ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field: a tool for mapping electron transfer pathways

A new phenomenon has been detected in the time-resolved electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pairs in photosynthetic reaction center proteins. The observed effects result from both increased resolution and orientational selectivity provided by high magnetic field EPR and are manifest as specific, derivative-type lines in the ENDOR spectrum. Importantly, the positions and amplitudes of these lines contain information on the interaction of a particular nucleus with both correlated electron spins. Thus, spin density delocalization in the protein environment between the donor and acceptor in the SCRP can be revealed via SCRP ENDOR, providing a unique opportunity to probe the electron-transfer pathways in natural and artificial photosynthetic assemblies.     

Structure of the charge separated state P865(+)Q(A)- in the photosynthetic reaction centers of Rhodobacter sphaeroides by quantum beat oscillations and high-field electron paramagnetic resonance: evidence for light-induced Q(A)- reorientation

The structure of the secondary radical pair, P865(+)Q(A)-, in fully deuterated and Zn-substituted reaction centers (RCs) of the purple bacterium Rhodobacter sphaeroides R-26 has been determined by high-time resolution and high-field electron paramagnetic resonance (EPR). A computer analysis of quantum beat oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the orientation of the various magnetic tensors of P865(+)Q(A)- with respect to a magnetic reference frame. The orientation of the g-tensor of P865(+) in an external reference system is adapted from a single-crystal W-band (95 GHz) EPR study [Klette, R.; T?rring, J. T.; Plato, M.; M?bius, K.; B?nigk, B.; Lubitz, W. J. Phys. Chem. 1993, 97, 2015-2020]. Thus, we obtain the three-dimensional structure of the charge separated state P865(+)Q(A)- on a nanosecond time scale after light-induced charge separation. Comparison with crystallographic data reveals that the position of the quinone is essentially the same as that in the X-ray structure. However, the head group of Q(A)- has undergone a 60 degrees rotation in the ring plane relative to its orientation in the crystal structure. Analysis suggests that the two different QA conformations are functionally relevant states which control the electron-transfer kinetics from Q(A)- to the secondary quinone acceptor QB. It appears that the rate-limiting step of this reaction is a reorientation of Q(A)- in its binding pocket upon light-induced reduction. The new kinetic model accounts for striking observations by Kleinfeld et al. who reported that electron transfer from Q(A)- to QB proceeds in RCs cooled to cryogenic temperature under illumination but does not proceed in RCs cooled in the dark [Kleinfeld, D.; Okamura, M. Y.; Feher, G. Biochemistry 1984, 23, 5780-5786].     

Orientation-resolving pulsed electron dipolar high-field EPR spectroscopy on disordered solids: I. Structure of spin-correlated radical pairs in bacterial photosynthetic reaction centers

Distance and relative orientation of functional groups within protein domains and their changes during chemical reactions determine the efficiency of biological processes. In this work on disordered solid-state electron-transfer proteins, it is demonstrated that the combination of pulsed high-field EPR spectroscopy at the W band (95 GHz, 3.4 T) with its extensions to PELDOR (pulsed electron-electron double resonance) and RIDME (relaxation-induced dipolar modulation enhancement) offers a powerful tool for obtaining not only information on the electronic structure of the redox partners but also on the three-dimensional structure of radical-pair systems with large interspin distances (up to about 5 nm). Strategies are discussed both in terms of data collection and data analysis to extract unique solutions for the full radical-pair structure with only a minimum of additional independent structural information. By this novel approach, the three-dimensional structure of laser-flash-induced transient radical pairs P(865)(*+)Q(A)(*-) in frozen-solution reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides is solved. The measured positions and relative orientations of the weakly coupled ion radicals P(865)(*+) and Q(A)(*-) are compared with those of the precursor cofactors P865 and QA known from X-ray crystallography. A small but significant reorientation of the reduced ubiquinone QA is revealed and interpreted as being due to the photosynthetic electron transfer. In contrast to the large conformational change of Q(B)(*-) upon light illumination of the RCs, the small light-induced reorientation of Q(A)(*-) had escaped previous attempts to detect structural changes of photosynthetic cofactors upon charge separation. Although small, they still may be of functional importance for optimizing the electronic coupling of the redox partners in bacterial photosynthesis both for the charge-separation and charge-recombination processes.     

Electronic structure description of a [Co(III)3Co(IV)O4] cluster: a model for the paramagnetic intermediate in cobalt-catalyzed water oxidation

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

"Glass transition" near 200 K in the bacterial photosynthetic reaction center protein detected by studying the distances in the transient P+Q(A)- radical pair

The transient radical pair P(+)Q(A)(-) in the photosynthetic reaction center from Rhodobacter sphaeroides R26 was studied over a wide temperature range using out-of-phase electron spin-echo envelope modulation (ESEEM) spectroscopy. This method is sensitive to the magnetic dipole-dipole interaction between the two electron spins of the pair and allows precise determination of the distance in the pair P(+)Q(A)(-). The out-of-phase data were complemented by normal in-phase ESEEM spectra from the two stable radicals of P(+) and Q(A)(-). The results seem to indicate that the radical pair undergoes a noticeable molecular motion around 200 K that may be characterized by a change in the distance in the pair by approximately 0.3 nm. As the two cofactors, P(+) and Q(A)(-), are held in a well-defined relative position by the reaction center protein, this means that the protein becomes flexible at 200 K. This effect may be ascribed to a dynamic glass transition around 200 K. The relation with the temperature dependence of the back reaction of P(+)Q(A)(-) is discussed.     

The Mn(2+)-bicarbonate complex in a frozen solution revisited by pulse W-band ENDOR

The coordination of bicarbonate to Mn (2+) is the simplest model system for the coordination of Mn (2+) to carboxylate residues in a protein. Recently, the structure of such a complex has been investigated by means of X-band pulse EPR (electron paramagnetic resonance) experiments ( Dasgupta, J. ; et al. J. Phys. Chem. B 2006, 110, 5099 ). Based on the EPR results, together with electrochemical titrations, it has been concluded that the Mn (2+) bicarbonate complex consists of two bicarbonate ligands, one of which is monodentate and other bidentate, but only the latter has been observed by the pulsed EPR techniques. The X-band measurements, however, suffer several drawbacks. (i) The zero-field splitting (ZFS) term of the spin Hamiltonian affects the nuclear frequencies. (ii) There are significant contributions from ENDOR (electron nuclear double resonance) lines of the M S not equal +/- (1)/ 2 manifolds. (iii) There are overlapping signals of (23)Na. All these reduce the uniqueness of the data interpretation. Here we present a high-field ENDOR investigation of Mn (2+)/NaH (13)CO 3 in a water/methanol solution that eliminates the above difficulties. Both Davies and Mims ENDOR measurements were carried out. The spectra show that a couple of slightly inequivalent (13)C nuclei are present, with isotropic and anisotropic hyperfine couplings of A iso1 = 1.2 MHz, T perpendicular1 = 0.7 MHz, A iso2 = 1.0 MHz, T perpendicular2 = 0.6 MHz, respectively. The sign of the hyperfine coupling was determined by variable mixing time (VMT) ENDOR measurements. These rather close hyperfine parameters suggest that there are either two distinct, slightly different, carbonate ligands or that there is some distribution in conformation in only one ligand. The distances extracted from T perpendicular1 and T perpendicular2 are consistent with a monodentate binding mode. The monodentate binding mode and the presence of two ligands were further supported by DFT calculations and (1)H ENDOR measurements. Additionally, (23)Na ENDOR resolved at least two types of (23)Na (+) in the Mn (2+)-bicarbonate complex, thus suggesting that the bicarbonate bridges two positively charged metal ions.     

Proton uptake of rhodobacter capsulatus reaction center mutants modified in the primary quinone environment

Flash-induced absorbance spectroscopy was used to analyze the proton uptake and electron transfer properties of photosynthetic reaction centers (RC) of Rhodobacter capsulatus that have been genetically modified near the primary quinone electron acceptor (Q(A)). M246Ala and M247Ala, which are symmetry-related to the positions of two acidic groups, L212Glu and L213Asp, in the secondary quinone electron acceptor (QB) protein environment, have been mutated to Glu and Asp, respectively. The pH dependence of the stoichiometry of proton uptake upon formation of the P+Q(A)- (H+/P+Q(A)-) and PQ(A) (H+/Q(A)-) (P is the primary electron donor, a noncovalently linked bacteriochlorophyll dimer) states have been measured in the M246Ala --> Glu and the M247Ala --> Asp mutant RC, in the M246Ala-M247Ala --> Glu-Asp double mutant and in the wild type (WT). Our results show that the introduction of an acidic group (Glu or Asp) in the QA protein region induces notable additional proton uptake over a large pH region (approximately 6-9), which reflects a delocalized response of the protein to the formation of Q(A)-. This may indicate the existence of a widely spread proton reservoir in the cytoplasmic region of the protein. Interestingly, the pH titration curves of the proton release caused by the formation of P+ (H+/P+: difference between H+/P+Q(A)- and H+/PQ(A)- curves) are nearly superimposable in the WT and the M246Ala --> Glu mutant RC, but substantial additional proton release is detected between pH 7 and 9 in the M247Ala --> Asp mutant RC. This effect can be accounted for by an increased proton release by the P+ environment in the M247Ala --> Asp mutant. The M247Ala --> Asp mutation reveals the existence of an energetic and conformational coupling between donor and acceptor sides of the RC at a distance of nearly 30A.     

Distance measurements between paramagnetic centers and a planar object by matrix Mims electron nuclear double resonance

Frequency-domain electron nuclear double resonance (ENDOR), two time-domain electron nuclear double resonance techniques, and electron spin echo envelope modulation spectroscopy are compared with respect to their merit in measurements of small hyperfine couplings to nuclei with intermediate gyromagnetic ratio such as 31P. The frequency-domain Mims ENDOR experiment is found to provide the most faithful line shapes. In the limit of long electron-nuclear distances of more than 0.5 nm, sensitivity of this experiment is optimized by matching the first interpulse delay to the transverse relaxation time of the electron spins. In the same limit, Mims ENDOR efficiency scales inversely with the sixth power of distance. Hyperfine splittings as small as 33 kHz can be detected, corresponding to an electron-31P distance of 1 nm. In systems, where a certain kind of nuclei is distributed in a plane, measurements of intermolecular hyperfine couplings can be analyzed in terms of a distance of closest approach of a paramagnetic center to that plane. By applying this technique to spin-labeled lipids in a fully hydrated lipid bilayer it is found that for a fraction of lipids, chain tilt angles can be 25 degrees larger than the mean tilt angle of the lipid chains. This model of all-trans hydrocarbon chains with a broad distribution of tilt angles is also consistent with orientation selection effects in high-field ENDOR spectra.     

Water Activity Regulates the Q(A)(-) to Q(B) electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides

We report on the effects of water activity and surrounding viscosity on electron transfer reactions taking place within a membrane protein: the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides. We measured the kinetics of charge recombination between the primary photoxidized donor (P(+)) and the reduced quinone acceptors. Water activity (aW) and viscosity (eta) have been tuned by changing the concentration of cosolutes (trehalose, sucrose, glucose, and glycerol) and the temperature. The temperature dependence of the rate of charge recombination between the reduced primary quinone, Q(A)(-), and P(+) was found to be unaffected by the presence of cosolutes. At variance, the kinetics of charge recombination between the reduced secondary quinone (Q(B)(-)) and P(+) was found to be severely influenced by the presence of cosolutes and by the temperature. Results collected over a wide eta-range (2 orders of magnitude) demonstrate that the rate of P(+)Q(B)(-) recombination is uncorrelated to the solution viscosity. The kinetics of P(+)Q(B)(-) recombination depends on the P(+)Q(A)(-)Q(B) <--> P(+)Q(A)Q(B)(-) equilibrium constant. Accordingly, the dependence of the interquinone electron transfer equilibrium constant on T and aW has been explained by assuming that the transfer of one electron from Q(A)(-) to Q(B) is associated with the release of about three water molecules by the RC. This implies that the interquinone electron transfer involves at least two RC substates differing in the stoichiometry of interacting water molecules.     

Enzyme control of small-molecule coordination in FosA as revealed by 31P pulsed ENDOR and ESE-EPR

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, which contains a phosphonate group. The active site of this enzyme consists of a high-spin Mn(2+) ion coordinated by endogenous ligands (a glutamate and two histidine residues) and by exogenous ligands, such as substrate fosfomycin. To study the Mn(2+) coordination environment of FosA in the presence of substrate and the inhibitors phosphonoformate and phosphate, we have used (31)P pulsed electron-nuclear double resonance (ENDOR) at 35 GHz to obtain metrical information from (31)P-Mn(2+) interactions. We have found that continuous wave (CW) (31)P ENDOR is not successful in the study of phosphates and phosphonates coordinated to Mn(2+). Parallel studies of phosph(on)ate binding to the Mn(2+) of FosA and to aqueous Mn(2+) ion disclose how the enzyme modifies the coordination of these molecules to the active site Mn(2+). Through analysis of (31)P hyperfine parameters derived from simulations of the ENDOR spectra we have determined the binding modes of the phosph(on)ates in each sample and discerned details of the geometric and electronic structure of the metal center. The (31)P ENDOR studies of the protein samples agree with, or improve on, the Mn-P distances determined from crystal structures and provide Mn-phosph(on)ate bonding information not available from these studies. Electron spin echo electron paramagnetic resonance (ESE-EPR) spectra have also been recorded. Simulation of these spectra yield the axial and rhombic components of the Mn(2+) (S = (5)/(2)) zero-field splitting (zfs) tensor. Comparison of structural inferences based on these zfs parameters both with the known enzyme structures and the (31)P ENDOR results establishes that the time-honored procedure of analyzing Mn(2+) zfs parameters to describe the coordination environment of the metal ion is not valid or productive.     

Pulsed electron nuclear double resonance studies of the photoexcited triplet state of pentacene in p-terphenyl crystals at room temperature

Pulsed electron nuclear double resonance (ENDOR) using a modified Davies-type [Phys. Lett. 47A, 1 (1974)] sequence is employed to study the hyperfine (HF) structure of the photoexcited triplet state of pentacene dispersed in protonated and deuterated p-terphenyl single crystals. The strong electron spin polarization and long phase memory time of triplet pentacene enable us to perform the ENDOR measurements on the S=1 spin system at room temperature. Proton HF tensor elements and spin density values of triplet pentacene are extracted from a detailed angular-dependent study in which the orientation of the magnetic field is varied systematically in two different pentacene planes. Analysis reveals that the pentacene molecule is no longer planar in the p-terphenyl host lattice. The distortion is more pronounced in the deuterated crystal where the unit cell dimensions are slightly smaller than those of the protonated crystal.     

Pulsed Electron-Nuclear Double Resonance in the Fourier Regime

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level molecular structural information. However, in molecules containing unpaired electron spins, NMR signals are difficult to measure directly. In such cases, data is obtained using the electron-nuclear double resonance (ENDOR) method, where nuclei are detected through their interaction with nearby unpaired electron spins. Unfortunately, electron spins spread the ENDOR signals, which challenges current acquisition techniques, often resulting in low spectral resolution that provides limited structural details. Here, we show that by using miniature microwave resonators to detect a small number of electron spins, integrated with miniature NMR coils, one can excite and detect a wide bandwidth of ENDOR data in a single pulse. This facilitates the measurement of ENDOR spectra with narrow lines spread over a large frequency range at much better spectral resolution than conventional approaches, which helps reveal details of the paramagnetic molecules’ chemical structure that were not accessible before.     

Effect of nitroxide radicals on chemically induced dynamic electron polarization of spin-correlated radical pairs in aqueous micellar solutions of sodium dodecyl sulfate

The multispin systems consisting of spin-correlated radical pairs (SCRPs) and stable nitroxide radicals, localized in micelles of sodium dodecyl sulfate (SDS), were studied by ESR and pulse laser photolysis techniques. In all the systems studied, the stable nitroxide radicals exert no effect on the shape of the ESR spectra of the SCRPs (in particular, on the shape of their antiphase structure) and on the decay kinetics of the ESR signal of the SCRPs. In the SDS micelles, the electron spin polarization transfer from the nonequilibrium electron spin states of the molecular triplets (SCRP precursors) is the most efficient mechanism of generation of the electron spin polarization in nitroxide radicals. The experimental data also show that the nitroxide radicals and SCRP radicals are most probably distributed uniformly in the micellar phase. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1390–1401, July, 2008.     

Cross‐Polarization Electron‐Nuclear Double Resonance Spectroscopy

Magnetic nuclei in the proximity of a paramagnetic center can be polarized through electron‐nuclear cross‐polarization and detected in electron‐nuclear double resonance (ENDOR) spectroscopy. This principle is demonstrated in a single‐crystal model sample as well as on a protein, the β2 subunit of E.coli ribonucleotide reductase (RNR), which contains an essential tyrosyl radical. ENDOR is a fundamental technique to detect magnetic nuclei coupled to paramagnetic centers. It is widely employed in biological and materials sciences. Despite its utility, its sensitivity in real samples is about one to two orders of magnitude lower than conventional electron paramagnetic resonance, thus restricting its application potential. Herein, we report the performance of a recently introduced concept to polarize nuclear spins and detect their ENDOR spectrum, which is based on electron‐nuclear cross polarization (eNCP). A single‐crystal study permits us to disentangle eNCP conditions and CP‐ENDOR intensities, providing the experimental foundation in agreement with the theoretical prediction. The CP‐ENDOR performance on a real protein sample is best demonstrated with the spectra of the essential tyrosyl radical in the β2 subunit of E.coli RNR.     

A triple resonance hyperfine sublevel correlation experiment for assignment of electron-nuclear double resonance lines

A new, triple resonance, pulse electron paramagnetic resonance (EPR) sequence is described. It provides spin links between forbidden electron spin transitions (DeltaM(S)=+/-1, DeltaM(I) not equal 0) and allowed nuclear spin transitions (DeltaM(I) = +/-1), thus, facilitating the assignment of nuclear frequencies to their respective electron spin manifolds and paramagnetic centers. It also yields the relative signs of the hyperfine couplings of the different nuclei. The technique is based on the combination of electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR)-detected NMR experiments in a way similar to the TRIPLE experiment. The feasibility and the information content of the method are demonstrated first on a single crystal of Cu-doped L-histidine and then on a frozen solution of a Cu-histidine complex.     

1H pulsed ENDOR and ESEEM evidence that the bis-imidazole complexes of iron(III) tetraphenylchlorin and tetraphenylporphyrin have the same order of g values, and the same electronic ground state.

The electronic structures of the bis-imidazole complexes of iron(III) tetraphenylporphyrin ([(TPP)Fe(ImH)(2)](+)) and iron(III) tetraphenylchlorin ([(TPC)Fe(ImH)(2)](+)) in frozen glassy solutions have been studied by the pulsed electron nuclear double resonance (ENDOR) technique of Mims and by electron spin-echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra have been used to determine the orientation of the imidazole ligand planes with respect to the g tensor axes. In the ENDOR spectra, the manifestations of the implicit TRIPLE effect described and explained earlier by Doan et al. (J. Am. Chem. Soc. 1996, 118, 7014) were seen. In this work, the explicit expressions describing this effect were derived for the first time and used to successfully simulate the proton ENDOR spectra at the low- (LF) and high-field (HF) edges of the EPR spectrum. Using pulsed ENDOR, we have been able to determine the spin density distributions in the pi-systems of both tetrapyrroles and show that [(TPC)Fe(ImH)(2)](+) has the electronic orbital ground state (d(xy)())(2)(d(xz)(),d(yz)())(3), the same as that known for [(TPP)Fe(ImH)(2)](+), and the largest principal g value corresponds to the g tensor axis 3, which is normal to the heme plane. For the TPP complex, the g tensor axis 1, corresponding to the smallest principal g value, was found to be at an angle phi(1) of 30-35 degrees from the N-Fe-N axis, with the ligand planes rotated by the angle of 20-25 degrees in the opposite direction. For the TPC complex, phi(1) was found to be about 25 degrees from the direction N(I)-Fe-N(III), where N(I) corresponds to the nitrogen of the saturated pyrrole ring. The ligand planes in this complex were found to be oriented at an angle of about 10 degrees in the opposite direction.     

Continuous-wave and pulse EPR study of the copper(II) complex of N-confused tetraphenylporphyrin: direct observation of a sigma metal-carbon bond

N-confused or inverted porphyrins, a family of porphyrin isomers that contain a confused pyrrole ring connected through its alpha and beta' positions in the macrocycle, exhibit unique physical and chemical properties, like, for instance, the ability to stabilize unusual oxidation states of metals due to the reactivity of the inverted pyrrole. In this Article, a combined multifrequency continuous-wave and pulse electron paramagnetic resonance (EPR) study of the copper(II) complex of N-confused tetraphenylporphyrin (TPP) is presented. By use of pulse EPR methods like ENDOR and HYSCORE, the magnetic interactions between the unpaired electron of the compound and the surrounding nitrogen nuclei were revealed. Through 13C labeling of the macrocycle, a detailed study of the carbon hyperfine interaction became possible and provided further insight into the character of the metal-carbon bond. The observed hyperfine couplings of the ligand atoms in the first coordination sphere showed the presence of a remarkably strong sigma Cu-C bond and allowed for a detailed analysis of the spin delocalization over the porphyrin macrocycle. Interestingly, it was found that the observed delocalization is approximately 11% larger than the corresponding one for CuTPP.     

A CIDEP study of photolysis benzophenone radicals in the microheterogeneous phase systems

The chemically induced dynamic electron polarization (CIDEP) of photolyzed benzophenone radicals in the microheterogeneous phase systems has been made by using the time-resolved electron spin resonance (TRESR) spectrometer. In the AEO_9/isoproanol/H_2O/C_8H_(18) liquid crystal system, the TRESR spectrum of Ph_2COH shows an unusual CIDEP of spin correlation radical pair (SCRP). The SCRP formation in these systems resulting from the character microenvironment and the related physical-chemistry property are discussed.     

Enthalpy/entropy driven activation of the first interquinone electron transfer in bacterial photosynthetic reaction centers embedded in vesicles of physiologically important phospholipids

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P(+)Q(A)(-)Q(B)-->P(+)Q(A)Q(B)(-) interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation ( approximately 500 meV) did not show large variation in vesicles of different lipids.