High-pressure tuning of primary photochemistry in bacterial photosynthesis: membrane-bound versus detergent-isolated reaction centers

While photosynthesis thrives at close to normal pressures and temperatures, it is presently well known that life is similarly commonplace in the hostile environments of the deep seas as well as around hydrothermal vents. It is thus imperative to understand how key biological processes perform under extreme conditions of high pressures and temperatures. Herein, comparative steady-state and picosecond time-resolved spectroscopic studies were performed on membrane-bound and detergent-purified forms of a YM210W mutant reaction center (RC) from Rhodobacter sphaeroides under modulating conditions of high hydrostatic pressure applied at ambient temperature. A previously established breakage of the lone hydrogen bond formed between the RC primary donor and the protein scaffold was shown to take place in the membrane-bound RC at an almost 3 kbar higher pressure than in the purified RC, confirming the stabilizing role of the lipid environment for membrane proteins. The main change in the multi-exponential decay of excited primary donor emission across the experimental 10 kbar pressure range involved an over two-fold continuous acceleration, the kinetics becoming increasingly mono-exponential. The fastest component of the emission decay, thought to be largely governed by the rate of primary charge separation, was distinctly slower in the membrane-bound RC than in the purified RC. The change in character of the emission decay with pressure was explained by the contribution of charge recombination to emission decreasing with pressure as a result of an increasing free energy gap between the charge-separated and excited primary donor states. Finally, it was demonstrated that, in contrast to a long-term experimental paradigm, adding a combination of sodium ascorbate and phenazine methosulfate to the protein solution potentially distorts natural photochemistry in bacterial RCs.

High-Pressure Modulation of Primary Photosynthetic Reactions

Photochemical charge separation is key to biological solar energy conversion. Although many features of this highly quantum-efficient process have been described, others remain poorly understood. Herein, ultrafast fluorescence barospectroscopy is used for the first time to obtain insights into the mechanism of primary charge separation in a YM210W mutant bacterial reaction center under novel surrounding modulating conditions. Over a range of applied hydrostatic pressures reaching 10 kbar, the rate of primary charge separation monotonously increased and that of the electron transfer to secondary acceptor decreased. While the inferred free energy gap for charge separation generally narrowed with increasing pressure, a pressure-induced break of a protein-cofactor hydrogen bond observed at ∼2 kbar significantly (by 219 cm^-1 or 27 meV) increased this gap, resulting in a drop in fluorescence. The findings strongly favor a model for primary charge separation that incorporates charge recombination and restoration of the excited primary pair state, over a purely sequential model. We show that the main reason for the almost threefold acceleration of the primary electron transfer rate is the pressure-induced increase of the electronic coupling energy, rather than a change of activation energy. We also conclude that across all applied pressures, the primary electron transfer in the mutant reaction center studied can be considered nonadiabatic, normal region, and thermally activated.

Mutation-Induced Changes in the Protein Environment and Site Energies in the (M)L214G Mutant of the Rhodobacter sphaeroides Bacterial Reaction Center

This work focuses on the low-temperature (5 K) photochemical (transient) hole-burned (HB) spectra within the P870 absorption band, and their theoretical analysis, for the (M)L214G mutant of the photosynthetic Rhodobacter sphaeroides bacterial reaction center (bRC). To provide insight into system-bath interactions of the bacteriochlorophyll a (BChl a) special pair, i.e., P870, in the mutated bRC, the optical line shape function for the P870 band is calculated numerically. On the basis of the modeling studies, we demonstrate that (M)L214G mutation leads to a heterogeneous population of bRCs with modified (increased) total electron-phonon coupling strength of the special pair BChl a and larger inhomogeneous broadening. Specifically, we show that after mutation in the (M)L214G bRC a large fraction (∼50%) of the bacteriopheophytin (HA) chromophores shifts red and the 800 nm absorption band broadens, while the remaining fraction of HA cofactors retains nearly the same site energy as HA in the wild-type bRC. Modeling using these two subpopulations allowed for fits of the absorption and nonresonant (transient) HB spectra of the mutant bRC in the charge neutral, oxidized, and charge-separated states using the Frenkel exciton Hamiltonian, providing new insight into the mutant's complex electronic structure. Although the average (M)L214G mutant quantum efficiency of P(+)QA(-) state formation seems to be altered in comparison with the wild-type bRC, the average electron transfer time (measured via resonant transient HB spectra within the P870 band) was not affected. Thus, mutation in the vicinity of the electron acceptor (HA) does not tune the charge separation dynamics. Finally, quenching of the (M)L214G mutant excited states by P(+) is addressed by persistent HB spectra burned within the B band in chemically oxidized samples.

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.

Identification of the intermediate charge-separated state P+betaL- in a leucine M214 to histidine mutant of the Rhodobacter sphaeroides reaction center using femtosecond midinfrared spectroscopy

Energy and electron transfer in a Leu M214 to His (LM214H) mutant of the Rhodobacter sphaeroides reaction center (RC) were investigated by applying time-resolved visible pump/midinfrared probe spectroscopy at room temperature. This mutant replacement of the Leu at position M214 resulted in the incorporation of a bacteriochlorophyll (BChl) in place of the native bacteriopheophytin in the L-branch of cofactors (denoted betaL). Purified LM214H RCs were excited at 600 nm (unselective excitation), at 800 nm (direct excitation of the monomeric BChl cofactors B(L) and B(M)), and at 860 nm (direct excitation of the primary donor (P) BChl pair (P(L)/P(M))). Absorption changes associated with carbonyl (C=O) stretch vibrational modes (9-keto, 10a-ester, and 2a-acetyl) of the cofactors and of the protein were recorded in the region between 1600 cm(-1) and 1770 cm(-1), and the data were subjected to both a sequential analysis and a simultaneous target analysis. After photoexcitation of the LM214H RC, P* decayed on a timescale of approximately 6.3 ps to P+BL-. The decay of P+BL- occurred with a lifetime of approximately 2 ps, approximately 3 times slower than that observed in wild-type and R-26 RCs (approximately 0.7 ps). Further electron transfer to the betaL BChl resulted in formation of the P+betaL- state, and its infrared absorbance difference spectrum is reported for the first time, to our knowledge. The fs midinfrared spectra of P+BL- and P+betaL- showed clear differences related to the different environments of the two BChls in the mutant RC.