Relaxation Time Determinations by Progressive Saturation EPR: Effects of Molecular Motion and Zeeman Modulation for Spin Labels
✍ Scribed by V.A Livshits; T Páli; D Marsh
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 253 KB
- Volume
- 133
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
The EPR spectra of nitroxide spin labels have been simulated as a function of microwave field, H 1 , taking into account both magnetic field modulation and molecular rotation. It is found that the saturation of the second integral, S, of the first harmonic in-phase absorption spectrum is approximated by that predicted for slowpassage conditions, that is, S ϳ H 1 / ͌ 1 ؉ PH 1 2 , in all cases. This result is independent of the degree of inhomogeneous broadening. In general, the fitting parameter, P, depends not only on the T 1 and T 2 relaxation times, but also on the rate of molecular reorientation and on the modulation frequency. Calibrations for determining the relaxation times are established from the simulations. For a given modulation frequency and molecular reorientation rate, the parameter obtained by fitting the saturation curves is given by 1/P ؍ a ؉ 1/␥ e 2 T 1 ⅐ T 2 eff , where T 2 eff is the effective T 2 . For molecular reorientation frequencies in the range 2 ؋ 10 7 -2 ؋ 10 8 s ؊1 , T 2 eff is dominated by the molecular dynamics and is only weakly dependent on the intrinsic T 2 0 , allowing a direct estimation of T 1 . For reorientation frequencies outside this range, the (T 1 T 2 ) product may be determined from the calibrations. The method is applied to determining relaxation times for spin labels undergoing different rates of rotational reorientation in a variety of environments, including those of biological relevance, and is verified experimentally by the relaxation rate enhancements induced by paramagnetic ions.