Properties of Rhodobacter sphaeroides Reaction Centers with the Ile→Tyr Substitution at Positions L177 and M206.

Studying pigment-protein interactions in the photosynthetic reaction centers (RCs) is important for the understanding of detailed mechanisms of the photochemical process. This paper describes spectral and photochemical characteristics, pigment composition, and stability of the Rhodobacter sphaeroides RCs with the I(L177)Y and I(M206)Y amino acid substitutions. The obtained data are compared with the properties of I(L177)H, I(L177)D, and I(M206)H RCs reported previously. It is shown that the I(L177)Y and I(M206)Y mutations cause a similar shift of the QYP band in the absorption spectra of the mutant RCs and do not affect the distribution of the electron spin density within the photo-oxidized P+ dimer. The differences in the position and amplitude of the QYB band in the I(L177)Y and I(M206)Y RCs were determined. The results indicate the possibility of new pigment-protein interactions in the vicinity of monomeric bacteriochlorophylls in the A and B chains, which might be of interest for future research.

Singlet-triplet Fission of Carotenoid Excitation in the Purple Phototrophic Bacteria Thermochromatium tepidum.

Singlet fission of carotenoid excitation is studied in purple phototrophic bacterium Thermochromatium tepidum. Using time-resolved EPR and magnetic field-induced modulation of fluorescence yield it is shown that the concept of intramolecular excitation fission developed in a number of publications is not supported by the experimental results. The obtained data favor intermolecular fission mechanism involving two carotenoid molecules.

Optically Addressable Silicon Vacancy-Related Spin Centers in Rhombic Silicon Carbide with High Breakdown Characteristics and ENDOR Evidence of Their Structure.

We discovered a family of uniaxially oriented silicon vacancy-related centers with S=3/2 in a rhombic 15R-SiC crystalline matrix. We demonstrate that these centers exhibit unique characteristics such as optical spin alignment up to the temperatures of 250°C. Thus, the range of robust optically addressable vacancy-related spin centers is extended to the wide class of rhombic SiC polytypes. To use these centers for quantum applications it is essential to know their structure. Using high frequency electron nuclear double resonance, we show that the centers are formed by negatively charged silicon vacancies V_{Si}^{-} in the paramagnetic state with S=3/2 that is noncovalently bonded to the neutral carbon vacancy V_{C}^{0} in the nonparamagnetic state, located on the adjacent site along the SiC symmetry c axis.

Singlet-triplet fission of carotenoid excitation in light-harvesting LH2 complexes of purple phototrophic bacteria.

The current generally accepted structure of light-harvesting LH2 complexes from purple phototrophic bacteria conflicts with the observation of singlet-triplet carotenoid excitation fission in these complexes. In LH2 complexes from the purple bacterium Allochromatium minutissimum, a drop in the efficiency of carotenoid triplet generation is demonstrated, which correlates with the extent of selective photooxidation of bacteriochlorophylls absorbing at ~850 nm. We conclude that singlet-triplet fission of carotenoid excitation proceeds with participation of these excitonically coupled bacteriochlorophylls. In the framework of the proposed mechanism, the contradiction between LH2 structure and photophysical properties of carotenoids is eliminated. The possibility of singlet-triplet excitation fission involving a third mediator molecule was not considered earlier.

Light-induced structural changes in photosynthetic reaction centres studied by ESEEM of spin-correlated D+QA- radical pairs.

Zn-substituted Rhodobacter sphaeroides R26 reaction centres (RCs) frozen in the dark and under illumination exhibit quite different recombination kinetics of the D+QA- radical pairs [Kleinfeld et al., Biochemistry, 23 (1984) 5780]. We have applied electron spin echo envelope modulation (ESEEM) of the spin-correlated D+QA- radical pairs to assess a possible light-induced change in the distance between the D and QA cofactors. The recombination kinetics and the field-swept spin-polarized EPR signal for the two preparations have been monitored by time-resolved EPR spectroscopy. For the samples frozen under illumination, a slight increase in the distance, 0.4+/-0.2 A, has been detected.

Formation of charge separated state P+OA- and triplet state 3P at low temperature in Rhodobacter sphaeroides (R-26) reaction centers in which bacteriopheophytin a is replaced by plant pheophytin a.

Low temperature optical and photochemical properties of Rhodobacter sphaeroides (R-26) reaction centers, in which bacteriopheophytin a has been replaced by plant pheophytin a, are reported. Modified reaction centers preserve the ability for photoinduced electron transfer from the primary electron donor P to the primary quinone acceptor QA at 80K. The triplet state ESR signal of modified reaction centers with prereduced QA at 10K shows an electron spin polarization pattern and ZFS parameters analogous to those for the triplet state 3P in non-treated reaction centers. It was found that at low temperature both P+QA- and 3P states are formed via a precursor radical pair P+I- in which I is the introduced plant pheophytin molecule. This shows that acceptor systems of bacterial and plant (photosystem II) reaction centers are mutually replacable in structural and functional aspects.