High-field EPR, ENDOR and ELDOR on bacterial photosynthetic reaction centers

We report on recent 95 and 360 GHz high-field electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and pulsed electron-electron double resonance (PELDOR) studies of wild-type and mutant reaction centers (RCs) from the photosynthetic bacteriumRhodobacter sphaeroides. Taking advantage of the excellent spectral and temporal resolution of EPR at 95 and 360 GHz, the electron-transfer (ET) cofactors radical ions and spin-correlated radical pairs were characterized by theirg- and hyperfine-tensor components, their anisotropicT ₂ relaxation as well as by the dipolar interaction between P ₈₆₅ ^?+ Q _A ^?? radical pairs. The goal of these studies is to better understand the dominant factors determining the specificity and directionality of transmembrane ET processes in photosynthetic RC proteins. In particular, our multifrequency experiments elucidate the subtle cofactor-protein interactions, which are essential for fine-tuning the ET characteristics, e.g., the unidirectionality of the light-induced ET pathways along the A branch of the RC protein. By our high-field techniques, frozen-solution RCs of novel site-specific single and double mutants ofR. sphaeroides were studied to modulate the ET characteristics, e.g., even to the extent that dominant B branch ET prevails. The presented multifrequency EPR work culminates in first 360 GHz ENDOR results from organic nitroxide radicals as well as in first 95 GHz high-field PELDOR results from orientationally selected spin-polarized radical pairs P ₈₆₅ ^?+ Q _A ^?? , which allow to determine the full geometrical structure of the pairs even in frozen-solution RCs.     

Transient and pulsed EPR spectroscopy on the radical pair state P 865 +. Q A −. to study light-induced changes in bacterial reaction centers

The radical pair state P ₈₆₅ ^+. Q _A ^?. (P₈₆₅: primary donor, Q_A: quinone acceptor) in Zn-substituted bacterial reaction centers is investigated using transient and pulsed EPR spectroscopy. For photoexcited samples not frozen in the dark but under continuous illumination, a prolonged lifetime of this radical pair state is observed in agreement with previous studies using time resolved optical spectroscopy. The transient EPR spectra revealed neither a different orientation of the quinone acceptor anion nor a change of itsg-anisotropy in the sample frozen in the charge separated state as compared with that frozen in the dark. The latter finding indicates a similar hydrogen bonding situation for Q _A ^?. in both samples. Changes observed in the transient EPR spectra are interpreted as result of contributions from spin-polarized Q _A ^?. which was generated in part of the sample while freezing under illumination. From the out-of-phase echo modulation pattern observed in the pulsed EPR measurements, it follows that the distance between P ₈₆₅ ^+. and Q _A ^?. is the same in dark frozen samples and in those frozen under continuous illuminaton. This is in contrast to the model suggested by Kleinfeld D., Okamura M.Y., Feher G.: Biochemistry23, 5780 (1984), in which an increased distance and a larger distribution of distances was suggested for samples frozen under illumination. The prolonged lifetime of the radical pair P ₈₆₅ ^+. Q _A ^?. is discussed in terms of differences in the relaxation behaviour of the protein.     

Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe²⁺-depleted transfer step from intermediate radical pair, P ₈₇₀ ⁺ Q _a ^? which is produced in an electron transfer step from intermediate radical pair, P ₈₇₀ ⁺ I^?. (P ₈₇₀ ⁺ is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I^? is the reduced bacteriopheophytin acceptor, and Q _a ^? is the reduced primary quinone acceptor.) The lifetime of P ₈₇₀ ⁺ I^? can be shortened relative to the lifetime of P ₈₇₀ ⁺ I^? in Fe²⁺-depleted RCs by substitution of Zn²⁺. We report the first observation of X-band and Q-band ESP EPR signals due to P ₈₇₀ ⁺ Q^? from bacterial reaction centers that contain Zn²⁺. Comparison of these signals to those observed from Fe²⁺-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model.     

The primary and secondary acceptors in bacterial photosynthesis III. Characterization of the quinone radicals Q A − ⋅ and Q B − ⋅ by EPR and ENDOR

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques were used to investigate the electronic structure of the primary (Q _A ^?? ) and secondary (Q _B ^?? ) ubiquinone electron acceptors in reaction centers (RCs) of the photosynthetic bacteriumRhodobacter sphaeroides. To reduce the EPR linewidth, the high-spin Fe²⁺ present in native RCs was replaced by diamagnetic Zn²⁺. Experiments were performed both on frozen solutions and single crystals at microwave frequencies of 9, 35 and 94 GHz. Differences in the EPR/ENDOR spectra were observed for Q _A ^?? and Q _B ^?? , which are attributed to different environments of the quinones in the RC. The differences exhibited themselves in: (i) the g-tensors, (ii) the¹⁷O and¹³C hyperfine coupling (hfc) constants of the quinones labeled at the carbonyl group, (iii) the¹H-hfcs of the quinone ring and (iv) the exchangeable protons hydrogen bonded to the carbonyl oxygens. From these results and from H/D exchange experiments, the following conclusions were drawn: both Q _A ^?? and Q _B ^?? have at least two hydrogen bonds of different strengths to the carbonyl oxygens. The hydrogen bonds for Q _A ^?? are stronger and more asymmetric than for Q _B ^?? . For Q _A ^?? the stronger bond (to O₄) was assigned to His(M219) and the weaker (to O₁) to Ala(M260). For Q _B ^?? the stronger bond (to O₄) was assigned to His(L190), with several weaker bonds (to O₁) to Ser(L223), Ile(224) and Gly(L225). From the temperature dependence of the hfcs of the exchangeable protons some dynamic properties of the RC were deduced. Hfcs with more distant nitrogens were observed by electron spin echo envelope modulation (ESEEM). For Q _A ^?? they were assigned to N_δ of His(M219) and to the peptide backbone nitrogen of Ala(M260) and for Q _B ^?? to N_δ of His(L190). These interactions indicate the extent of the electron wave function, which is important for the understanding of the electron transfer mechanism. Based on the magnetic resonance results, the function of the quinone acceptors in the reaction center is discussed.     

Magnetic-field-induced orientation of photosynthetic reaction centers as revealed by time-resolved W-band EPR of spin-correlated radical paris: Development of a molecular model

Spin-correlated radical pairs are the short-lived intermediates of the primary energy conversion steps of photosynthesis. In this paper, we develop a comprehensive model for the spin-polarized electron paramagnetic resonance (EPR) spectra of these systems. Particular emphasis is given to a proper treatment of the alignment of the photosynthetic bacteria by the field of the EPR spectormeter. The model is employed to analyze time-resolved W-band (94 GHz) EPR spectra of the secondary radical pair P ₇₀₀ ⁺ A ₁ ^? in photosystem I formed by photoexcitation of the deuterated and¹⁵N-substituted cyanobacteriumSynechococcus lividus. Computer simulations of the angular-dependent EPR spectra of P^700/+A_1/? provide values for the order parameter of the cyanobacterial cells and for the orientation of the membrane normal in a molecular reference system. The order parameter from EPR compares favorably with corresponding data from electron microscopy obtained for theS. lividus cells under similar experimental conditions. It is shown that high-field EPR of a magnetically aligned sample in combination with the study of quantum beat oscillations represents a powerful structural tool for the short-lived radical pair intermediates of photosynthesis.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A_0A (M688N_PsaA) and A_0B (M668N_PsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N_PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N_PsaA mutant. In particular, the P ₇₀₀ ^·+ decay kinetics in the M688N_PsaA mutant is significantly slower than in the WT or the M668N_PsaB mutant. The analysis of the out-of-phase electron–electron dipolar electron spin echo envelope modulation shows that in the M668N_PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair measured in the X-ray crystal structure. In the M688N_PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7–0.3, which agree well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair and the 24.6 Å distance for the P ₇₀₀ ^·+ A _1B ^·? radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron–sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors.     

Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

The secondary acceptor A₁ of the electron transport chain(s) of photosystem (PS) I is a phylloquinone (vitamin K₁, VK₁). Pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments at X-band frequencies were performed on the photoaccumulated acceptor radical A ₁ ^·? and the radical pair state P ₇₀₀ ^·+ A ₁ ^·? in PS I ofThermosynechococcus elongatus. The data obtained were compared with data from the respective radical anion of VK₁ in organic solvents. The unusualg tensor magnitude of A ₁ ^·? is explained by the hydrophobic binding pocket of this radical. The hyperfine couplings and the spin (and charge) density distribution is very different for A ₁ ^.? in PS I and VK ₁ ^·? in frozen alcoholic solution. This is attributed to a rather strong one-sided hydrogen bond to A ₁ ^·? . The presence of a hydrogen bond to A ₁ ^·? has only a minor effect ong. The hyperfine coupling constants of A ₁ ^·? determined from the radical pair spectra deviate only slightly from those derived from photoaccumulated A ₁ ^·? in PS I treated with dithionite at high pH. ENDOR resonances of the proton in a H bond were detected and an estimate of the strength and geometry of this bond to A ₁ ^·? was obtained. The significance of the hydrogen bond and other (hydrophobic) interactions of A₁ with the surrounding are briefly discussed.     

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P⁺A ₁ ^? and P⁺F _x ^? in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ₂ can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P⁺A ₁ ^? and P⁺F _x ^? in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P⁺F _x ^? in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A₀ to F_x with a time constant ofτ = 600 ps.     

Photo-Induced Electron Spin Polarization in Chemical and Biological Reactions: Probing Structure and Dynamics of Transient Intermediates by Multifrequency EPR Spectroscopy

In this minireview, modern multifrequency electron paramagnetic resonance (EPR) spectroscopy, in particular, at high magnetic fields, is shown to provide detailed information about structure, motional dynamics and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin?Cquinone donor?Cacceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $ {\text{P}}_{865}^{ \cdot + } $ $ {\text{Q}}_{\text{A}}^{ \cdot - } $ in frozen solution reaction centers from the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P₈₆₅Q_A. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the Q_A binding site does not change under photoreduction of the quinone??in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of ??classical?? spectroscopic and diffraction techniques for determining structure?Cdynamics?Cfunction relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.     

High-frequency EPR studies on cofactor radicals in photosystem I

Electron paramagnetic resonance (EPR) spectroscopy at W-band (94 GHz) is used to resolve theg-tensors of the radical ions of the primary chlorophyll donor P ₇₀₀ ^+? and the quinone acceptor A ₁ ^?? in photosystem I. The obtainedg-tensor principal values are compared with those of the isolated pigment radicals in organic solvents and the shifts are related to an impact of the protein environment. P ₇₀₀ ^+? has been investigated in photosystem I single crystals at 94 GHz. W-band EPR applied to the photoinduced radical pair P ₇₀₀ ^+? A ₁ ^?? is used to correctly assign theg-tensor axes of P ₇₀₀ ^+? to the molecular structure of the primary donor. Density functional theory calculations on a model of A ₁ ^?? in its binding pocket derived from the recent crystal structure of photosystem I were utilized to correlate experimental magnetic resonance parameters with structural elements of the protein.     

Theoretical investigation of Q A −· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

Density functional theory was used to calculate magnetic resonance parameters for the primary stable electron acceptor anion radical (Q _A ^?· ) in its binding site in the bacterial reaction center (bRC) ofRhodobacter sphaeroides. The models used for the calculations of the Q _A ^?· binding pocket included all short-range interactions of the ubiquinone with the protein surroundings in a gradual manner and thus allowed a decomposition and detailed analysis of the different specific interactions. Comparison of the obtained hyperfine and quadrupole couplings with experimental data demonstrates the feasibility and reliability of calculations on such complex biologically relevant systems. With these results, the interpretation of previously published 3-pulse electron spin echo envelope modulation data could be extended and an assignment of the observed double quantum peak to a specific amino acid is proposed. The computations provide evidence for a slightly altered binding site geometry for the Q_A ground state as investigated by X-ray crystallography with respect to the Q _A ^t-· anion radical state as accessible via EPR spectroscopy. This new geometry leads to improved fits of the W-band correlated-coupled radical pair spectra of Q _A ^?· -P ₈₆₅ ^+· compared to orientation data from the crystal structure. Finally, a correlation of the¹⁴N quadrupole parameters of His219 with the hydrogen bond geometry and a comparison with previous systematic studies on the influence of hydrogen bond geometry on quadrupole coupling parameters (J. Fritscher: Phys. Chem. Chem. Phys. 6, 4950–4956, 2004) is presented.     

ESEEM study of the location of spin-polarized chlorophyll-quinone radical pair in membrane-oriented spinach photosystems I and II complexes

Light-induced spin-polarized radical pairs, P700⁺A₁ ^? in spinach photosystem (PS) I particles and P680⁺Q_A ^? in Zn-substituted PS II core complexes, in oriented membranes were studied by pulsed electron paramagnetic resonance (EPR). Based on the determined distance of 25.2 ± 0.2 Å between P700 and A₁, the angular dependence of the spin-polarized electron spin echo envelope modulation (ESEEM) spectra on the magnetic field suggests that the angle between $R_{P700 - {\rm A}_1 } $ , the radius-vector connecting P700 and A₁, and the membrane normaln was 24 ± 4° in PS I particles. Obtained angle and distance of P700-A₁ axis suggested Q_K side in the molecular geometry of cofactors presented in a recent X-ray crystallography of cyanobacterial PS I reaction center to be an active branch of electron transfer. The distance between P680 and Q_A was determined to be 27.4 ± 0.3 Å for a nonoriented PS II. The angle between $R_{P680 - Q_{\rm A} } $ , the radius-vector connecting P680 and Q_A, andn was determined to be 21 ± 5°. The angle of P680-Q_A axis was close to that of 20° of P870-Q_A axis reported in X-ray analysis of the purple bacterial reaction center crystal.     

Structural analysis of three-spin systems of photosystem II by PELDOR

The pulsed electron electron double resonance (PELDOR) pulse sequence is applied to a three-spin system consisting of three radicals (Y _D ^· , Y _Z ^· and Q _A ^? ) generated in spinach PS II. The distance between Y_Z and Q_A has been determined to be 3.4 nm with the previously derived distances of the other radical pairs, 2.9 nm for Y _D ^· -Y _Z ^· and 3.9 nm for Y _D ^· -Q _A ^? . This distance has been derived from the Y _Z ^· -Q _A ^? radical pair trapped in Y_D-less mutants ofChlamydomonas reinhardtii. Furthermore the method was applied to the Y _D ^· -Q _A ^? -Chl _Z ⁺ system to find the unknown distance between Q_A and Chl_Z. The derived distance was 3.4 nm. A triangular configuration was found in the membrane system that gives the relative positions of the electron transfer components.     

Photosynthetic electron transport in the cyanobacteriumSynechocystis sp. PCC 6803: High-Field W-band and X-band EPR study of electron flow through photosystem I

In this work, by using the respective advantages of W- and X-band electron paramagnetic resonance (EPR) spectroscopy techniques to investigate electron transport processes, we have studied the light-induced redox transients of the primary electron donor P₇₀₀ and the secondary acceptor A₁ in photosystem (PS) I complexes of intact cyanobacterial cellsSynechocystis sp. PCC 6803. We found that the kinetic behavior of the cation radical P₇₀₀ ^·+ generated by illumination with continuous light, and the EPR intensity of the radical pair P₇₀₀ ^·+A ₁ ^·? generated upon laser pulse illumination strongly depend on the illumination prehistory (either the sample was frozen in the dark or during illumination). Both these processes were sensitive to the presence of electron transport inhibitors which block electron flow between the two photosystems. In line with our X-band EPR data on the kinetics of light-induced redox transients of P₇₀₀, our high-field W-band EPR study of the radical-pair state P₇₀₀ ^·+A ₁ ^·? shows that photosynthetic electron flow through the PS I reaction center is controlled both on the donor and on the acceptor side of PS I.     

Simultaneous electrochemical and electron paramagnetic resonance studies of fullerene anion radicals C 60 1− and C 60 3−

Simultaneous electrochemical and electron paramagnetic resonance experiments have been carried out on the reduced C₆₀ fullerene to examine theg-factor assignment of the radical species. C ₆₀ ^1? and C ₆₀ ^3? show the following EPR characteristics at room temperature: C ₆₀ ^1? :g _1?=2.0002±0.0001, 2ΔB _1s=0.17 mT, and C ₆₀ ^3? :g _{3?=2.0008±0.0002}, 2ΔB _1s=0.07 mT. EPR linewidths are apparently narrower compared to those in most of the spectra previously reported. Variable temperature EPR study on solution containing C ₆₀ ^1? has shown thatg _1? value is not while the linewidth is only slightly temperature dependent.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: An Investigation of Electron Transfer at Different Temperatures by Multifrequency Time-Resolved and CW EPR

The ambient temperature and low-temperature electron transfer properties of Photosystem I (PS I) from the M688N_PsaA and M668N_PsaB mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 are studied using transient electron paramagnetic resonance (EPR) and continuous-wave (CW) EPR. The two mutations are expected to alter the midpoint potentials of, and the reorganization energies around, the primary electron chlorophyll acceptors A_0A and A_0B, which should lead to a change in the yield and/or rate of electron transfer to the phylloquinone acceptors A_1A and A_1B, respectively. At ambient temperature it is known that both quinone acceptors are active in electron transfer. At low temperature there are at least two fractions that undergo either reversible or irreversible electron transfer. The EPR data of the two PS I variants are used to investigate the relationship between these low-temperature fractions and the ambient temperature electron transfer pathway. The results show that mutation in the PsaA-branch increases the rate of $ {\text{A}}_{{1{\text{A}}}}^{. - } $ to F_X electron transfer at ambient temperature, while the corresponding mutation in the PsaB-branch has no effect on the electron transfer rate observable by transient EPR. An analysis of the complete time/field datasets from both variants suggests that the yield of electron transfer in the branch carrying the mutation is reduced. The mutations have no effect on the low-temperature CW EPR spectra of the iron–sulfur clusters if the samples are frozen under illumination but they both cause a decrease in the yield of reduced F_A and F_B if the samples are frozen in the dark and then illuminated. The PsaA-branch mutation greatly reduces the intensity and changes the polarization pattern of the radical pair $ {\text{P}}_{700}^{ + } {\text{A}}_{1}^{. - } $ . Possible causes of the changes in the polarization pattern are discussed and it is suggested that the mutations introduce structural heterogeneity in the vicinity of the A₀ binding site. No clear correlation between the yield of electron transfer in a particular branch and the yield of stable charge separation is found.     

Interpretation of multifrequency transient EPR spectra of the P 700 + A0Q K − state in photosystem I complexes with a sequential correlated radical pair model: Wild type versus A0 mutants

Transient electron paramagnetic resonance (TR-EPR) spectra of the electron-hole pair state P ₇₀₀ ⁺ A₀Q _K ^? in photosystem I are numerically calculated. Parameter variation concerns mainly the exchange integralJ of the precursor spin pair state P ₇₀₀ ⁺ A ₀ ^? Q_K and its lifetime τ. A prominent emissive feature in the high-field region (P ₇₀₀ ⁺ part) of the EPR spectrum turns out to be diminished with increasing lifetime τ of the precursor pair state in the case of positive exchange couplingJ>0 (ferromagnetic type). Correspondingly, the emissive feature becomes more pronounced with increasing lifetime τ in the case of negative exchange couplingJ<0 (antiferromagnetic type). These results can be used to interpret the changes in the pattern observed in TR-EPR spectra comparing wild-type and specific A₀ mutants. The central ligating amino acid residue to the A₀ chlorophyll cofactor is mutated from native methionine (M) to leucine (L) in either the PsaA or the PsaB branch. Changes are observed only for the A-side mutant: M688L(PsaA). They are consistent with the following parameters in the precursor pair P ₇₀₀ ⁺ A ₀ ^? :J≈0.5÷1.0 mT and τ=1.5÷2 ns (as compared to τ～0.05 ns in the wild type).     

Light-induced EPR spectra of reaction centers fromRhodobacter sphaeroides at 80 K: Evidence for reduction of QB by B branch electron transfer in native reaction centers

Photosynthetic reaction centers (RCs) fromRhodobacter sphaeroides capture solar energy by electron transfer from primary donor D to quinone acceptor Q_B through the active A branch of electron acceptors. The light-induced electron paramagnetic resonance (EPR) spectrum from native RCs that had Fe²⁺ replaced by Zn²⁺ was investigated at cryogenic temperature (80 K, 35 GHz). In addition to the light-induced signal due to the formation of D^+.Q _A ^?. observed previously, a small fraction (ca. 5%) of the signal displayed very different characteristics. The signal was absent in RCs in which the Q_B was displaced by the inhibitor stigmatellin. Its decay time (τ=6 s) was the same as observed for D^+.Q _B ^?. in mutant RCs lacking Q_A, which is significantly slower than for D^+.Q _A ^?. (τ=30 ms). Its EPR spectrum was identical to that of D^+.Q _B ^?. . The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking Q_A (?=0.2%) and was temperature independent. These results are explained by direct photochemical reduction of Q_B via B branch electron transfer in a small fraction of native RCs.     

EPR studies on the di-nitrogen centers with nonequivalent atoms in a reddish-brown plastically deformed diamond

Two new di-nitrogen centers, which were labeled M2 and M3, were found together with known W7 and N4 centers in an unusual reddish-brown natural diamond. The following magnetic hyperfine interaction parameters (expressed in MHz) were determined for the two nitrogen atoms:A ₁ ⁽¹⁾ =117.95(5),A ₁ ⁽²⁾ =A ₁ ⁽³⁾ =84.48(5),A ₂ ⁽¹⁾ =7.1(1),A ₂ ⁽²⁾ =A ₂ ⁽³⁾ =6.6(1) for M2 andA ₁ ⁽¹⁾ =121.55(5),A ₁ ⁽²⁾ =A ₁ ⁽³⁾ =85.90(5),A ₂ ⁽¹⁾ =6.0(1),A ₂ ⁽²⁾ =5.4(1),A ₂ ⁽³⁾ =5.1(1) for M3. Hyperfine interaction tensors for the nitrogen atom N₁ with a larger interaction have axial symmetry about the <111> direction, but those for the other nitrogen atom, N₂, appear to be small, almost isotropic. Probable models of the M2 and M3 centers are suggested and discussed.