Subpicosecond equilibration of excitation energy in isolated photosystem II reaction centers.

Abstract

PhotosystemIH reaction centers have been studied by femtoend trent absorption spectroseopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy tanser can be observed in both directions between P680 and these accessory chlorins depending on which Is initily excited. This oxidizing potential is used to drive water splitting, which gives rise to oxygen evolution. The primary electron donor of PSII is thought to correspond to a spectral feature at 680 nm and is referred to as P680 (3), while a pheophytin (Ph) molecule functions as an electron acceptor (4-6). Studies of PSII core complexes binding 60 and 80 antenna chlorophylls have suggested that the primary radical pair P680+Ph-is formed at a rate of 4100 psfollowing the absorption ofa photon by antenna pigments (7).A similar conclusion was reached from photovoltage studies of larger PSII complexes (8). A kinetic model, in which there is a rapid (--1 ps) equilibration of excitation energy between the antenna pigments and P680, followed by the observed trapping of the excitation energy by radical pair formation in ""100 ps, has been proposed for this process (refs. 9 and 10; reviewed in ref. 2). This so-called trapping limited model (11) is valid when the rate of electron transfer from the primary electron donor is slower than energy transfer back to the antenna pigments. This model has also been applied to other photosynthetic antenna/reaction center complexes (12)(13)(14).However, previous studies have not been able to time resolve the energy-transfer processes that are predicted to cause the equilibration of excitation energy between the antenna and primary electron donor pigments prior to radical pair formation.We report here a study ofexcitation energy equilibration in the isolated Dl/D2/cytochrome b559 complex, which is the reaction center of PSII. This complex is much smaller than the isolated PSII core complexes discussed above, binding only six chlorophyll a and two Ph a pigments (15, 16). While several of these pigments are presumably involved in electron-transfer processes, these pigments will also function in an energy-transferring capacity before charge separation. Time-resolved fluorescence studies have determined that at least 94% of the chlorin pigments in our PSI1 reaction center preparation are able to transfer excitation energy to P680, resulting in a near unity quantum yield of the primary radical pair (17,18). In a previous study, we determined that when P680 is directly excited, Ph reduction occurs primarily at a rate of 21 ps-1 in isolated PSII reaction centers at room temperature (4).There have been several discussions of the similarities between the PSII reaction center of higher plants and the reaction center of purple bacteria (19,20). However, these two complexes are likely to be very different in terms oftheir energy-transfer kinetics when isolated from their antenna systems. The lowest SO-S, opticil transition for the pr...

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.…”

“…Our results indicate that the decay of P680* is due to an activated processes, unlike in the bacterial RC, and that the (nonactivated) intrinsic charge separation time is 300-400 fs at RT, which is much faster than in the bacterial system. Two other groups have reported on subpicosecond processes at RT, but did not relate these to charge separation processes: the London group ascribed a 100-fs time constant to equilibration between red and blue states of the (multimer) core of pigments (21,22) and a 500-fs time constant, which was mainly observed as a decay of the anisotropy, to energy transfer between two near-degenerate red states (41). Holzwarth and coworkers (27) mentioned the existence of a 250-fs time constant in their data and ascribed it to equilibration within a core of RC pigments.…”

“…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).…”

Site-Directed Mutations at D1-His198 and D2-His197 of Photosystem II in Synechocystis PCC 6803: Sites of Primary Charge Separation and Cation and Triplet Stabilization

“…The difference in the absorbance spectra of P A and B A poses a fundamental question as to the location and mechanism of primary charge separation in PSII reaction centers. The similarity of the P + Q A --PQ A difference spectrum observed here at 80 K in Synechocystis to that observed at both 5 K (30) and 77 K (54) in Synechococcus indicates that the absorbance spectra of P A and B A are practically invariant between 5 and 80 K and that P + is stabilized on P A at all temperatures including at 5 K. Given the difference in the energies of the lowest excited singlet states (31 meV), the localization of singlet excitation on B A (684 nm) over P A (672.5 nm) is favored by a factor of ∼10 31 at 5 K. As the rate of charge separation is slower (0.4-21 ps) (29,(100)(101)(102) than the rate at which energy is equilibrated within the central pigments of the reaction center (100-250 fs) (103,104), the excited state that drives charge separation must be on B A rather than on P A at low temperature ( Figure 9A,B). This situation is different from what is observed in wild-type bacterial reaction centers where the strong coupling between the P A and P B FIGURE 9: (A) Scheme indicating the various fates of the reaction center following center excitation at 5 and 298 K. The initiators of primary charge separation are B A * at 5 K and B A *, P A * and possibly Ph A * at 298 K. 3 P is stabilized on B A at 5 K but is delocalized at 298 K. P + is stabilized primarily on P A at all temperatures.…”

Light-Harvesting in Photosystem II

“…The consequence of this slow EET is that the primary charge transfer time should be extremely fast, i.e., around 300 fs, accompanied by a very large initial drop in free energy to explain the overall timeresolved results. It should be noted that at least in isolated RC complexes such a fast charge separation time was not observed (Groot et al 2005;Germano et al 2004;van Mourik et al 2004;Holzwarth et al 2006;Prokhorenko and Holzwarth 2000;Andrizhiyevskaya et al 2004;Wasielewski et al 1990;Durrant et al 1992;Pawlowicz et al 2008) and one might wonder whether this is realistic. On the other hand, it is possible that isolated RC complexes are ''slower'' than the ones in vivo (see also below).…”