A comparison of spin probe ESR, 2H- and 31P-nuclear magnetic resonance for the study of hexagonal phase lipids

Abstract

We have investigated the feasibility of the various possible magnetic resonance probes of lipids which form non-bilayer phases. As a model system we have used equimolar mixtures of phosphatidylethanolamine (PE) and cholesterol, which exhibit a thermotropic transition from a bilayer to a hexagonal phase. Variable temperature electron spin resonance (ESR) spin probe spectra were obtained using random dispersion and oriented lipid systems. Simultations of the ESR spectra were performed in order to aid in the interpretation of the experimental results for the oriented system. 31P- and 2H-nuclear magnetic resonance (NMR) studies were carried out using a deuterated PE. The ESR spin probes in the random dispersions show essentially no effect attributable to the phase transition. However, there are large, reversible effects in the temperature-dependent behaviour for the oriented system. The orientation dependence of the spectra above the transition temperature indicate that the hexagonal phase lipids may spontaneously assume a macroscopic organization on a flat surface. We find, however, that such an organization cannot be unambiguously assigned from the ESR spin probe spectra, and point out a potential difficulty in the interpretation of spin probe spectra in oriented systems. In contrast, the 2H-NMR method provides a reliable monitor of the phase transformation. Taken together, the 2H and 31P data indicate that the structure of the headgroup in PE is quite similar in both the bilayer and hexagonal phase. 2H-NMR should be very useful in probing the structural and dynamic characteristics of lipids in non-bilayer phases.