The free energy difference between the excited primary donor 1P∗ and the radical pair state P+H− in reaction centers of Rhodobacter sphaeroides

Elastic Vibrations in the Photosynthetic Bacterial Reaction Center Coupled to the Primary Charge Separation: Implications from Molecular Dynamics Simulations and Stochastic Langevin Approach

Primary electron transfer reactions in the bacterial reaction center are difficult for theoretical explication: the reaction kinetics, almost unalterable over a wide range of temperature and free energy changes, revealed oscillatory features observed initially by Shuvalov and coauthors (1997, 2002). Here the reaction mechanism was studied by molecular dynamics and analyzed within a phenomenological Langevin approach. The spectral function of polarization around the bacteriochlorophyll special pair PLPM and the dielectric response upon the formation of PL(+)PM(-) dipole within the special pair were calculated. The system response was approximated by Langevin oscillators; the respective frequencies, friction, and energy coupling coefficients were determined. The protein dynamics around PL and PM were distinctly asymmetric. The polarization around PL included slow modes with the frequency 30-80 cm(-1) and the total amplitude of 130 mV. Two main low-frequency modes of protein response around PM had frequencies of 95 and 155 cm(-1) and the total amplitude of 30 mV. In addition, a slowly damping mode with the frequency of 118 cm(-1) and the damping time >1.1 ps was coupled to the formation of PL(+)PM(-) dipole. It was attributed to elastic vibrations of α-helices in the vicinity of PLPM. The proposed trapping of P excitation energy in the form of the elastic vibrations can rationalize the observed properties of the primary electron transfer reactions, namely, the unusual temperature and ΔG dependences, the oscillating phenomena in kinetics, and the asymmetry of the charge separation reactions.

Energetics and kinetics of radical pairs in reaction centers from Rhodobacter sphaeroides. A femtosecond transient absorption study

Femtosecond transient absorption spectra on reaction centers from Rhodobacter sphaeroides wild type have been recorded with high time and wavelength resolution and a very high S/N ratio in the 500-940 nm range with a diode array system. The data have been analyzed by global analysis. Five lifetime components of 1.5, 3.1, 10.8, and 148 ps and long-lived (several nanoseconds) were required to fit the entire three-dimensional data surface adequately with a single set of lifetimes and decay-associated difference spectra (DADS). Up to 30 ps, there is little dispersion in the lifetimes, but in the longer time range (50-250 ps), a substantial variation in lifetime was observed, depending on detection wavelength. The data from the global analysis have been subjected to kinetic modeling comparing sequential kinetic schemes either including (reversible model) or excluding (forward model) back-reactions in the early electron transfer process(es). Thus, the molecular rate constants for the model(s) and the difference spectra of the pure intermediates [species-associated difference spectra (SADS)] were obtained. The data unequivocally confirm the necessity of an electron transfer intermediate with spectral characteristics of P+B-H prior to the formation of the P+BH- state (P is special pair, B is accessory chlorophyll, and H is pheophytin), irrespective of the model chosen. Besides being in much better agreement with the observation of long-lived fluorescence kinetics components, the reversible model results in SADS, in particular for the P+BH- state, that are in somewhat better agreement with expectations than for the pure forward model. For these and other reasons, the reversible model is preferred over the pure forward model. The electrochromic shifts of the H bands in the P+B- state and of the B bands in the P+H- state are revealed clearly in the spectra, thus supporting the assignments. Within the reversible model, the rate constant for the forward reaction in the first step P*-->P+B-H is slightly larger [k12 approximately (2.48 ps)-1] than for the second step P+B-H-->P+BH- [k23 approximately equal to (2.53 ps)-1], in contrast to the pure forward model. From the rate constants for the respective back-reactions, the free energy differences delta G relative to P* for the states P+B-H and P+BH- have been determined to be -41 and -91 meV, respectively. Thus, the free energy difference for the P+BH- state at early times after electron transfer is by a factor of 2-3 smaller than assumed so far. This has the important consequence that a quasi-equilibrium exists from about 10 ps until further electron transfer on the 200 ps time scale with a substantial percentage (approximately 16%) of the P+B-H state present. These results present the first direct evidence from transient absorption data, where the nature of the intermediate can be assigned, for the validity of the slow radical pair relaxation concept. The results have various consequences for understanding the mechanism of the overall electron transfer reaction and imply a much more active role of the protein in the early charge separation processes of the reaction center than assumed so far. The data are discussed in terms of current electron transfer theory. It is suggested that the two first-electron steps operate at a rate very close to the maximal possible rate.

Observation of multiple radical pair states in photosystem 2 reaction centers

Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.