Primary steps of electron and energy transfer in photosystem I: effect of excitation pulse wavelength.

Time-resolved differential spectra of photosystem I complex were obtained by the "pump-probe" technique with 25-fs pulses with maxima at 670, 700, and 720 nm. The ratio between the number of excited chlorophyll molecules of the antenna and of the reaction center was shown to depend on spectral characteristics of the pump pulses. In all cases, an ultrafast (<150 fs) formation of the primary radical pair P700(+)A(0)() was recorded. However, on excitation by pulses with maxima at 670 or 700 nm, detection of the charge separation was masked by the much more intensive bleaching at the chlorophyll Q(y) band due to excitation of the bulk antenna chlorophylls. We show that triggering the charge separation by 25-fs pulses centered at 720 nm allows to detect more clearly kinetics of formation of the primary and secondary ion-radical pairs. The findings help to explain possible reasons for discrepancies of kinetics of primary steps of electron transfer detected in different laboratories.