Yu. A. Grishin

Pulsed Orotron—A new microwave source for submillimeter pulse high-field electron paramagnetic resonance spectroscopy

A vacuum-tube device for the generation of pulsed microwave radiation in the submillimeter range (up to 380 GHz) is presented, designed for use as a source in a 360 GHz high-field/high-frequency electron paramagnetic resonance (EPR) spectrometer—the pulsed Orotron. Analogous to the known continuous wave (cw) version, in the pulsed Orotron microwave radiation is generated by the interaction of a nonrelativistic electron beam with a diffraction grating (stimulated Smith–Purcell radiation) in feedback with an open Fabry–Perot resonator construction. The presented design extends the cw Orotron by a gate electrode and a high-voltage pulsing unit to control the electron beam current. The generated pulses at 360 GHz have pulse lengths from 100 ns–10 μs and a pulse power of (22±5) mW. The output in a broader frequency band between 320 and 380 GHz ranges from 20 up to 60 mW. Within a 10 μs time slot, incoherent pulse trains of arbitrary duration can be generated. The pulsed Orotron has been incorporated in the qua...

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 anisotropicT2 relaxation as well as by the dipolar interaction between P865•+QA•− 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 P865•+QA•−, which allow to determine the full geometrical structure of the pairs even in frozen-solution RCs.

Techniques for EPR spectroscopy of pulsed electron double resonance (PELDOR): A review

Techniques for electron paramagnetic resonance (EPR) spectroscopy of pulsed electron double resonance (PELDOR) are reviewed, and the features of the EPR spectroscopy procedure are considered. Basic theoretical results demonstrating the capabilities of the method in investigating paramagnetic particles are presented. It is shown that distances between paramagnetic particles in chaotically oriented systems—pairs and supramolecular ensembles—can be determined in the range of 1.5–8.0 nm. By analyzing the kinetics of the PELDOR signal decay, it is possible, apart from the distance, to determine the distance distribution function, mutual orientation of particles, and number of paramagnetic particles in formation of aggregates. A general block diagram of the PELDOR spectrometer, as well as cm- and mm-wave band spectrometers available in Russian and foreign laboratories, is described. The PELDOR sensitivity and resolution are discussed, and the range of distances measurable using this method is determined. Methods and procedures for determining the experimental parameters used to interpret the kinetics of PELDOR signal decay and estimate the structural parameters of a system of paramagnetic centers under investigation are considered. The PELDOR method with a magnetic field jump is described. The PELDOR technique and its advantages and drawbacks are analyzed in comparison with other methods for determining distances.

Studies of various mechanisms of nuclear polarisation due to a resonant high-frequency field in radical reactions

Abstract Two different mechanisms of creation of nuclear polarisation in radical reaction products are studied by means of the high-frequency field influence on intermediate radical pairs (stimulated nuclear polarisation, SNP) and on intermediate short-lived radicals (dynamic nuclear polarisation, DNP). Criteria are formulated for distinguishing the contributions of DNP and SNP effects, and experimental demonstration of this possibility is presented for photoinduced reactions of quinones.

CIDNP-detected ESR of radical pairs in the photolysis of quinones

Abstract CIDNP-detected ESR spectra of short-lived radical pairs in the photolysis of quinones ( p -benzoquinone and duroquinone) have been studied. The influence of the saturating rf-field amplitude on the ESR spectra of radical pairs detected by this method has been demonstrated and investigated.

Submicrosecond field-jump device for pulsed high-field ELDOR

A field-jump device for fast stepping the electron paramagnetic resonance magnetic field around 3.4 T during pulsed electron-electron double resonance experiments at W-band (95 GHz) is described. Field jumps up to ±160 G and submicrosecond times for the full field-jump cycle allow precession frequency transfer experiments to be made for the determination of the nanometer distance and the orientation of nitroxide spin-label pairs in disordered samples.

CIDNP-detected ESR of short-lived radical pairs in solutions

Abstract CIDNP-detected ESR spectra of short-lived radical pairs in solution have been obtained for the first time in the example of the photolysis of benzoyl peroxide.

CIDNP detected ESR of short-lived biradicals

Abstract The influence of pumping the electron resonance transitions in intermediate radical particles on CIDNP effects has been observed for the first time. This phenomenon has been employed to take ESR spectra of short-lived biradicals in the photolysis of cyclododecanone in solution.

Stimulated nuclear polarization under induced coherent spin precession

Abstract The induced coherent spin precession of electrons in RPs has been studied experimentally by its manifestation in stimulated nuclear polarisation effects.

An improved TM110 resonator for continuous-wave ENDOR studies at X-band

An improved TM110 resonator for continuous-wave and time-resolved electron-nuclear double resonance (ENDOR) studies at X-band frequencies is described that has been designed with small samples and/or light excitation in mind. The filling factor is increased by reducing the resonator length to only 16 mm. The radio-frequency field is generated by either a conventional solenoid for stable samples or by a pair of coils for samples which require photoexcitation. This arrangement leaves the central volume of the resonator free for optimal sample illumination, which is achieved through a slit in the wall of the resonator. Microwave coupling is achieved by the incorporation of an iris in the top of the resonator, and magnetic field modulation is applied by external coils. The resonator’s high sensitivity is illustrated by a study of the temperature dependence of the continuous-wave ENDOR signal of the stable neutral flavin radical in DNA photolyase and an investigation of the photoexcited triplet state of free-base tetraphenylbacteriochlorin by time-resolved ENDOR.