Radical Pair Spectra Research Articles

Spin-Correlated Radical Pairs as Quantum Sensors of Bidirectional ET Mechanisms in Photosystem I.

Following light-generated electron transfer reactions in photosynthetic reaction center proteins, an entangled spin qubit (radical) pair is created. The exceptional sensitivity of entangled quantum spin states to weak magnetic interactions, structure, and local environments was used to monitor the directionality of electron transfer in Photosystem I (PSI). Electron paramagnetic resonance (EPR) spectra of radical pairs formed via each symmetric branch of cofactors, A or B, exhibit distinctive line shapes. By photochemical reduction and biochemical modification of PSI we created samples where the radical pair(s) from (1) only A branch, (2) only B branch, or (3) both A and B branches are detectable. These PSI samples were used to analyze the asymmetry of electron transfer as a function of temperature, freezing condition, and temperature cycling. The temperature dependency agrees with a dynamic model in which the conformational states of the protein regulate the directionality of electron transfer. High spectral resolution afforded by high-frequency (130 GHz) EPR, combined with extra resolution afforded by deuterated proteins, provides new mechanistic insight via structural and environmental sensitivity of the entangled electron spins of photogenerated radical pairs.

Theoretical Description of Pulsed RYDMR: Refocusing Zero-Quantum and Single Quantum Coherences

Abstract A theoretical description of pulsed reaction yield detected magnetic resonance (RYDMR) is proposed. In RYDMR, magnetic resonance spectra of radical pairs (RPs) are indirectly detected by monitoring their recombination yield. Such a detection method is significantly more sensitive than conventional electron paramagnetic resonance (EPR), but design of appropriate pulse sequences for RYDMR requires additional effort because of a different observable. In this work various schemes for generating spin-echo like signals and detecting them by RYDMR are treated. Specifically, we consider refocusing of zero-quantum coherences (ZQCs) and single-quantum coherences (SQCs) by selective as well as by non-selective pulses and formulate a general analytical approach to pulsed RYDMR, which makes an efficient use of the product operator formalism. We anticipate that these results are of importance for RYDMR studies of elusive paramagnetic particles, notably, in organic semiconductors.

Electron Transfer Pathways in a Tetrathiafulvalene-Aluminum(III) Porphyrin-Free-Base Porphyrin Triad Studied Using Electron Spin Polarization

The electron transfer pathways in two axially bound triads based on aluminum(III) porphyrin (AlPor) are investigated using the electron spin polarization patterns of the final radical pair state. In the triads, TTF-(Ph)n-py-AlPor-Ph-H2Por (n = 0, 1), free-base porphyrin (H2Por) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine on the opposite face of the porphyrin ring. Excitation of the triad at 532 nm leads to absorption by both AlPor and H2Por and to two possible charge separation pathways. In the liquid crystalline solvent 5CB, spin polarized transient electron paramagnetic resonance spectra of a weakly coupled radical pair are observed and assigned to the state \({\text{TTF}}^{ \cdot+ } {\text{H}}_{2} {\text{Por}}^{ \cdot- }\). The radical pair spectra are analyzed using a structural model of the complex to determine the dipolar coupling and relative orientation of the radicals. The inertia tensor calculated from the structural model is used to derive the principal values and axes of the order matrix. It is shown that the observed polarization pattern is only consistent with ferromagnetic exchange coupling in the radical pair. The spectra can be reproduced as the sum of two contributions originating in electron transfer from the excited singlet state of AlPor and from the excited the triplet state of H2Por. The latter is shown to account for the integral emissive net polarization observed in the spectra.

Role of the Hydrogen Bond from Leu722 to the A1A Phylloquinone in Photosystem I

Photosystem I (PS I) contains two molecules of phylloquinone that function as electron transfer cofactors at highly reducing midpoint potentials. It is therefore surprising that each phylloquinone is hydrogen bonded at the C(4) position to the backbone -NH of a Leu residue since this serves to drive the midpoint potential more oxidizing. To better understand the role of the H-bond, a PS I variant was generated in which L722(PsaA) was replaced with a bulky Trp residue. This change was designed to alter the conformation of the A-jk(1) loop and therefore change the strength of the H-bond to the PsaA-branch phylloquinone. Transient EPR studies at 80 K show that the A(1A) site in the PS I variant is fully occupied with phylloquinone, but the absence of methyl hyperfine couplings in the quinone contribution to the P(700)(*+)A(1)(*-) radical pair spectrum indicates that the H-bond has been weakened. In wild-type PS I, reduction of F(A) and F(B) with sodium dithionite causes a approximately 30% increase in the amplitude of the P(700)(*+)A(1)(*-) transient EPR signal due to the added contribution of the PsaB-branch cofactors to low temperature reversible electron transfer between P(700) and A(1A). In contrast, the same treatment to the L722W(PsaA) variant leads to a approximately 75% reduction in the amplitude of the P(700)(*+)A(1)(*-) transient EPR signal. This behavior suggests that A(1A) has undergone double reduction to phyllohydroquinone, thereby preventing electron transfer past A(0A). The remaining 25% of the P(700)(*+)A(1)(*-) radical pair spectrum shows an altered spin polarization pattern and pronounced methyl hyperfine couplings characteristic of asymmetric H-bonding to the phylloquinone. Numerical simulations of the polarization pattern indicate that it arises primarily from electron transfer between P(700) and A(1B). The altered reduction behavior in the L722W(PsaA) variant suggests that the primary purpose of the H-bond is to tie up the C(4) carbonyl group of phylloquinone in a H-bond so as to prevent protonation and hence lower the probability of double reduction during periods of high light intensity.

High-Resolution ESR Study of the H···CH3, H···CHD2, D···CH2D, and D···CD3 Radical Pairs in Solid Argon

High-resolution electron spin resonance (ESR) spectra of radical pairs of a hydrogen atom that coupled with a methyl radical (H...CH3, H...CHD2, D...CH2D, and D...CD3) were observed for X-ray irradiated solid argon containing selectively deuterium-labeled methanes, CH4, CH2D2, and CD4, at 4.2 K. The double-quartet 1H-hyperfine (hf) splittings of ca. 26 and 1.16 mT at the Deltam(s) = +/-1 and Deltam(s) = +/-2 transitions, which are one-half of the isotropic 1H-hf splittings of an isolated H-atom and a CH3 radical, were attributed to the H...CH3 pair. The 1H-hf splittings at the Deltam(s) = +/-1 transition were further split by the fine structure (fs) due to the electron dipole-dipole coupling. Because of the high-resolution spectra, three different sets of the fs splitting, d, are clearly resolved in the spectra of both the H...CH3 and the D...CD3 pairs. The separation distance (inter-spin distance), R, between the H-atom and the CH3 radical being in pairs was evaluated from the d values based on a point-dipole interaction model. For the case of the H...CH3 pair, the observed d values of 4.2, 4.9, and 5.1 mT yield the respective separations, R = 0.87, 0.83, and 0.82 nm, to probe the trapping site of the pair in an Ar crystalline lattice (fcc). For the pair with R = 0.87 nm, for example, we propose that the CH3 radical occupies a substitutional site and the counter H-atom occupies either the interstitial tetrahedral sites directed away from the CH3 radicals by a distance of 0.87 nm or the interstitial octahedral sites by a distance of 0.88 nm. When a mixture of CH4 and CD4 in a solid Ar matrix was irradiated, only two different radical pairs, H...CH3 and D...CD3, were observed. This result clearly demonstrates that the hydrogen atom and methyl radicals, which undergo a pairwise trapping, can originate from the same methane molecule.

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 (QA−·) in its binding site in the bacterial reaction center (bRC) ofRhodobacter sphaeroides. The models used for the calculations of the QA−· 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 QA ground state as investigated by X-ray crystallography with respect to the QAt-· 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 QA−·-P865+· 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.

Electronic Coupling Effects on Photoinduced Electron Transfer in Carotene−Porphyrin−Fullerene Triads Detected by Time-Resolved EPR

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C60 (Bahr et al., 2000) have been followed by time-resolved electron paramagnetic resonance. The electron-transfer process has been characterized in a glass of 2-methyltetrahydrofuran and in the nematic phase of two uniaxial liquid crystals (E-7 and ZLI-1167). In all the different media, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (C*+-P-C60*-), and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum. The weak exchange interaction parameter (J = +1.7 +/- 0.1 G) provides a direct measure of the dominant electronic coupling matrix element V between the C*+-P-C60*- radical pair state and the recombination triplet state 3C-P-C60. Comparison of the estimated values of V for this triad and a structurally related triad differing only in the porphyrin bridge (octaalkylporphyrin vs tetraarylporphyrin) explains in terms of an electronic coupling effect the approximately 6-fold variation of the recombination rate induced by the modification of the porphyrin bridge as derived by kinetic experiments (Bahr et al., 2000).

Photochemistry of Artificial Photosynthetic Reaction Centers in Liquid Crystals Probed by Multifrequency EPR (9.5 and 95 GHz)

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.

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.

Structure Dependence of Electron Spin Polarization in Zn−Porphyrin−Quinone Ensembles Oriented in a Liquid Crystal

Photoinduced intramolecular electron transfer was studied by time-resolved electron paramagnetic resonance spectroscopy in two photosynthetic model systems oriented in a nematic liquid crystal. The donor−acceptor assemblies consisted of differently substituted zinc porphyrins covalently linked to a quinone with different spacers, i.e., triptycyl and cyclohexylene. The orientation of the guest molecules with respect to the director of the liquid crystal was determined by line shape analysis of their triplet spectra and by molecular modeling. It is demonstrated that the polarized pattern of the radical pair spectrum is sensitive mainly to the spacer structure, but to some extent also to the peripheral substituents.

The spectral forms of transient ESR spectra of radical pairs and the origin of the antiphase structure

We show that transient electron spin resonance (ESR) spectra of short lived radical pairs can be written formally as a superposition of Lorentzian-like lines and the corresponding dispersive lines. The coefficients to these spectral components depend strongly on the frequency and amplitude of the microwave (mw) field and consequently a variety of spectral forms may result. The coefficients, and thus the spectral form, are determined by the interradical interaction induced rate of change of the longitudinal and transversal electron spin polarization. The longitudinal polarization is responsible for the well known chemically induced dynamic electron polarization (CIDEP) which changes the intensity of the lines without affecting the line shape. The transversal polarization is shown to be responsible for the antiphase structure (APS) in accordance with our previous proposition [J. Chem. Phys. 109, 8743 (1998)]. A simple one re-encounter model calculation illustrates and explains qualitatively most of the observed APS characteristics. Numerical calculations are used to discuss anomalously large APS splitting.

Composite structure of the stimulated nuclear polarization detected electron paramagnetic resonance spectra of the spin-correlated radical pairs in high magnetic fields

It is shown that the stimulated nuclear polarization detected EPR spectra of spin-correlated radical pairs in high magnetic fields consist of two types of component. There are lines which correspond to hyperfine components of the EPR spectra of radical pairs. In addition, narrow holes are expected. Their physical origin is degeneracy or avoided level crossing of the two energy levels in the rotating frame when the singlet-triplet dynamics in radical pairs exhibit peculiar behaviour. The linewidth of holes can be used to determine the mean lifetime of the spin-correlated radical pairs.

Transient electron paramagnetic resonance spectroscopy on green-sulfur bacteria and heliobacteria at two microwave frequencies

Spin polarized transient EPR spectra taken at X-band (9 GHz) and K-band (24 GHz) of membrane fragments of Chlorobium tepidum and Heliobacillus mobilis are presented along with the spectra of two fractions obtained in the purification of reaction centers (RC) from C. tepidum. The lifetime of P +· is determined by measuring the decay of the EPR signals following relaxation of the initial spin polarization. All samples except one of the RC fractions show evidence of light induced charge separation and formation of chlorophyll triplet states. The lifetime of P +· is found to be biexponential with components of 1.5 ms and 30 ms for C. tepidum and 1.0 and 4.5 ms for Hc. mobilis at 100 K. In both cases, the rates are assigned to recombination from F − X. The spin polarized radical pair spectra for both species are similar and those from Hc. mobilis at room temperature and 100 K are identical. In all cases, an emission/absorption polarization pattern with a net absorption is observed. A slight narrowing of the spectra and a larger absorptive net polarization is found at K-band. No out-of-phase echo modulation is observed. Taken together, the recombination kinetics, the frequency dependence of the spin polarization and the absence of an out-of-phase echo signal lead to the assignment of the spectra to the contribution from P +· to the state P +·F − X. The origin of the net polarization and its frequency dependence are discussed in terms of singlet–triplet mixing in the precursor. It is shown that the field-dependent polarization expected to develop during the 600–700 ps lifetime of P +·A −· 0 is in qualitative agreement with the observed spectra. The identity that the acceptor preceding F X and the conflicting evidence from EPR, optical methods and chemical analyses of the samples are discussed.

Spin coherence effects observed in the OD ESR spectra of radical pairs with strong hyperfine interaction

The optically detected ESR technique is used to record the spectra of a radical pair generated in the photolysis of 2,4,6-trimethylbenzoyl diphenylphosphine oxide in a micellar sodium dodecylsulfate solution. Along with the signals of the radicals of the pair, additional resonances have been detected in the spectrum, whose signal is inverted with respect to the fundamental ones. It is shown that the inverted lines consist of narrow and wide components, which arise from the effects of crossing and anticrossing of pair spin energy levels, respectively.

Chemically Induced Dynamic Electron Polarization Studies of Photochemical Reactions: Acetone in 2-Propanol and Triethylamine

Abstract The results of Chemically Induced Dynamic Electron Polarization (CIDEP) studies of acetone in 2-propanol and triethylamine are reviewed to show how CIDEP studies can reveal interesting details of the initial processes of photochemical reactions. After a brief description of various mechanisms that give rise to CIDEP, the following topics are discussed: a) spectral pattern and the main reacting state, b) CIDEP mechanisms and the temperature and frequency dependences of the spectra, c) direct EPR observation of the radical pair spectra in solution, d) time-developments of the CIDEP signals and spin and reaction dynamics of the initial processes of photochemical reactions, e) quantitative analyses of the magnitudes of CIDEP in terms of a stochastic Liouville equation, and f) some related applications. An overview of the remaining problems and the future outlook is also presented.

The effect of the spin exchange interaction on SNP and RYDMR spectra of geminate radical pairs

The characteristic features of the SNP and RYDMR spectra of radical pairs (RPs) in micelles and biradicals are analysed in a number of models. It is shown that recently found decrease of the SNP and RYDMR line splittings in these systems is, in general, explained by the fast ST dephasing (STD). Within a simple but reasonable model of spin-space dynamics of RPs, it is demonstrated that STD is most probably induced by the exchange interaction. Recently proposed high reactivity mechanism of the above-mentioned decrease is only a particular and rather artificial way to introduce fast STD which, in addition, is less effective than the exchange interaction mechanism.

Theory of multiquantum optically detected ESR spectra of radical pairs. I. General theory. Resonances in parallel radio-frequency field

A general theory of multiquantum optically detected ESR spectra of radical pairs has been developed neglecting the processes of internal spin relaxation. The theory has been used to calculate multiquantum resonances in radiofrequency field which is parallel to the Zeeman field. The position and form of spectral components at arbitrary values of nuclear partner spins have been studied assuming a Poisson life-time-distribution of radical pairs. A physical interpretation of the results obtained has been proposed. The position of the spectral lines of a resolved spectrum for a complex nuclear structure of radical pair partners has been established.

Theory of multiquantum optically detected ESR spectra of radical pairs. II. Resonances in perpendicular radio-frequency field

The previously developed general theory of multiquantum optically detected ESR spectra of radical pairs has been used to analyse multiquantum resonances in the radiofrequency field that is perpendicular to the Zeeman field. The cases of circular polarization (resonance and nonresonance rotation directions) have been considered. The position and form of spectral components at arbitrary values of nuclear spins have been studied assuming the Poisson lifetime distribution of radical pairs. A physical interpretation of the results obtained has been proposed. The resonances of the partners with a complex nuclear structure have been discussed. The nonresonance corrections in the spectra studied have been analysed. A simple method for their estimation has been proposed.

Stable radical pairs under the photolysis of 2,6-di- t-butyl quinone diazide in the single crystals of sterically hindered phenols: New type of double-spin probes

Intermediate radical products, which are formed when monocrystals of sterically hindered phenols containing 10 −3 M 2,6-di- t-butyl quinone diazide are photolysed, have been studied employing EPR. We have identified the formation of carbenes and several types of radical pairs. It is found that, with a rise in temperature, a red-shift is observed for the long-wave boundary involving the formation of radical pairs. The high stability of radical pairs made it possible to measure the thermal coefficients for the linear expansion of the monocrystals. Two types of asymmetric relaxation have been identified in the EPR spectra of radical pairs.

The CIDEP spectrum of a radical pair of the acetone ketyl radical in solution. Effect of deuteration

We report the CIDEP spectrum of a geminate radical pair of the acetone- d 6 ketyl radical in solution. Deuterated acetone shows a radical pair spectrum more clearly than protonated acetone and is found to be suitable for an investigation of the radical pair. It is also found that the radical pair spectrum of acetone- d 6 shows less M 1-dependent line broadenings than protonated acetone.