Rotational correlation time spin probe analysis

If the g- and hyperfine anisotropies are known from analysis of a solid-state spectrum, the line-width parameters (1, and yt can be used to compute the rotational correlation time, tr, a useful measure of freedom of motion. Line widths in ESR spectra of nitroxide spin labels, for example, have been used to probe the motional freedom of biological macromolecules.14 Since tr is related to the molecular hydrodynamic volume, Ft, and the solution viscosity, r, by a relationship introduced by Debye 15... 

Line Shape Analysis for Nitroxide Spin Probes. Nitroxide radicals as spin probes and labels are useful for the determination of the motional mechanism, rotational correlation time, to, and local polarity. Figure 2a demonstrates that the relative line widths and line heights depend on the rotational correlation time to, which is inversely proportional to the diffusion constant of rotational diffusion. 

The nature of the lipid-protein interactions can be further studied by the use of ST-ESR techniques. ST-ESR extends the useful timescale of ESR to > 10" S and can be used for the analysis of the amplitude and rate of the rotation of spin-labelled lipids. The rotational correlation time (Tq) which can readily be deduced from ST-ESR spectra, is inversely proportional to the rate of rotation of the spin probe. In the case of the PSII preparations the values for T are given in Table 1. In general all of these rotation rates are very slow indeed. 

An analysis of these data leads to the following conclusions. First, the correlation times of rotation of the Tempol spin probes located in the dense regions are virtually the same for isotropic and textured PHB samples. At the same time, the mobility of this radical in the loose regions is somewhat higher in textured matrices than in the isotropic ones. Second, the distribution of spin probes between dense and loose regions is virtually the same in both textured and isotropic PHB samples. 

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