Influence of the Electrochemical Properties of the Bacteriochlorophyll Dimer on Triplet Energy-Transfer Dynamics in Bacterial Reaction Centers

Application of Amber Suppression To Study the Role of Tyr M210 in Electron Transfer in Rhodobacter sphaeroides Photosynthetic Reaction Centers

The initial light-induced electron transfer (ET) steps in the bacterial photosynthetic reaction center (RC) have been extensively studied and provide a paradigm for connecting structure and function. Although RCs have local pseudo-C2 symmetry, ET only occurs along the A branch of chromophores. Tyrosine M210 is a key symmetry-breaking residue adjacent to bacteriochlorophyll BA that bridges the primary electron donor P and the bacteriopheophytin acceptor HA. We used amber suppression to incorporate phenylalanine variants with different electron-withdrawing/-donating capabilities at the position M210. X-ray data generally reveal no appreciable structural changes due to the mutations. P* decay and P+HA- formation are multiexponential (∼2 to 9, ∼10 to 60, and ∼100 to 300 ps) and temperature dependent. The 1020 nm transient-absorption band of P+BA- is barely resolved for a few variants at 295 K and for none at 77 K. The results indicate a change from two-step ET for wild-type RCs to the dominance of one-step superexchange ET for the mutants. Resonance Stark spectroscopy reveals that the free energy of P+BA- changes by -57 to +66 meV among the phenylalanine variants. Because P+BA- apparently lies above P* in all phenylalanine variants, the perturbations primarily affect the energy denominator for superexchange mixing. The findings deepen insight into primary ET in the bacterial RC.

Anomalous Temperature Dependence of the Triplet-Triplet Energy Transfer in Cereibacter sphaeroides I(L177)H Mutant Reaction Centers

In photosynthetic reaction centers, quenching of the primary donor triplet state by energy transfer to the carotenoid molecule provides efficient suppression of generation of singlet-excited oxygen, potent chemical oxidant. This process in the Cereibacter sphaeroides reaction centers is thermoactivated, and discontinues at temperatures below 40 K. In these reaction centers, substitution of amino acid residue isoleucine at the 177 position of the L-subunit with histidine results in the sharp decrease of activation energy, so that the carotenoid triplets are populated even at 10 K. Activation energy of the T-T energy transfer was estimated as 7.5 cm-1, which is more than 10-fold lower than activation energy in the wild type reaction centers. At certain temperatures, the energy transfer in the mutant is decelerated, which is related to the increase of effective distance of the triplet-triplet transfer. To the best of our knowledge, the described mutation presents the first reaction center modification leading to the significant decrease in activation energy of the T-T energy transfer to carotenoid molecule. The I(L177)H mutant reaction centers present a considerable interest for further studies of the triplet state quenching mechanisms, and of other photophysical and photochemical processes in the reaction centers of bacterial photosynthesis.

Quenching of bacteriochlorophyll a triplet state by carotenoids in the chlorosome baseplate of green bacterium Chloroflexus aurantiacus

To capture weak light fluxes, green photosynthetic bacteria have unique structures - chlorosomes, consisting of 104-5 molecules of bacteriochlorophyll (BChl) c, d, e. Chlorosomes are attached to the cytoplasmic membrane through the baseplate, a paracrystalline protein structure containing BChl a and carotenoids (Car). The most important function of Car is the quenching of triplet states of BChl, which prevents the formation of singlet oxygen and thereby provides photoprotection. In our work, we studied the dynamics of the triplet states of BChl a and Car in the baseplate of Chloroflexus aurantiacus chlorosomes using picosecond differential spectroscopy. BChl a of the baseplate was excited into the Qy band at 810 nm, and the corresponding absorption changes were recorded in the range of 420-880 nm. It was found that the formation of the Car triplet state occurs in ∼1.3 ns, which is ∼3 times faster than the formation of this state in the peripheral antenna of C. aurantiacus according to literature data. The Car triplet state was recorded by the characteristic absorption band T1 → Tn at ∼550 nm. Simultaneously with the appearance of absorption T1 → Tn, there was a bleaching of the singlet absorption of Car in the region of 400-500 nm. Theoretical modeling made it possible to estimate the characteristic time of formation of the triplet state of BChl a as ∼0.5 ns. It is shown that the experimental data are well described by the sequential scheme of formation and quenching of the BChl a triplet state: BChl a* → BChl aT → CarT. Thus, carotenoids from green bacteria effectively protect the baseplate from possible damage by singlet oxygen.

Highly Coupled Heterobicycle-Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C60

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole-fused porphyrin dimers and their mono- and bis-zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady-state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free-base and zinc porphyrin) reveals the occurrence of efficient singlet-singlet energy transfer (>95 % efficiency and rate constant >10¹²  s^-1 ) within the dyad. Further, donor-acceptor conjugates were formed by metal-ligand axial coordination of phenyl imidazole functionalized C₆₀ and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge-separated state was in the 30-40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π-spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Temperature dependence of nanosecond charge recombination in mutant Rhodobacter sphaeroides reaction centers: modelling of the protein dynamics

We investigated the influence of a range of factors-temperature, redox midpoint potential of an electron carrier, and protein dynamics-on nanosecond electron transfer within a protein. The model reaction was back electron transfer from a bacteriopheophytin anion, H_A^-, to an oxidized primary electron donor, P⁺, in a wild type Rhodobacter sphaeroides reaction center (RC) with a permanently reduced secondary electron acceptor (quinone, Q_A^-). Also used were two modified RCs with single amino acid mutations near the monomeric bacteriochlorophyll, B_A, located between P and H_A. Both mutant RCs showed significant slowing down of this back electron transfer reaction with decreasing temperature, similar to that observed with the wild type RC, but contrasting with a number of single point mutant RCs studied previously. The observed similarities and differences are explained in the framework of a (P⁺B_A^- ↔ P⁺H_A^-) equilibrium model with an important role played by protein relaxation. The major cause of the observed temperature dependence, both in the wild type RC and in the mutant proteins, is a limitation in access to the thermally activated pathway of charge recombination via the state P⁺B_A^- at low temperatures. The data indicate that in all RCs both charge recombination pathways, the thermally activated one and a direct one without involvement of the P⁺B_A^- state, are controlled by the protein dynamics. It is concluded that the modifications of the protein environment affect the overall back electron transfer kinetics primarily by changing the redox potential of B_A and not by changing the protein relaxation dynamics.