KINETICS OF ELECTRON-TRANSFER AND ELECTROCHROMIC CHANGE DURING THE REDOX TRANSITIONS OF THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX

The flash-induced kinetics of formation of the successive S-states of the oxygen-evolving complex were analyzed through absorption changes at 295 nm and in the blue region (440-424 nm). The 295 nm change monitors electron transfer from the charge-storing system towards the oxidized tyrosine Y-Z. The blue absorption changes are due to a local electrochromic shift that was previously shown to vary in size, both in response to electron transfer and to proton release from the catalytic center. The kinetics of proton release can thus be estimated by comparing the 295 nm and electrochromic responses. The half-times found for electron transfer (at pH 6.5) were 250 mu s for YZ+S0 --> YZS1, 55 mu s for YZ+S1 --> YZS2, 290 mu s for YZ+S2 --> YZS3 and 1.2 ms for YZ+S3 --> YZS0. The electrochromic kinetics are markedly biphasic during the YZ+S3 --> YZS0 transition, with a fast phase (t(1/2) approximate to 30 mu s) accounting for 40% of the total amplitude and a slow phase (t(1/2) approximate to 1.2 ms) concomitant with S-0 and O-2 formation. The fast electrochromic decay is accompanied by a lag in the electron transfer kinetics. This phase is interpreted as reflecting the electrostatically triggered expulsion of one proton from the catalytic center caused by the positive charge on Y-Z(+). This first step then allows the 1.2 ms reaction to take place. The electrochromic kinetics were also found to be globally faster than electron transfer for the YZ+S0 --> YZS1 transition above pH 6.5, suggesting similarly a Y-Z(+)-induced deprotonation.