title: Light-induced absorption spectra of the D1-D2-cytochrome b 559 complex of Photosystem II: Effect of methyl viologen concentration.
abstract: 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.
authors.affiliation: Estación Experimental de Aula Dei (C.S.I.C.), Avda. de Montañana, 1005, Apdo. 202, E-50080, Zaragoza, Spain.
authors.forename: I
authors.initials: I
authors.lastname: Yruela
authors.forename: E
authors.initials: E
authors.lastname: Torrado
authors.forename: M
authors.initials: M
authors.lastname: Roncel
authors.forename: R
authors.initials: R
authors.lastname: Picorel