Studies on lipid membranes by two-dimensional Fourier transform ESR: Enhancement of resolution to ordering and dynamics

Abstract

The first two-dimensional Fourier-transform electron spin resonance (2D-FT-ESR) studies of nitroxide-labeled lipids in membrane vesicles are reported. The considerable enhancement this experiment provides for extracting rotational and translational diffusion rates, as well as orientational ordering parameters by means of ESR spectroscopy, is demonstrated. The 2D spectral analysis is achieved using theoretical simulations that are fit to experiments by an efficient and automated nonlinear least squares approach. These methods are applied to dispersions of 1-palmitoyl-2oleoyl-sn-glycerophosphatidylcholine (POPC) model membranes utilizing spin labels 1-palmitoyl-2-(16-doxyl stearoyl) phosphatidylcholine and the 3-doxyl derivative of cholestan-3-one (CSL). Generally favorable agreement is obtained between the results obtained by 2D-FT-ESR on vesicles with the previous results on similar systems studied by continuous wave (cw) ESR on aligned samples. The precision in determining the dynamic and ordering parameters is significantly better for 2D-FT-ESR, even though the cw ESR spectra from membrane vesicles are resolved more poorly than those from well aligned samples. Some small differences in results by the two methods are discussed in terms of limitations of the methods and/or theoretical models, as well as possible differences between dynamic molecular structure in vesicles versus aligned membranes. An interesting observation with CSL/POPC, that the apparent homogeneous linewidths seem to increase in "real time," is tentatively attributed to the effects of slow director fluctuations in the membrane vesicles.

Electron Paramagnetic Resonance Investigation of Stratum Corneum Lipid Structure

“…The orienting potential in a lipid bilayer, U(X), determines the orientational distribution of molecules with respect to the local ordering axis of the membrane bilayer. It can be expressed as an expansion in generalized spherical harmonics [17,18],…”

Multifrequency Two-Dimensional Fourier Transform ESR: An X/Ku–Band Spectrometer

“…In particular, the increased ESR experiments over a broad range of frequencies would sensitivity of 2D FT ESR correlation spectroscopy to molecprovide greater insight into the subtle details of motional ular ordering and dynamics has made it possible to extend dynamics that could not be reliably extracted in an experiour knowledge about the dynamic molecular properties of ment at a single frequency. liquid crystals (3,4), polymers (5), and membrane vesicle dispersions (6), including peptide-lipid interactions (7).…”

On the Use of the Phase Memory TimeT2for the Quantitative Characterization of the Rotational Motions of Proteins in Lipid Bilayer Systems

“…In this orientation the spin autopeaks in the given Fourier spectra bear a strong resemHamiltonian exhibits an optimal senitivity to the rotational blance to the experimental spectra reported by Freed and comotions of the protein. Two possible modes of motions workers (3,18). The spectra show a marked hyperfine strucwithin the cone are considered below.…”

“…size Du h , where h denotes the x , y , or z axis. Note that The p/2 hard pulses were taken to be 5 ns long, correisotropic motion results when Du h is identical for all three sponding to experimental situations (18), and were divided axes. Any molecular reorientation involves rotations about into time slices of 0.25 ns for the computation of the echo all three axes.…”

“…The first question to be addressed is whether p/2 pulses and co-workers in studies of the behavior of CSL in lipid bilayers (18).…”