Application of EPR

Compared with the extensive application of NMR procedures, electron paramagnetic resonance (EPR) is used less frequently except for analysis of the state of metals in enzymes and coenzymes. There has, however, been increasing awareness of the role of radical-mediated reactions and some examples are used to illustrate its potential. 

Elucidation of the Unexpected Complexity of an Apparently Straightforward Metabolic Pathway 

Hydroxyl Radicals and Their Role in the Toxicity of H2O2 to Bacteria 

In a study directed to the analysis of the role of Fe and the generation of H2O2 in Escherichia coli (McCormick et al. 1998), hydroxyl radicals were specihcally trapped by reaction with ethanol to give the a-hydroxyethyl radical. This formed a stable adduct with a-(4-pyridyl-l-oxide)-iV-t-butyl nitroxide that was not formed either by superoxide or hydroxyl radicals. The role of redox-reactive iron is to use EPR to analyze the EPR-detectable ascorbyl radicals. 

This enzyme catalyzes the conversion of pyruvate to formate and acetyl CoA and is a key enzyme in the anaerobic degradation of carbohydrates in some Enterobacteriaceae. Using an enzyme selectively C-labeled with glycine, it was shown by EPR that the reaction involves production of a free radical at C-2 of glycine (Wagner et al. 1992). This was confirmed by destruction of the radical with O2, and determination of part of the structure of the small protein that contained an oxalyl residue originating from gly-734. 

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APPLICATION OF EPR SPECTROSCOPY TO THE STRUCTURAL AND FUNCTIONAL STUDY OF IRON-SULFUR PROTEINS... 

III. Application of EPR to the Structural Study of Iron-Sulfur Proteins... 

The applications of EPR spectroscopy reviewed in the present chapter are based on the sensitivity of the spectrum displayed by iron-sulfur centers to various characteristics, such as the redox state of the center, the distribution of the valences on the iron ions, the nature and the geometry of the ligands, and the presence of nearby paramagnetic species. Although considerable progress has been made during the past few years in the quantitative analysis of these various effects in the case of the conventional iron-sulfur centers described in Section II, the discovery of centers exhibiting unusual EPR properties as... 

Application of EPR Spectroscopy to the Structural and Functional Study of Iron-Sulfur Proteins Bruno Guigliarelli and Patrick Bertrand... 

The investigation of free radicals formed by high energy irradiation of polytetrafluoroethylene is an example of an early application of EPR to polymer radicals. 

For a more advanced treatment of the theory and applications of EPR spectroscopy, the reader is referred to the text by Weil et... 

Biomedical Aspects of Free Radicals Recent Developments Through the Application of EPR... 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

Due to this early work it became clear that the application of EPR techniques to the OEC would deliver very valuable information on the electronic and geometrical structure of this important enzyme. The work led to an enormous amount of publications in this field both on the native system and on related model compounds. These cannot all be discussed here in detail. In particular, the diverse field of synthetic Mn complexes serving as models for the OEC will not be reviewed the reader is referred to some review articles41 432 438 and a monograph edited by Pecoraro.439... 

Miscellaneous Applications of EPR to PS II. - A number of authors used EPR to detect changes of the S-state cycle and the tyrosine radicals and correlated this with other functional aspects of the OEC and PS II. In particular, the prominent S2 state MLS and the tyrosine signals have been used for these investigations. A representative fraction of such work and some other EPR applications from the last years are discussed below. 

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