Light-induced absorption spectra of the D1-D2-cytochrome b559 complex of Photosystem II:: Effect of methyl viologen concentration

The light-induced difference absorption spectra associated to the photo-accumulation of reduced pheophytin a were studied in the isolated D1-D2-Cyt b559 complex in the presence of variable methyl viologen concentrations and different illumination conditions under anaerobiosis. Depending on the methyl viologen/reaction centre ratio, the relative intensities of the spectral bands at 681.5 +/-0.5, 667.0 +/-0.5 and 542.5 +/-0.5 nm were modified. The reduced pheophytin a located at the D1-branch of the complex absorbs at 681.7 +/-0.5 nm, and at least two additional pigment species contribute to the Q(y) band of the difference absorption spectra with maxima at 667.0 +/-0.5 and 680.5 +/-0.5 nm. We propose the additional species correspond to a peripheral chlorophyll a and the pheophytin a located at the D2-branch of the complex, respectively. The blue absorbing chlorophyll at 667 nm is susceptible to chemical redox changes with a midpoint reduction potential of +470 mV. The Q x absorption bands of both pheophytins localised at the D2- and D1-branch of the D1-D2-Cyt b559 complex were at 540.7 +/-0.5 and 542.9 +/-0.5, respectively. The results indicated that the two pheophytin molecules can be photoreduced in the D1-D2-Cyt b559 complex in certain experimental conditions.