A monomeric bacteriochlorophyll triplet state (3B) in reaction centres of Rhodobacter sphaeroides R26, studied by absorption-detected magnetic resonance

Replacement or exclusion of the B-branch bacteriopheophytin in the purple bacterial reaction centre: The HB cofactor is not required for assembly or core function of the Rhodobacter sphaeroides complex

All of the membrane-embedded cofactors of the purple bacterial reaction centre have well-defined functional or structural roles, with the exception of the bacteriopheophytin (H(B)) located approximately half-way across the membrane on the so-called inactive- or B-branch of cofactors. Sequence alignments indicate that this bacteriochlorin cofactor is a conserved feature of purple bacterial reaction centres, and a pheophytin is also found at this position in the Photosystem-II reaction centre. Possible structural or functional consequences of replacing the H(B) bacteriopheophytin by bacteriochlorophyll were investigated in the Rhodobacter sphaeroides reaction centre through mutagenesis of residue Leu L185 to His (LL185H). Results from absorbance spectroscopy indicated that the LL185H mutant assembled with a bacteriochlorophyll at the H(B) position, but this did not affect the capacity of the reaction centre to support photosynthetic growth, or change the kinetics of charge separation along the A-branch of cofactors. It was also found that mutation of residue Ala M149 to Trp (AM149W) caused the reaction centre to assemble without an H(B) bacteriochlorin, demonstrating that this cofactor is not required for correct assembly of the reaction centre. The absence of a cofactor at this position did not affect the capacity of the reaction centre to support photosynthetic growth, or the kinetics of A-branch electron transfer. A combination of X-ray crystallography and FTIR difference spectroscopy confirmed that the H(B) cofactor was absent in the AM149W mutant, and that this had not produced any significant disturbance of the adjacent ubiquinol reductase (Q(B)) site. The data are discussed with respect to possible functional roles of the H(B) bacteriopheophytin, and we conclude that the reason(s) for conservation of a bacteriopheophytin cofactor at this position in purple bacterial reaction centres are likely to be different from those underlying conservation of a pheophytin at the analogous position in Photosystem-II.

Triplet energy transfer in bacterial photosynthetic reaction centres

[3-vinyl]-132-OH-bacteriochlorophyll a has been selectively exchanged against native bacteriochlorophyll a in the monomer binding sites at the A- and B-branch of the photosynthetic reaction centre from Rhodobacter sphaeroides. Transient absorption difference measurements were performed on these samples over a temperature range from 4.2 to 300 K with 20 ns time resolution. Specifically the decay of the primary donor triplet state, 3P870, as well as the rise and decay rates of the carotenoid triplet state, 3Car (spheroidene), were measured. The observed rates revealed a thermally activated carotenoid triplet formation corresponding to the decay of the primary donor triplet state. The activation energies for the triplet energy transfer process were 100(+/-10) cm-1 for reaction centers from wild-type Rhodobacter sphaeroides 2.4.1, with and without exchange of the monomeric bacteriochlorophyll on the electron transfer-active branch, BA. For reaction centers from Rhodobacter sphaeroides R26.1 with both monomers exchanged against [3-vinyl]-132-OH-bacteriochlorophyll a, and subsequent spheroidene reconstitution the activation energy was 460(+/-20) cm-1. These activation energies correspond to the energy difference between the triplet states of the accessory BChl monomer, BB, and the primary donor when native BChl a or [3-vinyl]-132-OH-BChl a is present in the BB binding site. In all samples the 3Car formation rates were bi-phasic over a large temperature range. A fast temperature-independent rate was observed on the wavelength of the carotenoid triplet-triplet absorption which dominated at very low temperatures. Additionally, a slower temperature-independent 3Car formation rate was observed at low temperatures which could be explained with the assumption of heterogeneity in the energy barrier (3BB) and/or the primary donor triplet state (3P870). A tunneling mechanism as proposed earlier by Kolaczkowski (PhD thesis, Brown University, 1989) is not only unnecessary but also incompatible with the available experimental data.

Protein modifications affecting triplet energy transfer in bacterial photosynthetic reaction centers

The efficiency of triplet energy transfer from the special pair (P) to the carotenoid (C) in photosynthetic reaction centers (RCs) from a large family of mutant strains has been investigated. The mutants carry substitutions at positions L181 and/or M208 near chlorophyll-based cofactors on the inactive and active sides of the complex, respectively. Light-modulated electron paramagnetic resonance at 10 K, where triplet energy transfer is thermally prohibited, reveals that the mutations do not perturb the electronic distribution of P. At temperatures > or = 70 K, we observe reduced signals from the carotenoid in most of the RCs with L181 substitutions. In particular, triplet transfer efficiency is reduced in all RCs in which a lysine at L181 donates a sixth ligand to the monomeric bacteriochlorophyll B(B). Replacement of the native Tyr at M208 on the active side of the complex with several polar residues increased transfer efficiency. The difference in the efficiencies of transfer in the RCs demonstrates the ability of the protein environment to influence the electronic overlap of the chromophores and thus the thermal barrier for triplet energy transfer.