Lipid-Gramicidin A' Interactions: Dynamic Structure of Boundary Lipid by 2D-ELDOR

Lipid-protein interactions have been a topic of major interest in many studies. The general picture that emerges is that membranes consist of a protein-containing lipid bilayer, where the membrane proteins are inserted such that their hydrophobic regions penetrate the lipid membrane thus fixing them to the bilayer. These proteins serve as receptors on the cell surface, being responsible for several cell functions such as signal transduction and ion transport. Although much is known about the dynamic structure of lipids in membranes, the lipid-protein interface is relatively less known. Several studies show that the lipids in protein-containing membranes can be divided in two classes: the bulk lipids and the boundary lipids, which are in close contact with the protein. ESR has been widely applied to investigations on membrane properties. One major limitation of conventional ESR studies on membranes has been their limited resolution. The two-dimensional Fourier Transform technique of 2D-ELDOR has proved to have enhanced spectral resolution to ordering and dynamics compared to conventional ESR. We have employed 2D-ELDOR to characterize the boundary lipid in equimolar mixtures of DPPC and GA. Gramicidin A' (GA) is a well-known peptide for studies of lipid-protein interactions. 2D-ELDOR provides a reliable and convenient way of studying the dynamic structure of lipids. The 2D-ELDOR spectra of the spin label 16-PC in 1/1 DPPC/GA vesicles is composed of two components, which are assigned to the bulk (sharp auto-peaks and cross-peaks) and to the boundary (broad auto-peaks) lipids, (cf. right Figure) The spectral fits of the experimental data indicate a regime of fast motions and very low ordering for the boundary lipids, whereas the boundary lipids show very high “y-ordering” and somewhat slower motions than the bulk lipids. The “y-ordering” represents a dynamic bending at the end of the boundary lipid acyl chain, (cf. left Figure) which can then coat the GA molecules.