Structure, paramagnetic species

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

This color transformation has been observed in dibenzo-p-dioxin (Structure I) and in its bromo, chloro, nitro, methyl, and ethyl derivatives in addition, the observed electron spin resonance (ESR) signals indicated the presence of paramagnetic species (2, 3). This phenomenon has been attributed to the formation of cation radicals in acid solution. 

Interactions between one or more nuclei and the unpaired electron yield a wealth of information concerning molecular structure. In addition, they have proven invaluable in the identification of paramagnetic species. As indicated in Table II many of the common elements have isotopes with nuclear magnetic spins which distinguish them from the other elements. If the isotopes of interest are not sufficiently abundant in the natural form, enriched samples may be purchased. The quantity used in surface studies is usually quite small, so relatively expensive isotopes such as 170 can be studied. In fact, it is possible to recover most of the isotope following an experiment, should the cost require it. 

Electron paramagnetic resonance (EPR) spectroscopy is yet another diagnostic tool for the detection of isomorphous substitution of Ti. Its sensitivity is very high, and investigations can be performed with samples even with very low contents of paramagnetic species. The spectra and g parameters are sensitive to the local structure and associated molecular distortions. Hence, it is an ideal tool to characterize Ti in titanosilicates. Ti in the + 4 oxidation state in titanosilicates is diamagnetic and hence EPR-silent. Upon contacting with CO or H2 at elevated... 

NMR and EPR techniques provide unique information on the microscopic properties of solids, such as symmetry of atomic sites, covalent character of bonds, strength of exchange interactions, and rates of atomic and molecular motion. The recent developments of nuclear double resonance, the Overhauser effect, and ENDOR will allow further elucidation of these properties. Since the catalytic characteristics of solids are presumably related to the detailed electronic and geometric structure of solids, a correlation between the results of magnetic resonance studies and cata lytic properties can occur. The limitation of NMR lies in the fact that only certain nuclei are suitable for study in polycrystalline or amorphous solids while EPR is limited in that only paramagnetic species may be observed. These limitations, however, are counter-balanced by the wealth of information that can be obtained when the techniques are applicable. 

It is inappropriate here to discuss details of nmr spectroscopy. However, the full possibilities for structural studies arise from the use of probes that bind to the molecule under study and perturb the nmr spectrum. These probes are generally paramagnetic species, in particular the lanthanide cations. In the nmr spectrum separate signals arise from each nucleus in the molecule, provided that the nucleus possesses a nonzero nuclear spin (e.g., H, 13C, 14N). The extent of the spectral perturbations of a given signal depends on the relative geometries of the paramagnetic species and the nucleus in question. Thus structural parameters can be obtained, in principle, for most atoms (nuclei) in a protein molecule.5... 

A complete coverage of the literature of EPR in photosynthesis is, not manageable and we will therefore give a rather subjective personal view of the field. The focus will be on major applications, i.e. where EPR was applied to get detailed information on structure and function of the particular paramagnetic species. The review contains selective work from the last 10 years with special emphasis on recent work that appeared in 2002 and 2003. The current work will be set in context of earlier results via critical review. 

EPR studies of paramagnetic species occurring in the bacterial reaction centre (bRC) have been comprehensively reviewed in this series by Weber45. This chapter covered the literature up to 1999, and also gave an introduction into structure and function of this interesting membrane protein. Due to an enormous international research effort (for leading references see ref. 3, 46-52 many of the questions about the structure and function of the bRC were already answered by the end of the last millennium. Advanced EPR techniques have... 

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