Wavelength and Intensity Dependent Primary Photochemistry of Isolated Photosystem II Reaction Centers at 5 deg C

The long wavelength absorption band of the isolated Photosystem II reaction center was directly excited at five wavelengths between 655 and 689 nm to study the effects of excitation wavelength and intensity on both excitation energy transfer and charge separation processes. Subpicosecond transient absorption measurements were made monitoring principally the bleach of the pheophytin a Q_x band at 544 nm. At all pump wavelengths, the kinetics require three exponentials (1-3, 10-25 and 50-100 ps) to be fit properly. The pump energy was varied by a factor of twenty-five (40-1000 nJ), with no apparent effect on either the rates or the amplitude ratios of the three components, although clear evidence of nonlinear behavior was observed at the higher excitation energies. The dependence of both the rates and amplitude ratios of the three components upon pump wavelength will be discussed in terms of excitation energy transfer occurring on a 30 ps timescale. Selective excitation into the short and long-wavelength sides of the composite Q_y band give identical transient spectra at 500 ps, indicating near-unity efficiency of excitation energy transfer. At 1 ps, the spectra are quite different, calling into question the extent of ultrafast (∼ 100 fs) excitation energy transfer. The time after the excitation pulse at which the transient crosses ΔA = 0 was found to be a highly sensitive measure of both the excitation energy and the identity of the pigment pool that had been excited.