Electrochemical oxidation of pigment cofactors in photosystem II reaction centers

Abstract

The oxidation of Chla and Pheoa in spinach D1-D2Cytb559 pigment-protein complex involved in plant photosynthesis was investigated by optical spectroelectrochemistry. To impose the potential a mixture of high-potential mediators (tris(1,10-phenanthroline) iron(II) and its mixed ligand complexes) was used. From 0.72 to 0.90V (vs.NHE) irreversible bleaching of b -carotene and Chla absorbing at 674nm occurs. Between 0.95 and 1.0V bleaching of the Chla Qy and Soret bands is partially reversible (up to 25%). The difference spetrum of the rereduced minus oxidized sample shows a maximum at 672-673nm, while the irreversibly oxidized component absorbs at 676-677nm. The loss of Soret band absorbance looks similar to that at potentials <0.90V, which suggests bleaching of Chla, not Pheoa. Further oxidation by prolonged exposure to potentials between 0.95 and 1.05V leads to irreversibly bleaching to a lower limit of 10-15% of residual absorbance, accompanied by a further blueshift of Qy band to 672nm. The absorbance difference spectra in the Soret region show that in the high potential range the disappearance of Pheoa dominates over that of Chla. The pigment content of our sample determined using the method of Eijckelhoff (Photosynth. Res. 1997, 52, 69) was 6 Chla and 1-2 ß-Car per 2 Pheoa. Based on these numbers we attribute the initial 40% irreversible bleaching below 0.9V to the oxidation of 3-4 Chla pigments with an absorbance maximum at 673-674nm. The maximum amount of reversibly oxidized pigment is about 20% of the total red band and may be associated with the two Chla molecules forming P680. The remaining bleaching at >1.0V is then associated with 0-1 Chla and the two Pheoa molecules. If our interpretation is correct, then our measurements constitute the first direct determination of the oxidative midpoint potential of P680, viz. 0.95-1.0V.