EPR moments for site-directed spin-labelling.

The absolute-value first moment and second moment of nitroxide EPR absorption spectra, which can be used for systematic analysis in site-directed spin-labelling, are analysed in terms of the rotational mobility and angular motional amplitude of the spin probe. Spectral simulations with the stochastic Liouville equation are used to give calibrations of second and absolute-value first moments as a function of isotropic rotational correlation time τR and axial order parameter Szz, over the entire range of sensitivity to these parameters. Both moments are additive for multicomponent spectra, e.g., for multiple conformations. The absolute-value first moment has the advantage over the central line width in that it does not overemphasize fast motional components, and over the second moment in that it is less sensitive to noise in the outer wings of the spectrum and to baseline cutoff.