Echo-Detected Electron Paramagnetic Resonance Spectra of Spin-Labeled Lipids in Membrane Model Systems

Abstract

The dynamics of spin-labeled lipid chains in the low-temperature phases of dipalmitoyl phosphatidylcholine (DPPC) membranes, with and without equimolar cholesterol, have been investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy. Echo-detected spectra from the two-pulse, primary spin-echo (pulse sequence: π/2-τ-π-τ-echo) are used to detect rapid angular motions, on the time scale of the phase memory time (T 2M ) that is in the nanosecond regime. Echo-detected spectra from the three-pulse, stimulated spin-echo (pulse sequence: π/2-τ-π/2-T-π/2-τ-echo) are used to detect slow angular motions, on the time scale of the spin-lattice relaxation time (T 1 ) that is in the microsecond regime. Spectra recorded at very low temperature (77 K) are used to correct the two-pulse echo spectra for instantaneous diffusion that arises from dipolar spin-spin interactions between different spin labels. Echo-detected inversion recovery spectra are used to correct the three-pulse echo spectra for intrinsic spin-lattice relaxation and large-scale spectral diffusion induced by nitrogen nuclear spin flips. The dependence of the echo-detected spectral line shapes on the two time delays, τ and T, can be simulated adequately by using a simple two-state model to represent the small-amplitude librational motions in the low-temperature membrane phases. The fast librational motion has isotropic character, no singly defined direction of the librational axis, and is segmental in nature, depending on chain position and also on the presence of cholesterol. The slow librational motion is of a more global, cooperative nature, being independent of chain position and cholesterol content.

Lipid Librations at the Interface with the Na,K-ATPase

“…These relaxation rates are averaged over the time interval from t 1 to t 2 , and are characterized by the maximum values, W L and W H , determined in the low-and high-field regions, respectively, of the ED-spectra. Calibration of the W L and W H relaxation rates in terms of the amplitude-correlation time product, ha 2 it c , of rotational motion from low-amplitude librations is taken from the results of spectral simulations (5,6,21). Here, a is the angular amplitude, t c is the correlation time, and angular brackets indicate an average over the librational motion.…”

“…The transmembrane sectors of the protein are interfaced with the lipid bilayer environment via a first shell of lipids with properties distinct from those of the bulk (1,2). Whereas the nanosecond rotational and conformational dynamics of the lipid chains interacting with transmembrane proteins are reasonably well studied (e.g., Marsh and Horváth (3)), little is known about their rapid librational motions such as are characterized for bilayer lipids by using pulse electron paramagnetic resonance (EPR) (4)(5)(6)(7). It is worthwhile to note that these equilibrium librational fluctuations, which persist to lower temperatures, are able to drive functionally important motions (i.e., on-pathway conformational transitions) at physiological temperatures, as embodied by the fluctuation-dissipation theorem (8,9).…”

Communication: Orientational self-ordering of spin-labeled cholesterol analogs in lipid bilayers in diluted conditions

“…The effect of instantaneous diffusion was employed to measure local concentrations of free radicals appearing in irradiated solids, [10][11][12] and was observed in different other applications, including studies of spin labels in biological systems, 13,14 transition metal ions in solids, 15 phosphorus donors in silicon. 16,17 The echo decay with increasing τ is induced by also spin relaxation of electrons and nuclei, and it depends on the electron spin concentration (due to relaxation of electron spins).…”

CW EPR, echo-detected EPR, and field-step ELDOR study of molecular motions of nitroxides ino-terphenyl glass: Dynamical transition, dynamical heterogeneity and β-relaxation

“…Spectra are free from the influence of so-called instantaneous diffusion that arises from spin-spin dipolar interaction according to the procedure proposed in ref. 38, where this influence is taken into account by measurement at low temperature (245 K) where temperature dependence is frozen. In Fig.…”

“…The explanation could be that the amplitude of motion is so high that the model is not valid. We tried the model which assumes angular jumps between two fixed positions [38], with a rectangular distribution of jump angles, which may be large. A satisfactory agreement has been achieved for different sets of the distribution width and times r c (data not shown).…”