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

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 2 M) 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.