Low-Temperature Studies of Electron Transfer to the M Side of YFH Reaction Centers from Rhodobacter capsulatus

High Yield of B-Side Electron Transfer at 77 K in the Photosynthetic Reaction Center Protein from Rhodobacter sphaeroides

The primary electron transfer (ET) processes at 295 and 77 K are compared for the Rhodobacter sphaeroides reaction center (RC) pigment-protein complex from 13 mutants including a wild-type control. The engineered RCs bear mutations in the L and M polypeptides that largely inhibit ET from the excited state P* of the primary electron donor (P, a bacteriochlorophyll dimer) to the normally photoactive A-side cofactors and enhance ET to the C₂-symmetry related, and normally photoinactive, B-side cofactors. P* decay is multiexponential at both temperatures and modeled as arising from subpopulations that differ in contributions of two-step ET (e.g., P* → P⁺B_B^- → P⁺H_B^-), one-step superexchange ET (e.g., P* → P⁺H_B^-), and P* → ground state. [H_B and B_B are monomeric bacteriopheophytin and bacteriochlorophyll, respectively.] The relative abundances of the subpopulations and the inherent rate constants of the P* decay routes vary with temperature. Regardless, ET to produce P⁺H_B^- is generally faster at 77 K than at 295 K by about a factor of 2. A key finding is that the yield of P⁺H_B^-, which ranges from ∼5% to ∼90% among the mutant RCs, is essentially the same at 77 K as at 295 K in each case. Overall, the results show that ET from P* to the B-side cofactors in these mutants does not require thermal activation and involves combinations of ET mechanisms analogous to those operative on the A side in the native RC.

Putative hydrogen bond to tyrosine M208 in photosynthetic reaction centers from Rhodobacter capsulatus significantly slows primary charge separation

Slow, ∼50 ps, P* → P⁺H_A^– electron transfer is observed in Rhodobacter capsulatus reaction centers (RCs) bearing the native Tyr residue at M208 and the single amino acid change of isoleucine at M204 to glutamic acid. The P* decay kinetics are unusually homogeneous (single exponential) at room temperature. Comparative solid-state NMR of [4′-¹³C]Tyr labeled wild-type and M204E RCs show that the chemical shift of Tyr M208 is significantly altered in the M204E mutant and in a manner consistent with formation of a hydrogen bond to the Tyr M208 hydroxyl group. Models based on RC crystal structure coordinates indicate that if such a hydrogen bond is formed between the Glu at M204 and the M208 Tyr hydroxyl group, the −OH would be oriented in a fashion expected (based on the calculations by Alden et al., J. Phys. Chem.1996, 100, 16761–16770) to destabilize P⁺B_A^– in free energy. Alteration of the environment of Tyr M208 and B_A by Glu M204 via this putative hydrogen bond has a powerful influence on primary charge separation.

Femtosecond stage of electron transfer in reaction centers of the triple mutant SL178K/GM203D/LM214H of Rhodobacter sphaeroides

Coherent processes in an initial phase of charge transfer in reaction centers (RCs) of the triple mutant S(L178)K/G(M203)D/L(M214)H of Rhodobacter sphaeroides were investigated by difference (light - dark) absorption spectroscopy with 18 fsec time resolution. Electron transfer in the B cofactor branch is activated in this mutant, while the A-branch electron transfer is slowed in comparison with native RCs of Rba. sphaeroides. A bulk of absorption difference spectra was analyzed in the 940-1060 nm range (stimulated emission of excited bacteriochlorophyll dimer P* and absorption of bacteriochlorophyll anions B(A)(-) and beta(-), where beta is a bacteriochlorophyll substituting the native bacteriopheophytin H(A)) and in the 735-775 nm range (bleaching of the absorption band of the bacteriopheophytin H(B) in the B-branch) in the -0.1 to 4 psec range of delays with respect to the moment of photoexcitation of P at 870 nm. Spectra were measured at 293 and 90 K. The kinetics of P* stimulated emission at 940 nm shows its decay with a time constant of approximately 14 psec at 90 K and approximately 18 psec at 293 K, which is accompanied by oscillations with a frequency of approximately 150 cm(-1). A weak absorption band is found at 1018 nm that is formed approximately 100 fsec after excitation of P and reflects the electron transfer from P* to beta and/or B(A) with accumulation of the P(+)beta(-) and/or P(+)B(A)(-) states. The kinetics of DeltaA at 1018 nm contains the oscillations at approximately 150 cm(-1) and distinct low-frequency oscillations at 20-100 cm(-1); also, the amplitude of the oscillations at 150 cm(-1) is much smaller at 293 than at 90 K. The oscillations in the kinetics of the 1018 nm band do not contain a 32 cm(-1) mode that is characteristic for native Rba. sphaeroides RCs having water molecule HOH55 in their structure. The DeltaA kinetics at 751 nm reflects the electron transfer to H(B) with formation of the P(+)H(B)(-) state. The oscillatory part of this kinetics has the form of a single peak with a maximum at ~50 fsec completely decaying at ~200 fsec, which might reflect a reversible electron transfer to the B-branch. The results are analyzed in terms of coherent nuclear wave packet motion induced in the P* excited state by femtosecond light pulses, of an influence of the incorporated mutations on the mutual position of the energy levels of charge separated states, and of the role of water HOH55 in the dynamics of the initial electron transfer.