ABSORBENCY DIFFERENCE SPECTRA OF THE S-STATE TRANSITIONS IN PHOTOSYSTEM-II CORE PARTICLES

Redox changes of the oxygen evolving complex in PS II core particles w ere investigated by absorbance difference spectroscopy in the UV-regio n. The oscillation of the absorbance changes induced by a series of sa turating flashes could not be explained by the minimal Kok model (Kok et al. 1970) consisting of a 4-step redox cycle, S-0 --> S-1 --> S-2 - -> S-3 --> S-0, although the values of most of the relevant parameters had been determined experimentally. Additional assumptions which allo w a consistent fit of all data are a slow equilibration of the S-3 sta te with an inactive state, perhaps related to Ca2+-release, and a low quantum efficiency for the first turnover after dark-adaptation. Diffe rence spectra of the successive S-state transitions were determined. A t wavelengths above 370 nm, they were very different due to the differ ent contribution of a Chi bandshift in each spectrum. At shorter wavel engths, the S-1 --> S-2 transition showed a difference spectrum simila r to that reported by Dekker et al. 1984b and attributed to an Mn(III) to Mn(IV) oxidation. The spectrum of absorbance changes associated wi th the S-2 --> S-3 transition was similar to that reported by Lavergne 1991 for PS II membranes. The S-0 --> S-1 transition was associated w ith a smaller but still substantial absorbance increase in the UV. Dif ferences with the spectra reported by Lavergne 1991 are attributed to electrostatic effects on electron transfer at the acceptor side associ ated with the S-state dependence of proton release in PS II membranes.