An orientation distribution model for interpreting ESR line shapes of ordered spin labels

Abstract

Partial macroscopic orientation of an ordered array of spin labels is simulated by assuming a Gaussian distribution of the labels and using the distribution function to weight the individual stick spectra that contribute to the final ESR spectrum. The orientation distribution function also includes provision for skewing the distribution, effectively introducing an angle of tilt of the spin label axes with respect to the plane of the partially oriented sample. Skewing the distribution 90° enables calculation of spectra that are characteristic of cylindrical symmetry. This orientation distribution is used in conjunction with models for rapid anisotropic motion. The computer simulations of biological samples with plane symmetry and cylindrical symmetry are presented for a model membrane system (egg lecithin multilayers) and axonal membranes (invertebrate nerve bundle). The distribution of orientations within the sample is determined, so that the partially oriented samples can be quantitatively described both as to average amplitude ofmotion and range of orientations in the spin-labeled regions. The FORTRAN listing of the computer program is included in the Appendix.