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The g-tensor of the neutral radical form of the flavin adenine dinucleotide cofactor FADH• of (6–4) photolyase from Xenopus laevis has been determined by very high-magnetic-field/high-microwave-frequency electron-paramagnetic resonance (EPR) performed at 360 GHz/12.8 T. Due to the high spectral resolution the anisotropy of the g-tensor could be fully resolved in the frozen-solution continuous-wave EPR spectrum. By least square fittings of spectral simulations to experimental data, the principal values of the g-tensor have been established: gX = 2.00433(5), gY = 2.00368(5), gZ = 2.00218(7). A comparison of very high-field EPR data and proton and deuteron electron-nuclear double resonance measurements yielded precise information concerning the orientation of the g-tensor with respect to the molecular frame. This data allowed a comparison to be made between the principal values of the g-tensors of the FADH• cofactors of photolyases involved in the repair of two different DNA lesions: the cyclobutane pyrim...

Hotly debated issues in the area of biological electron transfer (ET) involve questions of how long-range ET processes proceed along pathways that incorporate non-covalently linked cofactors interacting with their protein microenvironment. To contribute answers to such questions, time-resolved EPR (TREPR) experiments at various microwave frequency/magnetic field settings have been performed. In this overview, representative examples of this work from our laboratory are given which comprise transient intermediates of light-induced ET in (i) bacterial photosynthetic reaction centers and (ii) biomimetic model complexes. In these complexes the donor and acceptor components are tethered together either covalently via cyclohexenyl bridges or non-covalently via Watson-Crick base-pairing. The studies are aiming at broadening our knowledge of structure-dynamics-function relationships associated with ET processes. It is demonstrated that high-field TREPR opens new perspectives in elucidating complex photochemical E...

Semiempirical molecular orbital methods (PM3, INDO, ZINDO/S) have been used to calculate the effects of local electric fields and of hydrogen bonding on the g and hyperfine tensors of a nitroxide spin label model system. The results yield a linear correlation between the two principal tensor components g xx and A N zz at label sites of varying polarity. Hydrogen bonding with a single water molecule produces a constant shift of Δg xx ≅ −4 × 10−4. These theoretical results are used to interpret recent high field (3.4 T, 95 GHz) electron paramagnetic resonance investigations on site-directed spin labelled bacteriorhodopsin. This protein reveals a close correlation between proticity and polarity at the various label sites. The slope of the g xx versus A N zz dependence is affected strongly by polarity induced structural strains of the spin label.

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or...

The photoexcited triplet state of free-base porphycene randomly oriented in a glassy toluene matrix has been investigated by time-resolved EPR over the temperature range 4-200 K. The spectra show a remarkable temperature dependence. It is concluded that two triplet states are present in the EPR spectra: only one is present at 4 K, and the other is populated at higher temperatures. The energy difference between the two states is estimated to be 0.4 kJ mol-1. The behaviour is explained in terms of fast intramolecular NH tautomerism present in the porphycene macrocycle. As the temperature is raised the exchange between the two states increases and one effective state is observed. In addition, at even higher temperatures where the solvent is close to the liquid-glass transition, the triplet spectrum is affected by anisotropic relaxation caused by rotation of the porphycene molecule in the molecular plane.

We have used time-resolved electron-paramagnetic resonance (EPR) spectroscopy to investigate the triplet energy transfer in various covalently linked porphyrin dimers. For all these dimers the fine structure parameters are rather similar to the monomer values; thus according to the field-swept EPR spectra the triplet state appears to be localized on one monomeric subunit. However, for particular conformations, the time evolution of the EPR spectra reveals dynamic properties specific to a dimer. For these cases the hopping rates of the triplet exciton can be determined from the measured spin-lattice relaxation times. The values obtained for the different dimers can be related to structural parameters of the dimers and to the overlap of their electronic wavefunctions. Such overlap is essential for energy transfer via the exchange mechanism.

The interpretation of EPR and ENDOR measurements on an organic free radical which appears to be a universal concomitant of senescence in plants is discussed. On the basis of EPR spectra obtained at 95 GHz it is speculated that the radical is derived from a quinone.

Deuterium quadrupole coupling constants have been determined by distant ENDOR spectroscopy for a series of carboxylic acids in which deuterium is substituted at the H-bond position. The measured coupling constants cover a range of 40 to 175 kHz corresponding to a variation of the bridge (O-H … O) length of 2·45 to 2·70 A. Features of the electronic and geometrical structure which principally determine e 2 qQ and η of deuterium in these H-bond situations have been assessed. To aid this assessment a simple model is proposed on which theoretical calculations of the INDO MO type are based. Good agreement between experiment and theory is obtained. It is concluded that for calculation of the electric-field gradient tensor two and three-centre integrals in the vicinity of the H-bond cannot be neglected, and further, in the cases of short and symmetrical H-bonds, e 2 qQ can change sign. Our studies also indicate that in non-ionic systems the asymmetry parameter η is primarily determined by the molecule itself, ra...

Absolute g-tensor calculations for planar hydrocarbon and for non-planar phenyl substituted hydrocarbon radicals are reported. The relevant interactions determining g are discussed. Calculations are performed on the basis of a second-order perturbation expansion. The electronic wavefunctions are obtained from a simplified version of Hoffmann's extended Huckel model (SEH), where all valence electrons are taken into account explicitly. For planar systems the observed linear dependence of g on the energy of the half filled π orbital is well reproduced. A qualitative analysis of this dependence, making restrictive assumptions about the σ electrons, was given earlier by Stone. The calculations for non-planar model systems reproduce the g-factor anomalies which are observed for highly twisted phenyl substituted hydrocarbon radicals. The results show the necessity of direct π-σ mixing and are consistent with recent investigations of the proton hyperfine couplings in such systems.

The possibility of applying ENDOR in studies of hyperfine interactions in stable biradicals in liquid solution has been explored. The ENDOR spectra of two phenoxy biradicals are reported. Attempts to record ENDOR of some nitroxide and verdazyl biradicals were unsuccessful. The general structure of biradicals spectra and the relaxation processes which may affect the ENDOR effect are briefly discussed.

The E.S.R. spectrum of the o-dimesitoylbenzene anion-alkali cation radical shows unusually large isotropic alkali hyperfine splitting constants. We report a solution ENDOR study of this radical in which both alkali (85,87Rb) and proton ENDOR spectra were recorded. Both the alkali and proton intensities showed a strong dependence on the metal ion nuclear spin quantum number of the E.S.R. line being saturated. This dependence is attributed to strong flip-flop cross-relaxation induced by modulation of the isotropic alkali hyperfine splitting. The powder E.S.R. spectrum of the complex reveals a small anisotropy of the Rb hyperfine splitting tensor. This indicates a small metal non-s-contribution to the half-filled molecular orbital, which is consistent with the observed relaxation behaviour and the small g shift. The intensity variations in the alkali and proton ENDOR spectra were used to determine the relative signs of all hyperfine splitting constants, and the absolute signs of the hyperfine splitting const...