EXCITATION-ENERGY TRANSFER AND CHARGE SEPARATION IN THE ISOLATED PHOTOSYSTEM-II REACTION-CENTER

The nature of excitation energy transfer and charge separation in isol ated Photosystem II reaction centers is an area of considerable intere st and controversy. Excitation energy transfer from accessory chloroph yll a to the primary electron donor P680 takes place in tens of picose conds, although there is some evidence that thermal equilibration of t he excitation between P680 and a subset of the accessory chlorophyll a occurs on a 100-fs timescale. The intrinsic rate for charge separatio n at low temperature is accepted to be ca. (2 ps)(-1), and is based on several measurements using different experimental techniques. This ra te is in good agreement with estimates based on larger sized particles , and is similar to the rate observed with bacterial reaction centers. However, near room temperature there is considerable disagreement as to the observed rate for charge separation, with several experiments p ointing to a ca. (3 ps)(-1) rate, and others to a ca. (20 ps)(-1) rate . These processes and the experiments used to measure them will be rev iewed.