Abstract
A new method recently developed by J. S. Hyde and K. V. S. Rao ( J. Magn. Reson. 38, 313 (1980)) for the determination of rotational correlation times τ R in the slow-motional region is based on measuring EPR spin-label spectra at two different microwave frequencies. The utility of this method was proved by studying the dynamical state of spin-labeled substrate molecules bound to the endoplasmic hepatic cytochrome P-450 by use of X- and Q-band EPR spectroscopy. Under experimental conditions where only one slow-motional spin-label spectrum is observed, the new method exhibits advantages in comparison with other approaches. The experimental error in the determination of τ R however, increases significantly if (i) a free label spectrum is superimposed or (ii) the spin concentration (in many cases the protein concentration) is too low to produce a relatively noiseless Q-band spectrum. Membrane-bound proteins, e.g., maleimide spin-labeled liver microsomes, provide an appropriate reference system for such an analysis. The Hyde-Rao method is compared with a ST-EPR analysis of the same system.
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