ENDOR spin densities

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

Fig. 19 n-Spin-density distribution in diphenylcarbene [15 m= ] determined by ENDOR experiments and MO calculations. The o-spin density at the divalent carbon is in parentheses. 

ENDOR experiments can be performed in liquid solution, in which only the isotropic hfc s (Ajso) are detected. They are proportional to the spin density at the respective nucleus. Erom the assigned isotropic hfc s a map of the spin density distribution over the molecule can be obtained. In frozen solutions and powders the anisotropic hf interactions can also be determined. Eurthermore, the method allows the detection of nuclear quadrupole couplings for nuclei with 1 1. For dominant g anisotropy as found in many metal complexes the external magnetic field can be set to several specific g values in the EPR, thereby selecting only those molecules that have their g tensor axis along the chosen field direction. In such orientation-selected spectra only those hf components are selected that correspond to this molecular orientation ( single crystal-like ENDOR ). 

In this respect the possible presence of electron spin density at the iron in the Ni-Fe cluster is of importance. This question has been answered by ENDOR experiments performed on Fe enriched [NiFe] hydrogenase (Huyett et al. 1997). In this study no significant Fe hfc was detected in the NiA, NiB and NiC states (A57p s 1 MHz). This shows that in all relevant states of the enzyme the iron is in the low-spin Fe state. 

Vanadium(n) Complexes.—Dehydration of VSO. THjO has been shown to proceed via the formation of VS04,mH20 (where n = 6, 4, or 1) and V(OH)-(SO4), which were characterized by X-ray studies. The polarographic behaviour and the oxidation potential of the V -l,2-cyclohexanediamine-tetra-acetic acid complex, at pH 6—12, have been determined.Formation constants and electronic spectra have been reported for the [Vlphen),] " and [V20(phen)] complexes. The absorption spectrum of V ions doped in cadmium telluride has been presented and interpreted on a crystal-field model. The unpaired spin density in fluorine 2pit-orbitals of [VF ] , arising from covalent transfer and overlap with vanadium orbitals, has been determined by ENDOR spectroscopy and interpreted using a covalent model. " ... 

Oxidation of cytosine produces a radical with sites of unpaired spin density at Nl, N3, and C5. The cytosine cation has a pKn<4 and in solution deprotonates at NH2 [22]. In the solid state, Sagstuen et al. [24] assigned the primary oxidation radical observed in cytosine monohydrate as the Nl deprotonated cation, Cyt(Nl—H) . It is known from the ENDOR... 

Several recent papers have reported Density Functional Theory (DFT) calculations on the primary oxidation and reduction products observed in irradiated single crystals of the common nucleobases thymine [53], cytosine [54], guanine [55], and adenine [56]. The theoretical calculations include estimates of spin densities and isotropic and anisotropic hyperfine couplings which can be compared with experimental results (obtained from detailed EPR/ENDOR experiments). 

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies on frozen solutions of TDTA in tetrahydrofuran (THE) allowed estimates of its spin density distribution <2005JCD3838>. A comparison with that of TTTA <2001JMC1992> determined by similar methods shows that replacement of N of TTTA by isoelectronic C-H groups results in greater localization of spin density on the dithiazolyl ring. 

The papers reporting EPR/ENDOR studies of P + in special PS II samples show that the hfcs and the resulting spin density distribution is rather that of a disturbed monomeric Chi a+ than a dimeric species.201,343,346 This conclusion is, however, based on a comparison with monomeric Chi a 4 in solution219,343,347 which is known to be a poor model system.21,348 Another problem is the very highly oxidizing power of P + that leads to electron donation from other chloro-... 

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