Spin labeling fast-motional spectra

A second example is a 220 GHz EPR study of an oligonucleotide with a newly synthesized cytosine spin-label. The spectrum from a monomer in an aqueous solution exhibited a three-line pattern characteristic of a nitroxide in a fast motion regime. The anisotropic diffusion tensor estimated from this fast motion spectrum was consistent with the expected rapid motion of the probe molecule around its tether. 

Steinhoff et al. (1989) measured the temperature and hydration dependence of the ESR spectra of hemoglobin spin-labeled at cysteine )8-93. They observed the critical temperature near 200 K, as described above, and the sensitivity of the spectrum to hydration level. Spectrum simulations suggested that there were two types of motion in the dry protein, a fast vibration of the label within a limited motion cone upon the addition of water, a hydration-dependent motion assigned to the fluctuations of the protein, of correlation time 10 sec in samples of high hydration and at 300 K. The temperature dependence of the motional properties of a spin probe (TEMPONE), diffused into hydrated single crystals, closely paralleled the motional properties of the label. This was taken to be evidence for coupling between the dynamical properties of the protein and the adjacent solvent. 

The ESR spectra were analyzed quantitatively and the fraction of the fast (t. = 10 ), slow (r,. = 10 ), and very slow = I0 ") motions of the spin-label site (Tyr side-chain) were determined. A model is proposed for the heterogeneous structure of the swollen silk fibroin membrane. The ESR spectrum was complex, indicating the heterogeneous structure of the swollen membrane. 

The above discussion has considered the spin-label or spin-probe to be in a homogeneous system with a single correlation time. In some cases, however, the observed ESR spectrum is of neither the fast- nor the slow-motion type but is a mixture of the two (Figure 9.3). Such composite spectra arise when the spin-labels or probes simultaneously occupy two motionally distinct environments, one of which is significandy more restricting than the other. This behaviour may occur in situations such as adsorption of a polymer onto a solid [9], phase separations of polymers or polymer blends [25], polymer networks [26] or any other system where more than one phase is present. 

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