DIRECT MEASUREMENT OF THE EFFECTIVE RATE-CONSTANT FOR PRIMARY CHARGE SEPARATION IN ISOLATED PHOTOSYSTEM-II REACTION CENTERS

Transient absorption measurements of the pheophytin a anion band and a band bleach region using preferential excitation of P680 are performe d on isolated photosystem II reaction centers to determine the effecti ve rate constant for charge separation. A novel analysis of the Q(x) b and bleach region explicitly takes the changing background into accoun t in order to directly measure the rate of growth of the bleach. Both spectral regions reveal biphasic kinetics, with a ca. (8 ps)(-1) rate constant for the faster component, and a ca. (50 ps)(-1) rate constant for the slower component. We propose that the faster component corres ponds to the effective rate constant for charge separation from within the equilibrated reaction center core and provides a lower limit for the intrinsic rate constant for charge separation. The slower componen t corresponds to charge separation that is limited by slow energy tran sfer from a long-wavelength accessory chlorophyll a.