The results presented here suggest for the first time that the lumenal pH of thylakoid membranes regulates the migration of antenna, and hence the energy distribution, between the two photosystems, i.e. a low lumenal pH (pH 5.5) favors antenna migration from PS II to PS I. At pH 7.5, the deprotonation of LHC II antenna attached to PS I leads to back migration of LHC II to PS II.