Distinct populations in spin-label EPR spectra from nitroxides.

It is not possible to reproduce simulations of nitroxide spin-label EPR spectra published in J. Phys. Chem. 88, 3454-3465 (1984), when using the motional model and parameters given in that paper. Instead of the apparent two-component line shapes claimed, the spectra resemble single-component powder patterns expected from axially anisotropic, partial motional-averaging (a situation familiar for chain-labelled lipids in nonaligned fluid membranes). This is because: (i) the nitroxide z-axis is inclined at a fixed angle to the principal diffusion axis, and (ii) motion perpendicular to the principal diffusion axis is so slow as to approximate a powder distribution. The line shapes are compatible with simulations that use the same model for a complete range of nitroxide order parameters (Spectrochim. Acta A 53, 2235-2240, 1997), which are able to describe single-component experimental spectra from lipids spin-labelled at different chain positions in fluid bilayer membranes (Chem. Phys. Lipids 82, 7-14, 1996). Neither from simulation nor from experiment, is there any basis to assert that single-component nitroxyl EPR spectra resemble those containing two-components