Craig C. Schenck

Temperature dependence of electron transfer between bacteriopheophytin and ubiquinone in protonated and deuterated reaction centers of Rhodopseudomonas sphaeroides.

The rate of the electron-transfer reaction between bacteriopheophytin and the first quinone in isolated reaction centers of Rhodopseudomonas sphaeroides has an unusual temperature dependence. The rate increases about threefold with decreasing temperature between 300 and 25 K, and decreases abruptly at temperatures below 25 K. Partial deuteration of the reaction centers alters the temperature dependence of the rate constant. Qualitative features of the temperature dependence can be understood in the context of a theory of nonadiabatic electron transfer (Sarai, 1980. Biochim. Biophys. Acta 589:71-83). We conclude that very low-energy (10-50 cm-1) processes, perhaps skeletal vibrations of the protein, are important to electron transfer. Higher-energy vibrations, possibly involving the pyrrolic N--H bonds of bacteriopheophytin, also are important in this process.

Insights into the factors controlling the rates of the deactivation processes that compete with charge separation in photosynthetic reaction centers

Abstract This article addresses the factors that underlie the unity quantum yield of charge separation in the bacterial photosynthetic reaction center (RC). Of particular interest are the factors that suppress charge recombination and other deactivation processes which could compete effectively with charge separation. Studies on mutant RCs with enhanced charge recombination show that rates of decay processes can be increased tenfold or more upon a relatively small fractional increase in the free energy gap to the ground state. This behavior is opposite to that expected on the basis of Franck-Condon effects, where an increase in Δ G should decrease the deactivation rate. In the mutants, it is proposed that the enhanced deactivation rates of the intermediate charge-separated states result from (i) increased quantum mechanical mixing with electronic states at slightly higher energy that have inherently strong deactivation properties, and (ii) increased thermal repopulation of such strongly deactivated states. The thermal repopulation mechanism is proposed to play an important role in the heterodimer mutants studied here. This mechanism requires that the free energy gaps between the charge-separated and strongly deactivated states must decrease with temperature to be consistent with the weakly temperature-dependent deactivation behavior. Potential mediating states include the excited dimeric primary electron donor and a state involving the oxidized dimer and the reduced accessory bacteriochlorophyll molecule. It is concluded that increasing the free energy of charge-separated intermediates in order to minimize the Franck-Condon factors can enhance the deactivation rates and thus reduce the quantum yield of charge separation.

FREE-ENERGY DEPENDENCE OF THE RATE OF ELECTRON TRANSFER TO THE PRIMARY QUINONE IN BETA-TYPE REACTION CENTERS

Abstract Reaction centers of the beta-type mutants, (M)L214H and (M)L214H/(L)E104V, which contain a bacteriochlorophyll (denoted β) in place of the photoactive bacteriopheophytin, have been depleted of the native ubiquinone and reconstituted with a number of quinones. This system has allowed investigation of the rate versus free-energy relationship for electron transfer to the primary quinone in the activationless and inverted regions. The dependence of the rate on driving force is found to be much weaker in both mutants than in wild-type RCs. Analysis of the data using electron transfer theory shows that the essentially flat dependence of rate on free energy for the quinone-reconstituted beta-type mutants cannot be explained simply on the basis of increased driving force, but additionally requires a decrease in the reorganization energy. A decreased reorganization energy most likely derives from a change in the nature of the intermediary electron carrier in the mutants compared to wild-type RCs, in particular the involvement of the accessory bacteriochlorophyll molecule BChl L . The weak free-energy and temperature dependence of the electron transfer process are consistent with coupling to a range of high- to low-frequency vibrational modes of the cofactors and protein.

Site-Directed Mutations Affecting Primary Photochemistry in Reaction Centers: Effects of Dissymmetry in the Special Pair

The existence of the “special pair” (SP) of bacteriochlorophylls (BChl) was postulated nearly 20 years ago to describe the structural organization of the primary electron donor (P) of bacterial photosynthesis. The SP hypothesis was proposed based on magnetic resonance data measured for the oxidized radical cation of P in the bacterial reaction center (RC) protein (1,2). The SP concept was validated when detailed crystallographic data became available for the RCs from Rhodopseudomonas viridis and Rhodobacter sphaeroides (3–7). We now know that two strongly-interacting BChls with local C2 symmetry constitute the primary electron donor. Moreover, two monomeric BChls and two bacteriopheophytins (BPh) are related to each other about the same symmetry element. This structure reveals two prosthetic group chains, the L and M branches, that span the membrane.

Triplet state EPR of reaction centers from the HisL173→LeuL173 mutant of Rhodobacter sphaeroides which contains a heterodimer primary donor

Electron paramagnetic resonance (EPR) spectroscopy has been used to examine the triplet states in reaction centers of Rhodobacter sphaeroides which have undergone a genetic modification affecting the primary donor. Reaction centers containing the HisL173→LeuL173 substitution in the amino acid sequence have a primary donor which consists of a BChl-BPh heterodimer. The triplets formed in this heterodimer reaction center were compared with those formed in the wild-type reaction center which contains the BChl-BChl homodimer. Both reaction centers transfer triplet energy to the carotenoid under illumination at liquid nitrogen temperatures (∼90 K). However, the intensity of the carotenoid triplet signal is significantly decreased in the LeuL173 mutant compared with the wild-type reaction center. At 12 K, in wild-type reaction centers only the primary donor triplet is observed. The LeuL173 mutant exhibits a signal similar to that observed by Bylina et al. (1990) in HisM200→LeuM200 mutant reaction centers from Rb. capsulatus. The values of the zero-field splitting parameters of this triplet are discussed within the context of various models for the primary donor triplet state. No alteration in the ability of the carotenoid to quench the primary donor triplet state results from mutations at these sites.

Electric Field Effects on the PS Electron Transfer Kinetics and Spectra of Wild-Type and Heterodimer Mutant Rb. sphaeroides Reaction Centers

The intensity of fluorescence from an immobilized, isotropic sample of photosynthetic reaction centers (RCs) increases upon application of an electric field at 77 K [1], The change in fluorescence was found to be quadratic with the applied field strength, and the fluorescence in the field was found to become polarized [2]. The fluorescence increase is ascribed to a net decrease in the rate of the forward electron transfer reaction which competes with fluorescence from lP. The field alters the free energy change for electron transfer, ΔGet, and thus the rate because the energy of the dipolar product state P·HL·− [μ(P+·H·−) ~ 80 D] is sensitive to the field; the field may also affect the reorganization energy and the electronic coupling between 1P and P·+HL·−. In the original paper [1] we considered primarily effects of the field on the free energy. Subsequently our group [2] and Bixon and Jortner [3] considered possible additional effects of the electric field on the electronic coupling if dipolar states such as P·+BL·− mediate the interaction between 1P and P·+HL·−. The model of Bixon and Jortner [3] predicts that |ΔF/F| should be more than an order of magnitude greater than observed in a field of 106 V/cm (assuming that the local field correction f~1.2–1.5), and that the field dependence should be superquadratic, contrary to observation [2].