ENDOR triple resonance

M,h, F., Jones, M. R., and Lubitz, W. L., 1999, Reorientation of the acetyl group of the photoactive bacteriopheophytin in reaction centers of Rhodobacter sphaeroides An ENDOR/TRIPLE resonance study. Biospectroscopy, 5 35946. 

One key aspect of ENDOR spectroscopy is the nuclear relaxation time, which is generally governed by the dipolar coupling between nucleus and electron. Another key aspect is the ENDOR enhancement factor, as discussed by Geschwind [294]. The radiofrequency frequency field as experienced by the nucleus is enhanced by the ratio of the nuclear hyperfine field to the nuclear Zeeman interaction. Still another point is the selection of orientation concept introduced by Rist and Hyde [276]. In ENDOR of unordered solids, the ESR resonance condition selects molecules in a particular orientation, leading to single crystal type ENDOR. Triple resonance is also possible, irradiating simultaneously two nuclear transitions, as shown by Mobius et al. [295]. 

A refinement of the ENDOR experiment is electron-nnclear-nnclear triple resonance, now commonly denoted TRIPLE. In TRIPLE experiments one monitors the effect of a simnltaneons excitation of two nnclear spm transitions on the level of the EPR absorption. Two versions, known as special TRIPLE (ST) and general TRIPLE (GT), are rontinely perfonned on connnercially available spectrometers. 

Mdbius K, Plato M and Lubitz W 1982 Radicals in solution studied by ENDOR and TRIPLE resonance spectroscopy P/rys. Rep. 87 171-208... 

Mdbius K, Lubitz W and Plato M 1989 Liquid-state ENDOR and TRIPLE resonance Advanced EPR in Biology and Biochemistry ed A J Hoff (Amsterdam Elsevier) ch 13, pp 441-99... 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

In collaboration with Fabian Gerson at the University of Basel, the radical anion of 72 was also studied by ESR, ENDOR, and triple-resonance spectroscopy. The results obtained thus far indicated that the odd electron of the radical anion of 72 was delocalized exclusively between the para-linked benzene rings, and was consistent with the result procured from a similar study on the parent [2.2]paracy-clophane. " ... 

Upon treatment of 2,5-dihydro-1,2,3,5-thiatriazole 1-oxides with sodium or potassium (4.20.5.1) in 1,2-dimethoxyethane 2,5-dihydro-l,2,3,5-thiatriazol-5-yl radicals are formed which have been studied by means of ESR, ENDOR, and general triple resonance spectroscopy <90JCS(P2)1619>. 

Dihydro-l, 2,3,5-thiatriazole 1-oxide radicals were generated from the thiatriazole oxides upon hydrogen abstraction in the reaction with thermally formed bis(4-methylphenyl)aminyl (4.20.5.1). Only radicals with aryl substituents in positions two and four are persistent, radicals with alkyl groups in these positions could not be detected. The radicals were investigated by ESR, ENDOR, and general triple resonance spectroscopy <92MRC84>. 

The ENDOR technique refers to electron-nuclear double resonance. This consists of the effect on a partially saturated ESR line of simultaneously irradiating the sample with a radiofrequency to induce nuclear resonance transitions of hyperfine coupled nuclei. It may enable one to obtain information about signs of coupling constants. ELDOR is the technique corresponding to electron-electron double resonance. Such techniques, coupled with TRIPLE resonance, have been utilized and well described in a discussion of pyridine and 4,4-bipyridyl radical anion ESR spectra measured in sodium/liquid ammonia (80JMR<41)17). 

Because of the inherent non-planar structure of helicenes it seemed of interest to examine the spin distribution in helicene radical anions. For the mono anion of hexahelicene a set of 8 hyperfine splitting constants (hfsc s) and 38 = 6561 ESR lines can be expected. Such a spectrum will be poorly resolved. Indeed, it was not possible to determine hfsc s from the ESR-spectrum of hexahelicene 132). Using the ENDOR technique which reduces the amount of lines the eight hfsc s could be deduced, however, and the relative signs could be determined l33) by the triple resonance technique. 

The data on electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and general triple resonance spectroscopy for 2,5-dihydro-l,2,3,5-thiatriazole-5-yl radicals was reviewed by Holm and Larsen <1996CHEC-II(4)733>. There are no new data on this subject. 

Another method that is important for structure assignment is the electron-nuclear-nuclear triple resonance (TRIPLE) spectroscopy (Endeward et al., 1998 Makinen et al., 1998), which is an extension of the ENDOR method. In the general TRIPLE experiment, transitions of different nuclei are driven simultaneously. One ENDOR transition is irradiated saturating rf power at a constant frequency, while the entire ENDOR frequency range is swept to obtain the TRIPLE spectrum. 

Endeward, B., Plato, M., Will, S., Vogel, E., Szyczewski, A. ,Moebius, K.. (1998) Liquid-phase EPR, ENDOR, and TRIPLE resonance studies on corrole and isocorrole cation radicals, Applied Magnetic Resonance 14, 69-80. 

Lubitz, W., Lendzian, F., Plato, M., Scheer, H., and Moebius, K. (1997) The bacteriochlorophyll a cation radical revisited. An ENDOR and triple resonance study, Applied Magnetic Resonance 13, 531-551. 

The radical cations from several substituted A V -diphenyl viologens (PV) have also been investigated by means of EPR spectroscopy as well as electron nuclear double resonance (ENDOR) and triple resonance spectroscopies. 

J. C. Evans, A. G. Evans, N. H. Nouri-Sorkhabi, A. Y. Obaid, and C. C. Rowlands, An electron spin resonance, ENDOR, and TRIPLE resonance study of methyl-substituted AGV-diphenyl-4,4-bipyridylium dichloride radical cations,./. Chem. Soc., Perkin Trans. 2 1985, 315-318. 

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