Time-dependent thermodynamics during early electron transfer in reaction centers from Rhodobacter sphaeroides.

Abstract

The temperature dependence of fluorescence on the picosecond to nanosecond time scale from the reaction centers of Rhodobacter sphaeroides strain R-26 and two mutants with elevated P/P+ midpoint potentials has been measured with picosecond time resolution. In all three samples, the kinetics of the fluorescence decay is complex and can only be well described with four or more exponential decay terms spanning the picosecond to nanosecond time range. Multiexponential fits are needed at all temperatures between 295 and 20 K. The complex decay kinetics are explained in terms of a dynamic solvation model in which the charge-separated state is stabilized after formation by protein conformational changes. Many of these motions have not had time to occur on the time scale of initial electron transfer and/or are frozen out at low temperature. This results in a time- and temperature-dependent enthalpy change between the excited singlet state and the charge-separated state that is the dominant term in the free energy difference between these states. Long-lived fluorescence is still observed even at 20 K, particularly for the high-potential mutants. This implies that the driving force for electron transfer on the nanosecond time scale at low temperature is less than 200 cm-1 (25 meV) in R-26 reaction centers and even smaller on the picosecond time scale or in the high-potential mutants.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…Contributions to complex kinetic profiles from dynamic effects involving motions of the chromophores, the protein, or both accompanying or following the decay of P* or P + H L -also have been discussed. 18,22,65,74,[104][105][106][107] In our previous work, we proposed the scheme for D LL given in Figure 2B, wherein the charge-separated intermediates (states other than P + Q A -and P + Q B -) lie above P* in free energy. This scheme derived from (1) the current view of the free energies of the intermediates in wild type, (2) the measured P/P + potential in D LL being at least 100 meV higher than in wild type, and (3) the lack of spectral signatures for formation of a chargeseparated state from P* in D LL at 295 K. In the simplest interpretation, the data presented here for D LL in either Deriphat glass or LDAO glass at 77 K are similar to the results for D LL at 295 K and can be reconciled with the model shown in Figure 2B, namely, with the charge-separated intermediates lying above P* and P* f ground state (via internal conversion) being the only decay route of P*.…”

“…[55][56][57] Additionally, calculations generally place the P + B -state higher in free energy than the P + H -state on both L and M branches by 0.15-0.25 eV, consistent with the difference in redox potentials of bacteriochlorophyll and bacteriopheophytin in vitro. [58][59][60][61] Calculations and experiments have provided estimates or bracketed ranges for the free energies of the charge-separated states in the wild-type RC: P + B L -0.05-0.1 eV below P*; 29,50,51,[62][63][64][65][66][67][68][69][70] P + H L -∼0.25 eV below P* when relaxed; [71][72][73][74][75] P + B M -0.1-0.2 eV above P* and P + H M -below P* by no more than ∼0.15 eV and probably within 0.1 eV. [50][51][52][76][77][78][79] Systematic efforts to manipulate the free energy differences of the L-and M-branch charge-separated states by site-directed mutagenesis of key amino acids, including those near B M and B L , have led to mutant RCs in which electron transfer to the M branch competes effectively with charge separation to the L branch, yielding P + H M -(reviewed in ref 80).…”

Specific alteration of the oxidation potential of the electron donor in reaction centers from Rhodobacter sphaeroides.

“…Since the expected increase in the energy of the P+I-state by up to 260 meV is larger than the P*/P+I-energy difference of 120-200 meV estimated from the P* fluorescence decay (reviewed in ref. 30), it is surprising that the forward electron transfer reaction can even occur in the mutants. However, the midpoint potentials derived from oxidation-reduction titrations are equilibrium measurements and thus pertain to fully relaxed systems, whereas measurements of the energy difference indicate that it evolves over the time of the initial electron transfer (30).…”

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

“…The most favourable fit to the full set of experimental data from 690-1060 nm required five lifetimes. A combination of the model for RC kinetics from (5,6,8,9,12,18) and the idea of radical pair relaxation (5,18,(38)(39)(40) gave a five-component kinetic scheme ( Figure 5) that offered a satisfactory description of the data. The SADS of the two excited states and three radical pair states involved in this scheme were consistent with what is known about the spectra of these states.…”