F. Lendzian

The electronic structure of the primary donor cation radical in Rhodobacter sphaeroides R-26: ENDOR and TRIPLE resonance studies in single crystals of reaction centers

Abstract The electron spin density distribution of the cation radical of the primary donor, D+, a bacteriochlorophyll a dimer was determined by ENDOR and TRIPLE resonance experiments performed on single crystals of reaction centers (RCs) of Rhodobacter sphaeroides R-26. Nine isotropic proton hyperfine coupling constants (hfcs) were obtained and from the angular dependence of the hfcs in three crystallographic planes, five complete hyperfine (hf) tensors were determined. Theoretical hf tensors were calculated by the all-valence-electron SCF molecular orbital method RHF-INDO/SP using the X-ray structure data of the dimer D and its amino acid environment. A comparison of the directions of the principal axes of the experimental and calculated hf tensors enabled us to identify the hfcs with specific protons on the two bacteriochlorophyll halves DL and DM of the dimer. The result shows that the unpaired valence electron is unequally distributed over the dimer halves, favoring DL by approx. 2:1. This ratio has been obtained from the proton hfcs of rotating methyl groups, which directly reflect the π-spin densities at the corresponding positions in the two macrocycles, DL and DM. It was further confirmed by recent 15N-ENDOR experiments on RC single crystals (Lendzian, F., Bonigk, B., Plato, M., Mobius, K. and Lubitz, W. (1992) in The Photosynthetic Bacterial Reaction Center II (Breton, J. and Vermeglio, A., eds.), pp. 89–97, Plenum Press, New York). The observed asymmetry of D+ is attributed to the difference in energies of the highest filled molecular π-orbitals of the monomeric halves, DL and DM, which is caused by differences in the structure of the two bacteriochlorophylls and/or their environment. Possible implications of this asymmetry for the electron transfer in the RC are discussed.

Molecular orbital investigation of dimer formations of bacteriochlorophyll a. Model configurations for the primary donor of photosynthesis

Abstract Three stable dimer configurations of bacteriochlorophyll a are predicted by a self-consistent-field molecular orbital calculation (all valence electrons, restricted Hartree-Fock, INDO-parametrization, perturbational treatment of spin polarization) including geometry optimization by energy minimization. Spin-density distributions of the cation radicals are also calculated and compared with results from magnetic resonance experiments. The largest binding energy is obtained from a strong overlap of the π-systems combined with the formation of two symmetrical MgO bonds between the monomeric components. This structure is compatible with the experimental spin density distribution and very similar to the recently determined X-ray structure of the bacteriochlorophyll b dimer in the photosynthetic reaction center of Rps. viridis. Some possible perturbations by the protein environment are discussed.

ESR, ENDOR and TRIPLE resonance studies of the primary donor radical cation P960+• in the photosynthetic bacterium Rhodopseudomonas viridis

The light-induced radical cation of the primary electron donor P960+• in photosynthetic reaction centers from Rhodopseudomonas viridis has been investigated by ESR, ENDOR and TRIPLE techniques. Both the comparison with the cation radical of monomeric bacteriochlorophyll b (BChl b) and with molecular-orbital calculations performed on P960+• using the results of an X-ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960+•. The possible relevance of this result for the primary electron transfer step in the reaction center is briefly discussed.

Time-resolved CIDEP-enhanced endor on short-lived radicals

Abstract On two photolytically generated transient radicals showing CIDEP effects. ENDOR experiments have been performed by integrating the ESR signal between 4 and 5 μs after the irradiating laser flash. Different ENDOR effects are observed for a net effect electron polarization in p -benzosemiquinone and for a multiplet polarization in benzyl radicals.

Triplet electron transfer and time-resolved EPR: Detection of the radical-pair state of a covalently linked porphyrin—quinone

Abstract Triplet electron transfer is observed in a covalently linked porphyrin-quinone in ethanol at temperatures between 160 and 130 K. Both, the primary porphyrin triplet state T PQ and the radical pair + (PQ − ) T created by light-induced charge separation, are observed by time-resolved EPR. The latter is identified by its g factor and dipolar splitting.

Endor and triple resonance in solutions of the chlorophyll a and bis(chlorophyll)cyclophane radical cations

Abstract The radical cations of chlorophyll a (Chl a) and bis ( chlorophyll )cyclophane (BCP), a synthetic model dimer of chlorophyll, were studied by ENDOR and TRIPLE resonance in liquid solution. The measured hyperiine coupling constants (hfcs) of Chl a + were compared with the results of molecular orbital calculations. By comparing the hfcs of BCP + and Chl a + a localization of the unpaired electron on one macrocycle of BCP + is deduced.

Time-resolved ESR and endor during the photolysis of ω,ω-dimethoxy-ω-phenyl-acetophenone

Abstract The ketone ω,ω-dimethoxy-ω-phenyl-acetophenone (DMPA) is an efficient initiator in UV-curing polymerization processes started by photochemically generated radicals. The short-lived radical intermediates created during the photolysis of DMPA have been investigated by time-resolved ESR and ENDOR spectroscopy taking advantage of a strong CIDEP effect.

Proton endor study of the photoexcited triplet state PT in Rps. sphaeroides R-26 photosynthetic reaction centres

The photoexcited triplet state PT of Rhodopseudomonas sphaeroides R-26 has been investigated by ENDOR measurements performed on frozen photosynthetic reaction centre solutions. For the first time hyperfine data could be obtained for PT. These data indicate a delocalisation of the triplet state over two bacteriochlorophyll a molecules.

Computer analysis of spectra with strongly overlapping lines. Application to TRIPLE resonance spectra of the chlorophyll a cation radical

Abstract To obtain more information from spectra with strongly overlapping lines, a computer program named COMPASS was developed. After the lineshape functions are fixed, the positions, widths, and areas of the lines are adjusted by a special least-squares method. As an example the analysis of four magnetic TRIPLE resonance spectra of the chlorophyll a cation radical is presented. Three more lines than in an earlier analysis are revealed by this deconvolution method. This agrees well with the predictions of earlier molecular orbital calculations.

The bacteriopheophytin a anion radical. A solution endor and triple resonance study

Abstract From electrolytically generated bacteriopheophytin a anion radical in solution, 10 proton and 4 nitrogen hyperfine coupling constants, including signs, have been measured by ESR, ENDOR and TRIPLE resonance techniques. The experimental results are compared to those from MO calculations and possible assignments to molecular positions discussed.