Steinhoff HJ
A method using nitroxide radical spin labels for determining both the isotropic
rotational correlation time tau 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 tau
R. A new averaging parameter S (tau R), calculated on the basis of the
principal values of the tensor A', permits the evaluation of the rotational
correlation time tau 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.