Absorbance difference spectra upon charge transfer to secondary donors and acceptors in Photosystem II

Abstract

Charge-transfer reactions to secondary electron donors (Z, M) and acceptors (Q A, Q B) in Photosystem II particles isolated from a thermophilic cyanobacterium Synechococcus sp. (Schatz, G.H. and Witt H.T. (1984) Photobiochem. Photobiophys. 7, 1–14) were analyzed by measurements of fluorescence yield and absorbance changes in the millisecond time domain induced by repetitive flashes. (1) The electron-transfer reaction Q − AQ B → Q AQ − B was found to occur with kinetic phases of 0.2 ± 0.1 ms and 1.5 ± 0.5 ms half-time. At 10 ms after flashes an equilibrium distribution of Q − A Q B Q A Q − B of about 15 85 in oxygen-evolving and of about 25 75 in Tris-treated PS II particles was reached. (2) The absorbance difference spectra were determined for (Q − A - Q A), (Q − B - Q B), (Z + - Z) and for (S 4 - S 0), the transition associated with oxygen evolution. In the ultraviolet region they show that these electron-acceptors and -donors are the same as in spinach PS II. In the visible region all the difference spectra contain major contributions by electrochromic bandshifts due to electrostatic interaction of the reduced acceptors or oxidized donors with nearby reaction center pigments. Upon electron transfer from Q − A to Q B electrochromic bandshifts due to interaction with pheophytin a disappeared almost completely. Bandshifts observed in the (Z + - Z) and (S 4 - S 0) spectra were attributed to chlorophyll a.