title: Anisotropic motion effects in CW non-linear EPR spectra: relaxation enhancement of lipid spin labels.
abstract: Continuous-wave (CW) EPR measurements of enhancements in spin-lattice (T(1)-) relaxation rate find wide application for determining spin-label locations in biological systems. Often, especially in membranes, the spin-label rotational motion is anisotropic and subject to an orientational potential. We investigate here the effects of anisotropic diffusion and ordering on non-linear CW-EPR methods for determining T(1) of nitroxyl spin labels. Spectral simulations are performed for progressive saturation of the conventional in-phase, first-harmonic EPR signal, and for the first-harmonic absorption EPR signals detected 90 degrees -out-of-phase with respect to the Zeeman field modulation. Motional models used are either rapid rotational diffusion, or strong-jump diffusion of unrestricted frequency, within a cone of fixed maximum amplitude. Calculations of the T(1)-sensitive parameters are made for both classes of CW-experiment by using motional parameters (i.e., order parameters and correlation times), intrinsic homogeneous and inhomogeneous linewidth parameters, and spin-Hamiltonian hyperfine- and g-tensors, that are established from simulation of the linear CW-EPR spectra. Experimental examples are given for spin-labelled lipids in membranes.
authors.affiliation: Centre of Photochemistry, Russian Academy of Sciences, 117427, Moscow, Russian Federation.
authors.forename: V A
authors.initials: VA
authors.lastname: Livshits
authors.forename: B G
authors.initials: BG
authors.lastname: Dzikovski
authors.forename: D
authors.initials: D
authors.lastname: Marsh
mesh_terms: D016880:Anisotropy; D004578:Electron Spin Resonance Spectroscopy; D016014:Linear Models; D008962:Models, Theoretical; D010713:Phosphatidylcholines / Q000737:chemistry*
chemical_list: D010713:Phosphatidylcholines