Spin-label information content

In this section we provide an outline of the kinds of information that can be obtained from a nitroxide radical spin-label spectrum. 

In this class of situations, all that is wanted is the amplitude of the signal. An example of this is spin-immunoassay [52], where the signal intensity is proportional to the amount of material under assay. The experimenter may be faced with the problem of obtaining the best possible signal-to-noise ratio without regard to lineshape distortions. The following is a check-list of options that can be considered. 

In the range of characteristic rotational motions of s, the total spectral 

The main experimental problem is that very small field modulation amplitudes must be employed in order to avoid distortion of the lineshape, which results in a poor signal intensity. It is noted that lineshape distortion is a linear process, transferring information to higher harmonics of the field modulation frequency. In principle, this information could be collected and used to correct for this distortion, but practical ways have not yet been developed. 

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More recently, homonuclear correlation techniques relying on J-couplings have also been developed [86, 87, 89] and applied to the assignment of spin systems of amino-acid residues in uniformly labeled proteins and peptides [90]. These have, in some cases, a higher information content than the comparable dipolar-mediated experiments, as relayed correlations throughout the continuous 13C-13C network are more easily realized at high B0 fields [90]. 

The effect of wQ on the enzyme was examined by comparing the EPR spectra of lysine-labeled tryptophanase. Although the spin label is nonspecific, it can still provide molecular-level information about the enzyme under different conditions. EPR spectra of the spin-labeled enzyme in bulk water and in reverse micelles are shown in Figure 6. Much broader spectra were obtained in reverse micelles, and the calculated rotational correlation time of the attached label (101 increased with w0. Thus, the enzyme-bound spin label became more constrained as the water content of the reverse micelle decreased. Since the rotational correlation time of the entire protein in bulk water calculated from the Stokes-Einstein equation is about 200 nsec, the motion of the spin label was still rapid relative to the tumbling rate of the enzyme. Therefore, broadening of the spectrum was apparently caused by a change in the local dynamics of tryptophanase rather than by a decrease in the enzyme s overall rotation rate. The tumbling rate of the enzyme could have decreased as well, however. 

EPR is used extensively to detect, identify and follow the fate of radicals involved in polymerization or polymer degradation processes. It is also used extensively to characterize polymeric materials in terms of their morphology, heterogeneity, structural transformations, chain dynamics, and so on [2-5]. For this purpose, one can take advantage of the stable paramagnetic centers present in material to be examined (e.g., residual post-polymerization radicals, or TMIs used as catalytic centers or stabilizers). In most cases, however, external spin probes are added, including spin-labeled macromolecules (see Sections 23.2.1.5 and 23.2.2.1) [6]. The sensitivity and high content of structural information contained in the spin Hamiltonian parameters allow to obtain valuable - and often unique - data on the studied systems [7]. 

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