The radical cation of bacteriochlorophyll b. A liquid-phase endor and triple resonance study

The previous termradical cationnext term of bacterioehlorophyll b (BChl b) is investigated by ENDOR and TRIPLE resonance in liquid solution. The experimental hyperfine coupling constants, ten proton and three nitrogen couplings, are compared with the predictions from advanced molecular-orbital calculations (RHF INDO/SP). The detailed picture obtained of the spin density distribution is a prerequisite for the investigation of the primary electron donor previous termradical cationnext term in BChl b containing photosynthetic bacteria

EPR, ENDOR, and TRIPLE resonance studies of modified bacteriochlorophyll cation radicals

A series of substituted bacteriochlorophyll molecules, all used in reconstitution experiments of reaction centers of Rhodobacter sphaeroides (Struck et al. Biochim. Biophys. Acta 1991, 1060, 262-270), were characterized by EPR, electron-nuclear double (ENDOR), and electron-nuclear-nuclear triple (TRIPLE) resonance spectroscopy in their monomeric radical cation states. Effects of different substituents at position 3 in the porphyrin macrocycle were considered, especially for two «crosslinks» between plant and bacterial chlorophylls. These are 3-vinylbacteriochlorophyll where the «bacteria» acetyl group at position 3 was substituted by vinyl and 3-acetylchlorophyll where the «plant» vinyl group was substituted by acety

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

Correlation of structural and spectroscopic properties of a photosynthetic reaction center

Polarized spectra of absorption and light-induced absorbance changes are presented for the crystallized reaction centers of Rhodopseudomonas viridis. We find that a model based on extended dipole interaction between all six pigments is capable of interpreting detailed features such as the contributions from the individual pigments to the various absorption peaks. Even though the pigments are arranged in approximate C2 symmetry, the optical spectra together with the calculations reflect deviations from this symmetry, which may be important in understanding the electron pathway

Conformational relaxation following reduction of the photoactive bacteriopheophytin in reaction centers from Blastochloris viridis. Influence of mutations at position M208

AbstractThe photochemically trapped bacteriopheophytin (BPh) b radical anion in the active branch (ΦA−) of reaction centers (RCs) from Blastochloris (formerly called Rhodopseudomonas) viridis is characterized by 1H-ENDOR as well as optical absorption spectroscopy. The two site-directed mutants YF(M208) and YL(M208), in which tyrosine at position M208 is replaced by phenylalanine and leucine, respectively, are investigated and compared with the wild type. The residue at M208 is in close proximity to the primary electron donor, P, the monomeric bacteriochlorophyll (BChl), BA, and the BPh, ΦA, that are involved in the transmembrane electron transfer to the quinone, QA, in the RC. The analysis of the ENDOR spectra of ΦA− at 160 K indicates that two distinct states of ΦA− are present in the wild type and the mutant YF(M208). Based on a comparison with ΦA− in RCs of Rhodobacter sphaeroides the two states are interpreted as torsional isomers of the 3-acetyl group of ΦA. Only one ΦA− state occurs in the mutant YL(M208). This effect of the leucine residue at position M208 is explained by steric hindrance that locks the acetyl group in one specific position. On the basis of these results, an interpretation of the optical absorption difference spectrum of the state ΦA−QA− is attempted. This state can be accumulated at 100 K and undergoes an irreversible change between 100 and 200 K [Tiede et al., Biochim. Biophys. Acta 892 (1987) 294–302]. The corresponding absorbance changes in the BChl Qx and Qy regions observed in the wild type also occur in the YF(M208) mutant but not in YL(M208). The observed changes in the wild type and YF(M208) are assigned to RCs in which the 3-acetyl group of ΦA changes its orientation. It is concluded that this distinct structural relaxation of ΦA can significantly affect the optical properties of BA and contribute to the light-induced absorption difference spectra

0105 v.2 Probing the Surrounding of a Cobalt(II) Porphyrin and its Superoxo Complex by EPR Techniques

Abstract. Octaethylporphyrinato-cobalt(II), Co(II)OEP, was studied by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) in frozen solutions of methanol, tetrahydrofuran and pyridine diluted into chloroform. Oxygenation led to the respective paramagnetic superoxo complexes Co(III)OEPpy$$O 2 # . The EPR spectra demonstrate strong differences in the axial ligation states ((base-on)/(base-off)) and ease of the oxygenation process. Additional ENDOR studies with partial orientation selection along the principal g tensor axes are performed for resolution of the 1 H, 14/15 N and 59 Co hyperfine (hf) coupling constants. This allows a comparison of the electron spin density distribution of the superoxo complex and its precursor. The data are interpreted in the framework of a more rigorous and detailed theoretical configuration interaction model than previously presented in the literature. The theoretical treatment shows that the structure of the superoxo complex is best described in a three-orbital model with contributions from the cobalt 3d, 4s, and oxygen p orbitals. The analysis reproduces the experimental g and Co hf data yielding the relative energies of the MOs and the MO coefficients for the description of the spin density distribution in the Co(II) complex and its superoxo complex. To demonstrate the generality of the approach and possible applications, a comparison is made with the vitamin B 12r system. Furthermore, it provides detailed insight into the electronic and geometric changes resulting from axial ligation and oxygenation of the Co(II)OEP complex; this information can be used for predictions of the catalytic activity

Integration of Catalysis with Storage for the Design of Multi-Electron Photochemistry Devices for Solar Fuel

Decarbonization of the transport system and a transition to a new diversified energy system that is scalable and sustainable, requires a widespread implementation of carbon-neutral fuels. In biomimetic supramolecular nanoreactors for solar-to-fuel conversion, water-splitting catalysts can be coupled to photochemical units to form complex electrochemical nanostructures, based on a systems integration approach and guided by magnetic resonance knowledge of the operating principles of biological photosynthesis, to bridge between long-distance energy transfer on the short time scale of fluorescence, ~10−9 s, and short-distance proton-coupled electron transfer and storage on the much longer time scale of catalysis, ~10−3 s. A modular approach allows for the design of nanostructured optimized topologies with a tunneling bridge for the integration of storage with catalysis and optimization of proton chemical potentials, to mimic proton-coupled electron transfer processes in photosystem II and hydrogenase

An improved coupling design for high-frequency TE011 electron paramagnetic resonance cavities

In high-frequency electron paramagnetic resonance (EPR) spectroscopy the sample is usually accommodated in a single-mode cylindrical TE011 microwave cavity. This cavity stands out in terms of flexibility for various types of EPR experiments due to convenient control of its resonance frequency and easy waveguide-to-cavity microwave coupling. In continuous wave and in pulsed EPR it is, however, essential to be able to vary the coupling efficiency over a large range. We present a new mechanical design to vary the microwave coupling to the cavity using a movable metal sphere. This coupling sphere is shifted in the plane of the iris wall inside the coupling waveguide. The design allows for a compact and robust construction of the EPR probehead that can be easily accommodated inside a limited space of helium flow cryostat. The construction details and characterization of the coupling element for 95 GHz (W-band) EPR as well as for 34 GHz (Q-band) are presented. © 2013 American Institute of Physics