Simulation of electron spin resonance spectra of spin-labeled fatty acids covalently attached to the boundary of an intrinsic membrane protein. A chemical exchange model

Abstract

The ESR spectrum of a spin-labeled fatty acid covalently attached to rhodopsin in reconstituted systems with high lipid to protein ratio exhibits only narrow lines at 37°C, but has the appearance of the superposition of broad and narrow components at 5°C [ J. Davoust, B. M. Schoot, and P. F. Devaux, Proc. Natl. Acad. Sci. USA 76, 2755 (1979) ]. In this report it is shown that a temperature-dependent chemical exchange between two states (protein associated and lipid bilayer) can account for the results. The model implies that the covalently attached long-chain spin label explores more than just one layer of lipid around the protein. The boundary layer, or first layer, is associated with a high order and a strong immobilization of the probe at the time scale of ESR; thus τ c > 10 −7 sec; in the bulk lipid phase, the order is low, and the effective correlation time is approximately 10 −9 sec. The two states are spectroscopically well differentiated at low temperature (5°C) in the slow exchange regime. As the temperature increases, exchange effects tend to average the boundary layer spectrum with that of the fluid lipid spectrum. This phenomenon gives rise at 20°C to an apparent boundary layer state with an effective correlation time of 10 −8 sec. This latter value is equal to the inverse of the exchange frequency at 20°C. At 35°C, both components collapse, indicating that the exchange frequency is greater than the anisotropy of the hyperfine tensor of nitroxide. The rate of exchange of the acyl chains at the boundary of intrinsic proteins can be inferred from the efficiency of this spectral averaging. This rate of exchange is found to be comparable to the hopping frequency of freely diffusible phospholipids. The persistence of an immobilized component at high temperature, which can be observed after optical bleaching, must be associated with trapped, nonexchangeable, lipids within protein aggregates. The results of this article are used to discuss the various reports in the literature on lipid-protein interactions based on covalently attached or freely diffusible spin-labeled lipids as well as deuterated lipids.