Anisotropic spin label mobilities in azurin from 95 GHz electron paramagnetic resonance spectroscopy

The anisotropy of the motion of spin labels in proteins is determined by high field (95 GHz) EPR, yielding information on protein structure and dynamics. Spin labels at three positions in azurin are compared. At position 27, isotropic rotation, at positions 12 and 118 preferential rotation about the nitroxide N–O bond is found. The mode of motion reflects the spin label dynamics and the protein environment better than the isotropic rotational correlation times from 9 GHz EPR. A method to reliably measure protein samples under physiological conditions on a commercial 95 GHz EPR spectrometer is introduced.