The information in EPR spectra

Both Fourier-transform (FT) and continuous-wave (CW) EPR spectrometers are available. The FT-EPR instrument is like an FT-NMR spectrometer except that pulses of microwaves are used to excite electron spins in the sample. The layout of the more common CW-EPR spectrometer is shown in Fig. 13.38. It consists of a microwave source (a klystron or a Gimn oscillator), a cavity in which the sample is inserted in a glass or quartz container, a microwave detector, and an electromagnet with a field that can be varied in the region of 0.3 T. The EPR spectrum is obtained by monitoring the microwave absorption as the field is changed, and a typical spectrum (of the benzene radical anion, CgHj) is shown in Fig. 13.39. The peculiar appearance of the spectrum, which is in fact the first 

To begin to interpret the EPR spectra of organic radicais that can foim during bioiogicai processes we need to compare the spectrum of the sample with that of a free eiectron. 

Equation 13.10 gives the resonance frequency for a transition between the nis=— and nts = + levels of a free electron in terms of the value 2.0023. The magnetic moment of an unpaired electron in a radical also interacts with an external field, but the value is different from that of a free electron on account of local magnetic fields induced in the molecular framework of the radical. Consequently, the resonance condition is normally written as 

The deviation of from e = 2.0023 depends on the ability of the apphed field to induce local electron currents in the radical, and therefore its value gives some information about electronic structure. In that sense, the value plays a similar role in EPR as the shielding constant plays in NMR. Because -values differ very little from g in many radicals (for example, 2.003 for H, 1.999 for NO2,2.01 for CIO2), their main use in biochemical apphcations is to aid the identification of the species present in a sample. 

Recent EPR studies have shown that the amino acid tyrosine participates in a number of biological electron transfer reactions, including the oxidation of water to Oj in plant photosystem II, the reduction of Oj to water in cytochrome c oxidase, and the reduction of ribonucleotides to deoxyribonucleotides catalyzed by the enzyme ribonucleotide reductase. During the course of these electron transfer reactions, a tyrosine radical forms (4). The center of the EPR spectrum of the tyrosine radical in cytochrome c oxidase of the bacterium P. denitrificans occurs at 344.50 mT in a spectrometer operating at 9.6699 GHz (radiation belonging to the X band of the microwave region). Its -value is therefore 

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