Abstract
A method using nitroxide radical spin labels for determining both the isotropic rotational correlation time τ R and the environmental polarity of the label is described. By means of a least square fitting method, the values of an effective hyperfine tensor A′ and of an effective g value tensor g′ of randomly oriented spin labels are determined from X-band EPR spectra on the basis of an effective time-independent Hamiltonian. The traces of the tensors deliver the information about the environmental polarity of the label and are not dependent on the rotational correlation time τ R. A new averaging parameter S ( τ R), calculated on the basis of the principal values of the tensor A′ , permits the evaluation of the rotational correlation time τ R in a very wide time range between 10 −10 and 10 −6 s. An application of this method to spin-labeled methemoglobin over a large temperature range and in environments of different polarity is discussed.
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