Spectroscopic characterization of the acceptor state Q −A and the donor state S2 of Photosystem II of spinach in the blue, red and near-infrared

Abstract

With Photosystem II particles prepared from spinach, partial difference spectra were measured of the primary bound plastoquinone Q A and of the donor state S2. Extinction coefficients were estimated by comparison with the absorption change of a redox-active dye, N, N, N′, N′-tetramethyl- p-phenylenediamine. The spectrum of Q − A − Q A was better resolved than in earlier studies (Van Gorkom, H.J. (1974) Biochim. Biophys. Acta 347, 439–442; Schatz, G.H. and Van Gorkom, H.J. (1985) Biochim. Biophys. Acta 810, 283–294; Dekker, J.P. (1985) Thesis, University of Leiden) and is seen to be largely composed of shifts of two pigment bands in opposite directions, presumably a pheophytin red shift and a chlorophyll blue shift, as in bacterial reaction centers (Clayton, R.K. and Straley, S.C. (1972) Biophys. J. 12, 1221–1234). The spectrum of S2–S1 in the red suggests blue shifts of two chlorophyll bands. It resembles, but is not identical to, the S2–S1 spectrum earlier reported for Photosystem II of Synechococcus (Saygin, O. and Witt, H.T. (1985) FEBS Lett. 187, 224–226). Earlier measurements of S2-associated absorbance in the near-infrared (Dismukes, G.C. and Mathis, P. (1984) FEBS Lett. 178, 51–54) are not confirmed. The observed changes in this range are probably largely scattering changes. This conclusion is reached on the basis of theoretical considerations