Primary Electron Transfer Kinetics in Membrane-Bound Rhodobacter sphaeroides Reaction Centers:  A Global and Target Analysis

Abstract

Absorbance difference kinetics were measured on quinone-reduced membrane-bound wild type Rhodobacter sphaeroides reaction centers in the wavelength region from 690 to 1060 nm using 800 nm excitation. Global analysis of the data revealed five lifetimes of 0.18, 1.9, 5.1, and 22 ps and a long-lived component for the processes that underlie the spectral evolution of the system. The 0.18 ps component was ascribed to energy transfer from the excited state of the accessory bacteriochlorophyll (B*) to the primary donor (P*). The 1.9 ps component was associated with a state involving a BChl anion absorbing in the 1020 nm region. This led to the conclusion that primary electron transfer is best described by a model in which the electron is passed from P* to the acceptor bacteriopheophytin (H L ) via the monomeric bacteriochlorophyll (B L ), with the formation of the radical pair state P + B L-. An analysis assuming partial direct charge separation from B* [Van Brederode, M. E., Jones, M. R., and Chem. Phys. Lett. 268,[143][144][145][146][147][148][149] was also consistent with the data. Within the framework of a five component model, the 5.1 and 22 ps lifetimes were associated with charge separation and relaxation of the P + H L -radical pair state respectively, providing a description which adequately accounted for the complex kinetics of decay of P*. Alternatively, by assuming that the 5.1 and 22 ps components originate from a single component with a multi-exponential decay, a simpler analysis with only four components could be employed, resulting in only a small increase (7%) in the weighted root mean square error of the fit. In both descriptions part of the decay of P* proceeds with a lifetime of about 2 ps. The relative merits of these alternative descriptions of the primary events in light-driven electron transfer are discussed. Similar measurements on YM210H mutant reaction centers revealed four lifetimes of 0.2, 3.1, and 12 ps and a long-lived component. The 3.1 and 12 ps lifetimes are ascribed to multi-exponential decay of the P* state. The differences with the WT data are discussed.The reaction center (RC) of purple bacteria is responsible for the conversion of light energy into a transmembrane electrical potential. The structures of the RC from two species of purple bacteria, Rhodopseudomonas (Rps.) Viridis and Rhodobacter (Rb.) sphaeroides, have been solved to high resolution (1, 2). They reveal an assembly of protein subunits and redox cofactors that are arranged around an axis of approximate 2-fold symmetry. The primary donor of electrons (P) is a pair of excitonically-coupled bacteriochloropyll a (BChl) molecules positioned close to the periplasmic face of the protein. The remaining redox cofactors, two monomeric BChls (B), two molecules of bacteriopheophytin a (H), and two ubiquinones (Q), are arranged around the symmetry axis in two branches that span the membrane dielectric. Excitation of P initiates a sequence of electron transfer reactions that result in the separation of charge across the ...

Multiple pathways for ultrafast transduction of light energy in the photosynthetic reaction center of Rhodobacter sphaeroides

“…5A, solid line) and the remaining spectrum is compared with the inverse ground state absorbance spectrum normalized on the P absorbance bleach. This double difference spectrum shows a strong bleach on the blue side of the B absorbance band and bleach of P groundstate absorbance, thus resembling the P ϩ B A Ϫ difference spectrum as reported in the literature (7,(9)(10)(11). This finding indicates that the SADS obtained after 796-nm excitation represents a mixture of about equal (ϳ60:40%) contribution of P* and P ϩ B A Ϫ .…”

“…It now is becoming accepted that electron transfer from P* to P ϩ H A Ϫ proceeds via the anion of the accessory Bchl, B A , located between P and H A . At room temperature P ϩ B A Ϫ is formed as a short-lived, intermediate with a lifetime of 0.9-1.5 ps (7)(8)(9)(10)(11). Charge separation is completed by electron transfer from P Until recently it generally was accepted that excitation of the RC bacteriopheophytins or monomeric Bchls resulted in downhill energy transfer to P within a few hundreds of femtoseconds, forming P* that drives charge separation (12)(13)(14)(15).…”

Ultrafast Dynamics and Spectroscopy of Bacterial Photosynthetic Reaction Centers

“…The timescales shown in Fig. 19 are compatible with those discussed in the literature [2,10,16,21,23,25,31,36,39,51].…”

Tuning electron transfer by ester‐group of chlorophylls in bacterial photosynthetic reaction center

“…2). In our computation, rotating the H A acetyl-group by 180°in the darkadapted (light-exposed) structure down-shifted (up-shifted) E m (B A ) by 48 mV (45 mV), which corroborates spectroscopic studies [26,31]. Thus, rotation of B A ester-group together with H A acetyl-group might be the only effective way to tune the E m of B A in such a hydrophobic region.…”