Light-induced Fourier transform infrared spectrum of the cation radical P680+

Primary electron donor(s) in isolated reaction center of photosystem II from Chlamydomonas reinhardtii

Isolated reaction centers (RCs) from wild-type Chlamydomonas (C.) reinhardtii of Photosystem II (PSII), at different levels of intactness, were studied to provide more insight into the nature of the charge-separation (CS) pathway(s). We argue that previously studied D1/D2/Cytb559 complexes (referred to as RC680), with ChlD1 serving as the primary electron donor, contain destabilized D1 and D2 polypeptides and, as a result, do not provide a representative model system for the intact RC within the PSII core. The shapes of nonresonant transient hole-burned (HB) spectra obtained for more intact RCs (referred to as RC684) are very similar to P(+)QA(-) - PQA absorbance difference and triplet minus singlet spectra measured in PSII core complexes from Synechocystis PCC 6803 [Schlodder et al. Philos. Trans. R. Soc. London, Ser. B2008, 363, 1197]. We show that in the RC684 complexes, both PD1 and ChlD1 may serve as primary electron donors, leading to two different charge separation pathways. Resonant HB spectra cannot distinguish the CS times corresponding to different paths, but it is likely that the zero-phonon holes (ZPHs) observed in the 680-685 nm region (corresponding to CS times of ∼1.4-4.4 ps) reveal the ChlD1 pathway; conversely, the observation of charge-transfer (CT) state(s) in RC684 (in the 686-695 nm range) and the absence of ZPHs at λB > 685 nm likely stem from the PD1 pathway, for which CS could be faster than 1 ps. This is consistent with the finding of Krausz et al. [Photochem. Photobiol. Sci.2005, 4, 744] that CS in intact PSII core complexes can be initiated at low temperatures with fairly long-wavelength excitation. The lack of a clear shift of HB spectra as a function of excitation wavelength within the red-tail of the absorption (i.e., 686-695 nm) and the absence of ZPHs suggest that the lowest-energy CT state is largely homogeneously broadened. On the other hand, in usually studied destabilized RCs, that is, RC680, for which CT states have never been experimentally observed, ChlD1 is the most likely electron donor.

Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: pheophytin is the primary electron acceptor

The mechanism and kinetics of electron transfer in isolated D1/D2-cyt(b559) photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of approximately 1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the approximately 200-ps component represents the electron transfer to the Q(A) acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are approximately 3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chl(acc D1). Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170-180 ns(-1), and the rate constant of secondary electron transfer is 120-130 ns(-1).

Fourier transform infrared study of the cation radical of P680 in the photosystem II reaction center: evidence for charge delocalization on the chlorophyll dimer

A Fourier transform infrared (FTIR) difference spectrum of the primary electron donor (P680) of photosystem II upon its photooxidation (P680+/P680) was obtained in the frequency region of 1000-3000 cm-1. The reaction center (RC) complex (D1-D2-Cytb559) was used for the measurements in the presence of ferricyanide as an exogenous electron acceptor. Control measurements of electronic absorption (300-1200 nm) showed that illumination of the RC complex at 150 K induced major oxidation of P680 concomitant with oxidation of a carotenoid and an accessory chlorophyll (Chl). Illumination at 250 K also specifically bleached one of the two beta-carotene molecules bound to the RC complex, and the sample thus treated exhibited little formation of a carotenoid cation on subsequent illumination at 150 K. The P680+/P680 FTIR difference spectrum (with minor contamination of Chl+/Chl) was measured at 150 K using this partially carotenoid-deficient RC complex. The spectrum showed a broad positive band centered at approximately 1940 cm-1, which could be ascribed to an infrared electronic transition of P680+ analogous to that previously observed in various bacterial P+. This finding indicates that a positive charge is delocalized over (or hopping between) the two Chl molecules in P680+. The low intensity of this electronic band compared with that of the bacterial band could have three possible explanations: weak resonance interaction between the constituent Chl molecules, an asymmetric structure of P680+, and the difference in Chl species. Bands in the C=O stretching region (1600-1750 cm-1) were interpreted in comparison with resonance Raman spectra of the RC complex. The negative peaks at 1704 and 1679 cm-1 were proposed as candidates for the keto C9=O bands of P680. The observation that neither of these bands agreed with the main keto C9=O band at 1669 cm-1 in the previous 3P680/P680 FTIR spectrum [Noguchi et al. (1993) Biochemistry 32, 7186-7195] led to the idea that the triplet state migrates to a Chl (designated as ChlT) different from P680 at low temperatures. Based on these results, structural models of Chl molecules including P680 and ChlT and their coupling in the cation, triplet, and Qy singlet states are discussed.