Stimulated nuclear polarization - a new method for studying the mechanisms of photochemical reactions (original) (raw)
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Chemical Physics Letters, 1998
The low magnetic field electron-nuclear resonance transitions, detected via changes in the nuclear polarization of radical reaction products, have been investigated. Due to the mixing of the wavefunctions in low magnetic fields such transitions are allowed as electron resonance transitions but at the same time lead to significant changes of nuclear polarization. The photolysis of 2,4,6-trimethylbenzoylphosphonic acid dimethyl ester has been used as a model reaction. It proceeds via the Ž. intermediate dimethoxyphosphonyl radical which has a large hyperfine constant 69.6-70 mT. It has been shown that this effect can be applied for the investigation of low field electron and nuclear polarization. q 1998 Elsevier Science B.V.
Chemical Physics Letters, 1986
Two different mechanisms of creation of nuclear polarisation in radical reaction products are studied by means of the h&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.
The Journal of Chemical Physics, 1999
The method of 13 C chemically induced dynamic nuclear polarization in a switched external magnetic field ͑SEMF CIDNP͒ has been applied for the first time in an experimental investigation of micellized radical pairs ͑RP͒. Using the examples of three photochemical reactions it has been shown, that SEMF CIDNP allows the investigation of the kinetics of short-lived micellized RPs with high time-resolution in low and intermediate magnetic fields. The experimental kinetics have been analyzed and simulated on the basis of a previously developed theory ͓Parnachev et al., J. Chem. Phys. 107, 9942 ͑1997͔͒. It has been demonstrated that such an analysis provides information on the rates of radical escape from the micelle, on electron relaxation and on the rate of S -T Ϫ transitions. The analysis of the estimated rates of S -T Ϫ transitions showed that the exchange interaction is essentially anisotropic in the RPs studied.
Chemical Physics Letters, 1997
Photoconductivity-detected magnetic resonance has been performed successfully as a new investigation method for transient radical-ion pairs. The chemical reaction of radical-ion pairs is controlled by microwave radiation. The photoionization system of TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine) in alcohol is employed. The data show the broadened linewidth of the radical-ion pair spectrum that may be influenced by the microwave field and/or the dynamic motion of the radicals. This observation method is a powerful way of studying the dynamics of transient radical-ion pairs formed in photochemical and photobiological reactions that involve electron transfer reactions.
Time-resolved CIDNP: an NMR way to determine the EPR parameters of elusive radicals
Physical Chemistry Chemical Physics, 2011
Chemically Induced Dynamic Nuclear Polarization (CIDNP) of the diamagnetic products of radical reactions is exploited for the purpose of determination of the hyperfine coupling constants (HFCCs) of the radical intermediates. A simple proportionality relation between geminate CIDNP of a nucleus and its HFCC at the radical stage is established. The applicability range of this relation is determined: the relation is fulfilled in the case of a large difference in g-factor between the radicals involved and for the situation where the number of magnetic nuclei in the system is sufficiently large. The validity of the relation was confirmed by CIDNP experiments on radical pairs with precisely known HFCCs. Using the proportionality relation we were able to measure the HFCCs in various short-lived radicals of the amino acids histidine and tryptophan and of the S-N-centered cyclic radical of methionine derived from the methionine-glycine dipeptide in aqueous solution.
1H Dynamic Nuclear Polarization Based on an Endogenous Radical
The Journal of Physical Chemistry B, 2012
We demonstrate a 15-fold enhancement of solid-state NMR signals via dynamic nuclear polarization (DNP) based on a stable, naturally occurring radical in a protein: the flavin mononucleotide (FMN) semiquinone of flavodoxin. The linewidth of flavodoxin's EPR signal suggests that the dominant DNP mechanism is the solid effect, consistent with the field-dependent DNP enhancement profile. The magnitude of the enhancement as well as the bulk-polarization build-up time constant (τ B) with which it develops are dependent on the isotopic composition of the protein. Deuteration of the protein to 85 % increased the nuclear longitudinal relaxation time T 1n and τ B by factors of five and seven, respectively. Slowed dissipation of polarization can explain the twofold higher maximal enhancement than that obtained in proteated protein, based on the endogenous semiquinone. In contrast, the long τ B of TOTAPOL-based DNP in non-glassy samples was not accompanied by a similarly important long T 1n , and in this case the enhancement was greatly reduced. The low concentrations of radicals occurring naturally in biological systems limit the magnitude of DNP enhancement that is attainable by this means. However, our enhancement factors of up to 15 can nonetheless make an important difference to the feasibility of applying solid-state NMR to biochemical systems. We speculate that DNP based on endogenous radicals may facilitate MAS NMR characterization of biochemical complexes and even organelles, and could also serve as a source of additional structural and physiological information.
Time-domain magnetic resonance studies of short-lived radical pairs in liquid solution
Faraday Discussions of the Chemical Society, 1984
Magnetic resonance spectra of radical-ion pairs possessing lifetimes as short as 12 ns have been obtained using a new time-resolved optically detected magnetic resonance technique. Short-lived radical pairs are produced by a laser flash. The transient optical absorbance of the radical pairs or the triplet products resulting from their collapse is monitored as a function of time in the presence of high-power 9.1 GHz radiation as a magnetic field i s swept. At resonance the microwaves induce transitions among the radical-pair energy levels that are observed as changes in the population of either the radical pair or the triplet products resulting from radical-pair collapse. These resonances can be used to obtain radical-pair structure and dynamics. Radical-ion pairs produced in the reaction-centre protein from the photosynthetic bacterium R. sphaeroides and radical-ion pairs resulting from the photoreduction of anthracene by N, N-diethylaniline in acetonitrile are discussed. All experiments are performed at ambient temperature in liquid solution.
Theory of optically detected magnetic resonance spectra of radical pairs
Chemical Physics
The characteristic features of optically detected magnetic resonance spectra of radical pairs associated with dynamic nature of the signal have been studied theoreticaliy. The dependence of the spectral line width and shape upon me microwave intensity and the radical pair lifetime 113~ been determined. It has ken shown that a time delay in the signal detection can narrow the spectrzd hypesfine components observed to a size less than the natural width md thus make it possible to detect resolved spectra of short-lived radicals.
Biophysical Chemistry, 2005
The effects of either static or pulsed magnetic fields on the reaction rate of Fremy's salt-ascorbic acid were studied directly by EPR spectroscopy. Radical pair mechanism (RPM) accounts for the magnetic field effects, but the expected amounts are so small that they need to be observed with particular care with EPR technique. The method is based on the resolution of a pair of EPR signals by the addition of a stationary field gradient, where the signals are coming from the exposed and control capillary sample. To this purpose, a suitable device for the gradient generation was used. Others improvements were the strictly keeping of the same boundary temperature condition in the capillary pairs, obtained by a refrigerating system controlled by a thermocouple, and the use of a pair of Helmholtz coils to generate an external high homogeneous magnetic field. By this experimental set up, we found that the magnetic field induce the decrease of the studied radical reaction rate. This EPR approach is a significant alternative to the spectrophotometric one. Moreover, it offers the advantage to detect both the radicals and/or intermediates involved in the reaction. D