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

Abstract

Despite the apparent similarity between the plant Photosystem II reaction center (RC) and its purple bacterial counterpart, we show in this work that the mechanism of charge separation is very different for the two photosynthetic RCs. By using femtosecond visible-pump-mid-infrared probe spectroscopy in the region of the chlorophyll ester and keto modes, between 1,775 and 1,585 cm(-1), with 150-fs time resolution, we show that the reduction of pheophytin occurs on a 0.6- to 0.8-ps time scale, whereas P+, the precursor state for water oxidation, is formed after approximately 6 ps. We conclude therefore that in the Photosystem II RC the primary charge separation occurs between the "accessory chlorophyll" Chl(D1) and the pheophytin on the so-called active branch.

Fig. 1.

Time traces at 1,657 (▪), 1,681 (○), and 1,711 (▵)cm^–1 upon excitation at 669 nm. The solid line through the data points is a fit with t₁ = 0.2 ps, t₂ = 3 ps, t₃ = 32 ps, t₄ = 2 ns, and t₅ = infinite; instrument response is 150 fs. The time scale is linear up to 3 ps and logarithmic thereafter. The rms error of the fit is 15 μOD.

Fig. 2.

Lower curves show the evolution of the time-dependent absorption changes in the PSII RC upon 669-nm excitation. The lifetimes of the spectra are 0.2 ps (black), 3 ps (red), 32 ps (green), 2 ns (blue), and infinite (cyan). A smooth line is drawn through the measuring points as a guide for the eye. The Chl*/Chl (black line) and H*/H (red line) difference spectra in THF measured 10 ps after excitation at 530 nm are shown (upper curves) for comparison.

Fig. 3.

EADS upon 669-nm (black lines) and 681-nm excitation (red lines). The lifetimes of the spectra are 3 and 0.6 ps (a); 32 and 21 ps (b), 2 ns and 500 ps (c); infinite and infinite (d), for the 669- and 681-nm data sets, respectively. In d, the spectrum from steady-state Fourier transform IR experiments (blue line) is reproduced from refs. and for comparison.

Fig. 4.

Modeling of the PSII RC kinetics. (a) Five-compartmental model used to fit the kinetics of Streak emission data (38) and the mid-IR data upon 681-nm excitation simultaneously. BC and PC denote radiative excited states, and RP1, -2, and -3 denote radical pair states. The initial excitation into BC and PC is 1:1. Note that because only two excited state compartments are used, each consists of several pigments. Transition rates are indicated. The eigen-values of this model (which correspond to the lifetimes obtained in the measurements) are 0.8, 6, 36, and 200 ps and 8 ns. (b) The concentration profiles of each of the compartments in time (log-linear scale).

Fig. 5.

Spectra of P+/P and as reported in refs. and , spectra of the states PC, BC, and RP1, resulting from target analysis of the 681-nm data set.