I. Yruela et al., Light-induced absorption spectra of the D1-D2-cytochrome b559 complex of Photosystem II: Effect of methyl viologen concentration, PHOTOSYN R, 67(3), 2001, pp. 199-206
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
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 per
ipheral chlorophyll a and the pheophytin a located at the D2-branch of the
complex, respectively. The blue absorbing chlorophyll at 667 nm is suscepti
ble to chemical redox changes with a midpoint reduction potential of +470 m
V. 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 cond
itions.