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-acc umulation of reduced pheophytin a were studied in the isolated D1-D2-Cyt b5 59 complex in the presence of variable methyl viologen concentrations and d ifferent illumination conditions under anaerobiosis. Depending on the methy l 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 red uced pheophytin a located at the DI-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 periphera l chlorophyll a and the pheophytin a located at the D2-branch of the comple x, respectively. The blue absorbing chlorophyll at 667 nn is susceptible to chemical redox changes with a midpoint reduction potential of +470 mV The Qx 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, respecti vely. The results indicated that the two pheophytin molecules can be photor educed in the D1-D2-Cyt b559 complex in certain experimental conditions.