Deuterium NMR study of lipid organisation in Acholeplasma laidlawii membranes

Abstract

THE most convincing studies of molecular order in model membranes are undoubtedly those involving deuterium nuclear magnetic resonance (NMR) of labelled lipids in the lamellar liquid crystalline phase^1–6. The relevance of these results to biological membranes has not yet been established, however; there is no a priori reason to expect the state of organisation of lipids in the lamellar liquid crystal to resemble closely that of a natural membrane. We report here the first detailed study of acyl chain order in the lipids of a natural biological membrane—the plasma membrane of the microorganism Acholeplasma laidlawii. Specifically deuterated fatty acids were incorporated biosynthetically into the membrane lipids as described previously⁷. The ²H-NMR spectra of the membrane demonstrate the co-existence of at least two different kinds of lipid. Signals from the more fluid lipid are most prominent at temperatures near and above the growth temperature (37 °C). The more solid-like lipid, which predominates at lower temperatures, may be similar to the gel state of lamellar phases and could include lipid associated with the membrane protein. For the more fluid phase, the plot of order parameter against position of deuteration strongly resembles that for egg yolk² and dipalmitoyl³ phosphatidylcholine. Incorporation of cholesterol into the membrane of A. laidlawii increases the average acyl chain order and also increases the spread of order parameters, similar to its effect on egg yolk phosphatidylcholine⁶. These results represent a major justification for the use of lamellar liquid crystals as models for the lipid in biological membranes.