Sub-picosecond Equilibration of Excitation Energy in Isolated Photosystem II Reaction Centers Revisited:  Time-Dependent Anisotropy

Abstract

Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centers. As reported previously [Durrant, J. R.; et al. Proc. Natl. Acad. Sci. U.S.A. 1992 , 89, 11632−11636], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Q y band bleach/stimulated emission is dominated by energy transfer processes with a 100 ± 50 fs time constant. In contrast, depolarization of this signal occurs with a time constant of 400 ± 30 fs, from an initial anisotropy of ∼0.4 to a value of ∼0.15 at 1.5 ps. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction center absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction center excited states occurs on a sub-picosecond time scale. Transient data collected for pheophytin Q x absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process. These results are discussed in the context of the PS II multimer model and are shown to be in good agreement with this model.

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

“…This temperature dependence is in fact rather weak and corresponds to an activation energy of only Ϸ1.5-2 nm. This suggests that the decay of P680* is not due to energy transfer to blue (Ϸ670-675 nm) states, but rather to a near-degenerate state, in agreement with the suggestion in Merry et al (41). This state can, however, not be the trap state, as was suggested in in ref.…”

“…This state can, however, not be the trap state, as was suggested in in ref. 41, since the results discussed in the previous section show that the trap-to-P680 transfer is much slower than 2.6-0.4 ps. Furthermore, the spectral changes associated with the 2.6 3 0.4-ps time constant are rather large and this suggests that a transition occurs to a state of low oscillator strength, or dark state, whereas the trap state has a more or less comparable oscillator strength to that of P680 (9,13).…”

“…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.…”

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).…”

“…The ''spectroscopic absence'' of a special pair and the fact that in the PSII RC the Hs absorb light in the same spectral region as the Chls led to the proposal of the ''multimer'' model (6), in which the interactions between all of the chlorins are about equal, and consequently the excitation may get localized on any of the chlorins, depending on the specific realization of the disorder. This model was used to interpret the spectroscopic data of the PSII RC (6) and later the dynamics of energy and electron transfer (7,8).Excitation of the PSII RC with a short laser flash leads to ultrafast charge separation followed by the formation of the radical pair P D1 ϩ H D1 Ϫ , consisting of the H molecule in the D 1 branch, H D1 , and one of the Chl forming the special pair, P D1 (9). Electron transport stops at the H molecule because the final quinone acceptors are missing in these preparations.…”