Distinct Populations in Spin-Label EPR Spectra from Nitroxides

Two-component nitroxide spin-label electron paramagnetic presonance (EPR) spectra are important to the analysis of lipid-protein interactions, phase separation in lipid membranes, and conformational changes in proteins. A paper published in this journal offers an interpretation of such spectra based on simulations with single-site models. It is not possible to reproduce those simulations published in Meirovitch , E. ; [ Analysis of Protein Lipid Interactions Based on Model Simulations of Electron Spin Resonance Spectra . J. Phys. Chem. 1984 , 88 , 3454 - 3465 ] that might conceivably be taken to resemble two-component line shapes, when using the motional model and parameters given in that paper. Instead of the apparent two components, the spectra resemble single-component powder patterns expected from axially anisotropic, partial motional-averaging (a situation familiar for chain-labeled 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 [ Schorn , K. ; Extracting Order Parameters from Powder EPR Lineshapes for Spin-Labelled Lipids in Membranes . Spectrochim. Acta, Part A 1997 , 53 , 2235 - 2240 ], which are able to describe single-component experimental spectra from lipids spin-labeled at different chain positions in fluid-bilayer membranes [ Schorn , K. ; Lipid Chain Dynamics in Diacylglycerol-Phosphatidylcholine Mixtures Studied by Slow-Motional Simulations of Spin Label ESR Spectra . Chem. Phys. Lipids 1996 , 82 , 7 - 14 ]. Neither from simulation nor from experiment is there any basis to assert that single-component nitroxyl EPR spectra resemble those containing two components.