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 Q_x band bleach region using preferential excitation of P680 are performed on isolated photosystem II reaction centers to determine the effective rate constant for charge separation. A novel analysis of the Q_x band bleach region explicitly takes the changing background into account 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 corresponds 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 component corresponds to charge separation that is limited by slow energy transfer from a long-wavelength accessory chlorophyll a.