WAVELENGTH AND INTENSITY-DEPENDENT PRIMARY PHOTOCHEMISTRY OF ISOLATEDPHOTOSYSTEM-II REACTION CENTERS AT 5-DEGREES-C

The long wavelength absorption band of the isolated Photosystem II rea ction 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. Subpi cosecond transient absorption measurements were made monitoring princi pally 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 5 0-100 ps) to be fit properly. The pump energy was varied by a factor o f twenty-five (40-1000 nJ), with no apparent effect on either the rate s or the amplitude ratios of the three components, although clear evid ence of nonlinear behavior was observed at the higher excitation energ ies. The dependence of both the rates and amplitude ratios of the thre e components upon pump wavelength will be discussed in terms of excita tion energy transfer occurring on a 30 ps timescale. Selective excitat ion into the short and long-wavelength sides of the composite Q(y) ban d give identical transient spectra at 500 ps, indicating near-unity ef ficiency of excitation energy transfer. At 1 ps, the spectra are quite different, calling into question the extent of ultrafast (similar to 100 fs) excitation energy transfer. The time after the excitation puls e at which the transient crosses Delta A = 0 was found to be a highly sensitive measure of both the excitation energy and the identity of th e pigment pool that had been excited.