EPR signals

When the applied magnetic field is swept to bring the sample into resonance, MW power is absorbed by the sample. This changes the matching of the cavity to the waveguide and some power is now reflected and passes via the circulator to the detector. This reflected radiation is thus the EPR signal. 

The practical goal of EPR is to measure a stationary or time-dependent EPR signal of the species under scrutiny and subsequently to detemiine magnetic interactions that govern the shape and dynamics of the EPR response of the spin system. The infomiation obtained from a thorough analysis of the EPR signal, however, may comprise not only the parameters enlisted in the previous chapter but also a wide range of other physical parameters, for example reaction rates or orientation order parameters. 

Figure Bl.15.7. Transient EPR. Bottom time-resolved EPR signal of the laser-flash-indueed triplet state of pentaeene in /j-terphenyl. BpO.085 mT. Top initially, the transient magnetization M is aligned along B z. In the presenee of a MW magnetie field B the magnetization preeesses about BJ x (rotating frame representation).

ELDOR is tlie acronym for electron-electron double resonance. In an ELDOR experiment [28] one observes a rednction in the EPR signal intensity of one hyperfme transition that results from the saturation of another EPR transition within the spin system. ELDOR measurements are still relatively rare bnt the experiment is fimily established in the EPR repertoire. 

It is important to emphasize that direet studies sueh as those earned out on the eyelopropylmethyl radieal ean be done with low steady-state eoneentrations of the radical. In the case of the study of the eyelopropylmethyl radical, removal of the source of irradiation leads to rapid disappearance of the EPR spectrum, because the radicals react rapidly and are not replaced by continuing radical formation. Under many conditions, the steady-state concentration of a radical intermediate may be too low to permit direct detection. Failure to observe an EPR signal, therefore, cannot be taken as conclusive evidence against a radical intermediate. 

The [SsNa]" radical cation (5.14, E = S) has been well characterized in solution by its five-line (1 2 3 2 1) EPR signal. It is prepared by oxidation of S4N4 with reagents such as AsEs or HSOsE. ... 

The multifrequency EPR and Mdssbauer properties of the [FesSJ in C. vinosum NiFe-hydrogenase are particularly interesting since they provide evidence of magnetic interactions with nearby paramagnetic species (151, 154, 155). The magnetically isolated form exhibits a well-resolved, almost axial EPR signal, g = 2.018, 2.016, 2.002, indicative of minimal conformational heterogeneity. However, a com-... 

In 1964, Rieske and co-workers reported the observation of an EPR signal around g = 1.90 in the cytochrome bci complex (1). They succeeded in the isolation of the iron sulfur protein that gave rise to the EPR signal and showed that it contained a [2Fe-2S] cluster. Over the... 

During oxidation of the MoFe protein the P clusters are the first to be oxidized at about -340 mV. This redox potential was first measured (40) using Mossbauer spectroscopy and exhibited a Nemst curve consistent with a two-electron oxidation process. It is possibly low enough for this redox process to be involved in enzyme turnover (see Section V). No additional EPR signal was observed from this oxidized form at this time. However, later a weak signal near g = 12 was detected and was finally confirmed, using parallel mode EPR... 

Yet further oxidation removes at least one more electron from each P cluster with an +90 mV to yield a protein oxidized by a total of at least eight electrons and with EPR signals from mixed spin states of S = I and S = I (42, 47). The combined integrations of these signals demonstrated that their intensity was equivalent to that of the FeMoco EPR signals in the same preparations. This provided the first evidence (47) that MoFe proteins contained equivalent numbers of FeMoco centers and P clusters and that P clusters contained 8 Fe atoms. Previously it had been considered that the P clusters were fully reduced Fe4S4 clusters and thus that there were two P clusters for every FeMoco center per molecule. 

In general there are few reproducible data on binding of reducible substrates to the isolated MoFe proteins. However, the S = EPR signal from the FeMoco centers of Kpl is pH dependent, the g values changing with a pKa of 8.7 (50). Of course, the proton is a substrate of nitrogenase however, there is no direct evidence for the proton associated with the pKa being bound directly to FeMoco. Nevertheless, this pKa can be perturbed by addition of the analog substrate acety-... 

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