ENDOR studies

Almost simultaneously, Lindahl and co-workers proposed that Cluster C is the CO oxidation site based on EPR and ENDOR studies of the cyanide adduct of the enzyme (134). That proposal was based on the premise that CO and cyanide compete for the same binding site. Additionally, Xia and Lindahl have shown that, by mild SDS treatment, they can partially dissociate CODH/ACS, which is a tetra-meric enzyme with an subunit composition, into an isolated a subunit and an form (135). The form has the same level of CO oxidation activity as the native protein indicating that the a subunit is not involved in CO oxidation and that the /8 subunit must contain the clusters required for CO oxidation (135). In addition, CO2 alters the g values of the Credi form of the enzyme (136). 

Fe Q-band ENDOR study of the isotopically enriched Ni-C state of D. gigas and D. desulfuricans hydrogenases and Ni-B state of D. desulfuricans revealed a weak coupling between the Fe and the nickel atoms when the enzyme was in the Ni-A forms while no coupling was observed for the Ni-B form (186). A careful analysis of linewidth of Ni-A and Ni-B EPR signals detected in Fe enriched and nonenriched hydrogenase samples indicated that hyperfine interactions are lost in the spectral linewidth and, hence, nonde-tectable. 

Jeevarajan, A. S., L. D. Kispert et al. (1993b). An ENDOR study of carotenoid cation radicals on silica-alumina solid support. Chem. Phys. Lett. 209 269-274. 

Konovalova, T. A. and L. D. Kispert (1998). EPR and ENDOR studies of carotenoid-solid Lewis acid interactions. J. Chem. Soc. Faraday Trans. 94 1465-1468. 

Konovalova, T. A., S. A. Dikanov et al. (2001a). Detection of anisotropic hyperfine components of chemically prepared carotenoid radical cations ID and 2D ESEEM and pulsed ENDOR study.. /. Phys. Chem. B 105 8361-8368. 

Lawrence, J., A. L. Focsan et al. (2008). Pulsed ENDOR studies of carotenoid oxidation in Cu(II)-substituted MCM-41 molecular sieves. J. Phys. Chem. B 112 1806-1819. 

Piekara-Sady, L., A. S. Jeevarajan et al. (1995). ENDOR study of the (7,7 -dicyano)- and (7 -phenyl)-7 -apo-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel. J. Chem. Soc. Faraday Trans. 91 2881-2884. 

Wu, Y., L. Piekara-Sady et al. (1991). Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem. Phys. Lett. 180 573-577. 

C. Finazzo, J. Harmer, B. Jaun, E.C. Duin, F. Mahlert, R.K. Thauer, S. Van Doorslaer and A. Schweiger, Characterization of the MCRred2 form of methyl-coenzyme M reductase A pulse EPR and ENDOR study, J. Biol. Inorg. Chem., 2003, 8, 586. 

In powder samples with broad EPR lines, large Zeeman modulation amplitudes have to be applied to improve the sensitivity. Such amplitudes often produce microphonic noise in the cavity and cause an uncertainty in the orientation selection in single crystal-like ENDOR spectra (Sect. 4.1). A modulation technique which avoids these problems in powder ENDOR studies has been proposed by Hyde et al.32). In this scheme the Zeeman modulation is replaced by a 180° modulation of the phase of the microwave signal. 

Relative signs of the hyperfine splittings of two nuclei. In single crystal ENDOR studies the relative signs between hfs constants of different nuclei may sometimes be determined from the cross-term (3.18). If this second order term between nuclei I and K is... 

Since the unpaired electron in transition metal complexes is generally localized near the central ion and the ligand atoms in the first coordination sphere, summation in (5.5) over these nuclei is often sufficient. In this approximated form, the point-dipole model has frequently been applied in ENDOR studies of transition metal complexes to determine the proton positions from their hfs tensors (Sect. 6). In some cases the accuracy of this method has turned out to be significantly higher than that of an X-ray diffraction analysis62,130 131). 

An extensive EPR and ENDOR study of Cu(TPP) and Ag(TPP) (Fig. 32) doped into (H20)Zn(TPP) single crystals has been published by Brown and Hoffman66,171. The complete set of hfs and quadrupole tensors for the 14N nuclei and the hfs tensors of the Ag(II) and Cu(II) ions and of the pyrrole protons are reported. A detailed analysis of the magnetic parameters (Table 6.1), which are measured with high accuracy, has been presented using the standard MO treatment66. ... 

A single crystal nitrogen and proton ENDOR study of Cu(salen) (Fig. 33) doped into Ni(salen) has been published by Kita et al.173,174). The magnetic parameters obtained for this Schiff base complex are summarized in Table 7. 

In frozen solution EPR spectra of copper proteins, ligand and copper hf interactions are only partially resolved, or not at all. It is therefore striking that only few ENDOR studies on copper proteins have been published so far17,199-201). Nevertheless, these few ENDOR results already demonstrate the power of the double resonance technique to probe the coordination environment of the copper-containing sites in these types of proteins. 

A similar ENDOR study on VO(H)-doped TGS601 will be discussed in Sect. 6.6... 

A single crystal ENDOR study comprising 1H-, 13C-, 14N- and 59Co-ENDOR investigations of the Schiffbase compound Co(acacen) (Fig. 42) doped into Ni(acacen) 1/2H20 was therefore undertaken by Rudin et al.12,59,61,219). As illustrated in Fig. 2b, ENDOR spectroscopy allows hf and quadrupole interactions of all the magnetic ligand... 

The ENDOR work on heme compounds is an impressive example of a systematic study on an important class of biological compounds. One disadvantage is that most experiments had to be performed with the B0 field parallel to the complex normals, so that the full hfs and quadrupole tensors could usually not be evaluated. A single crystal ENDOR study on aquo-Mb is in preparation to rectify this lack of information257. ... 

The 8-Fe proteins contain two four-iron-sulfur cubes which are separated (center-to-center) by about 12 A. The EPR spectra from the fully reduced proteins are typical for a 4-Fe center with intercluster spin-spin coupling. An ENDOR study on the 8-Fe ferredo-xin from C. pasteurianum in the fully reduced state has been reported by Anderson et al.273. Since the 57Fe-56Fe difference spectra are only poorly resolved, analysis of the ENDOR data turned out to be difficult. All eight iron atoms are assumed to have similar AFe tensors with principal values Afe = 25 MHz, AFe = 29 MHz and AFe = 33 MHz. These coupling parameters suggest that the reducing electrons are delocalized over the four irons of each of the two nearly identical 4-Fe clusters. 

A single crystal ENDOR study on V(bz)2 diluted into 2,2-paracyclophane is in preparation, which will allow to get deeper insight into the dynamic behavior of the benzene rings in these types of compounds281. ... 

The proton ENDOR study of the chromyl ethyleneglycolate anion in ethanol reported by Mohl et al.293 presents the first successful adaptation of the ENDOR technique to a transition metal complex in liquid solutions. The aim of this work was to characterize the ENDOR relaxation behavior and to find optimum conditions for ENDOR detection. Two proton ENDOR lines with a hf splitting of ajj, = 1.74 MHz were observed. This is in agreement with a previous EPR study294 which had shown that all eight protons are equivalent. The optimum microwave and rf fields are both proportional to (Tr(g - geO rAj 2)1/2, where A denotes the dipolar part of the proton hfs tensor. For the chromyl ethyleneglycolate anion these two values have been calculated to — 8 10-6T and B = 2.7 mT. According to Mohl et al.293, successful proton... 

Davydov, R., Makris, T.M., Kofman, V., Werst, D.E., Sligar, S.G. and Hoffman, B. M. (2001) Hydroxylation of camphor by reduced oxy-cytochrome P450cam mechanistic implications of EPR and ENDOR studies of catalytic intermediates in native and mutant enzymes. Journal of the American Chemical Society, 123, 1403-1415. 

Fan, C. L., Teixeira, M., Moura, J., Moura, I., Huynh, B. H., Legall, J., Peck, H. D. and Hoffman, B. M. (1991) Detection and characterization of exchangeable protons bound to the hydrogen-activation nickel site of Desulfovibrio gigas hydrogenase - a H-1 and H-2 Q-band ENDOR study./. Amer. Chem. Soc., 113, 1-24... 

Fan C,Teixeira M, Moura J, et al. 1991. Detection and characterization of exchangeable protons bound to the hydrogen-activating nickel site of Desulfovibrio gigas hydrogenase a and Q-band ENDOR study. J Am Chem Soc 113 20. ... 

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