Wavelength and intensity dependent primary photochemistry of isolated Photosystem II reaction centers at 5°C

Abstract

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 Qx 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 Qy 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 A A = 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.

Charge separation in the reaction center of photosystem II studied as a function of temperature

“…The lifetimes from our fit at 240 K differ somewhat from those reported at RT, where the initial decay is described by lifetimes of 100-250 fs, Ϸ1-3 ps and 15-20 ps (20)(21)(22)(23)(24)(25)(26)(27), in which the amplitude of the 1-to 3-ps component is usually small relative to the other components. First of all we note that in a complex system like PSII when one process is not fitted correctly, other components will easily be influenced as well.…”

“…However, it is not clear whether or not the 1.9-ps time constant at 1.4 K and the 3-or 20-ps processes at RT can be attributed to the same physical process. Furthermore, most recent studies (19,20,22,27,31) conclude that radical pair formation in the PSII RC is too complicated to assign one kinetic component to charge separation.…”

“…However, it is not clear whether or not the 1.9-ps time constant at 1.4 K and the 3-or 20-ps processes at RT can be attributed to the same physical process. Furthermore, most recent studies (19,20,22,27,31) conclude that radical pair formation in the PSII RC is too complicated to assign one kinetic component to charge separation.Low temperature subpicosecond measurements are a valuable method to obtain more information on the primary processes since, due to slowing down of uphill energy transfer, the kinetics can be related to one of the primary processes in a more straightforward manner. The particular temperature dependence of the kinetics will yield information on their origin.…”

“…A straightforward interpretation of the results from pump-probe measurements in terms of pigment to pigment energy transfer and charge separation rates has proven difficult, mainly due to the congested absorption spectrum of the PSII RC. At room temperature (RT), most groups observed kinetics with lifetimes of 1-3 ps, 10-20 ps and Ͼ1 ns (14)(15)(16)(17)(18)(19)(20) as well as 100-600 fs (21)(22)(23)(24)(25)(26)(27). At very low temperature (4 K) the lifetime of singlet-excited P680 (P680*) was estimated to be 1.9 ps by transient hole-burning spectroscopy (28).…”

Initial electron donor and acceptor in isolated Photosystem II reaction centers identified with femtosecond mid-IR spectroscopy

“…Electron transport stops at the H molecule because the final quinone acceptors are missing in these preparations. In contrast to the bacterial RC, in the PSII RC the initial dynamics are highly multiexponential with lifetimes in the order of 100 and 300-400 fs, and 3, 10, and 30 ps (7,(10)(11)(12)(13)(14)(15)(16)(17)(18). These dynamics all seem to represent a mixture of energy transfer, excited state decay, radical pair-formation and relaxation, in part due to the shallow equilibria between the excited pigments and the initial radical pair state(s).…”