HF EPR Spectra of Spin Labels in Membranes

Abstract

Simulation of the high-field EPR spectra from spin-labelled lipids in membranes is treated. The lateral ordering of the lipid chains is characterised by g xx −g yy anisotropy and can be analysed by motional narrowing theory. Spin Hamiltonian tensors required for simulation must be corrected for changes in environmental polarity within the membrane. At very high field, the EPR spectra are sensitive only to the local segmental motion of the spin-labelled lipid chains; overall tumbling of the chain axis is at the rigid limit. Spectra at conventional EPR frequencies, however, are additionally sensitive to the slow overall lipid chain motion. Stochastic Liouville simulations of conventional low-field EPR spectra that use partially averaged spin-Hamiltonian tensors obtained from high-field simulations are able to describe the slow-motional component. Strategies to analyse residual slow-motional sensitivity of 94-GHz spin-label spectra are presented within the context of a multifrequency approach.