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 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.
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