Librational Dynamics of Spin-Labeled Membranes at Cryogenic Temperatures From Echo-Detected ED-EPR Spectra

Methods of electron spin echo of pulse electron paramagnetic resonance (EPR) spectroscopy are increasingly employed to investigate biophysical properties of nitroxide-labeled biosystems at cryogenic temperatures. Two-pulse echo-detected ED-spectra have proven to be valuable tools to describe the librational dynamics in the low-temperature phases of both lipids and proteins in membranes. The motional parameter, alpha(2)tau(C), given by the product of the mean-square angular amplitude, alpha(2), and the rotational correlation time, tau(C), of the motion, is readily determined from the nitroxide ED-spectra as well as from the W-relaxation rate curves. An independent evaluation of alpha(2) is obtained from the motionally averaged N-14-hyperfine splitting separation in the continuous wave cw-EPR spectra. Finally, the rotational correlation time tau(C) can be estimated by combining ED- and cw-EPR data. In this mini-review, results on the librational dynamics in model and natural membranes are illustrated.