The use of wide-line [ 14N]nitrogen NMR as a probe in model membranes

Abstract

1. Introduction A major aim of bioiogi~al membrane research is to understand the relationship between membrane struc- ture and function at the molecular level. The presently accepted fluid mosaic model [ 1] indicates that the membrane is basically an anisotropic fluid composed of a lipid bilayer in which proteins are embedded. At the molecular level it is necessary to describe both the average conformation of the molecules and their molecular dynamics. There is both translation and rotation of the lipid and protein molecules within the bilayer structure. For the lipids in particular, there are many internal degrees of freedom. Of the many techniques [Z] being used to probe the molecular environment within membranes, nuclear magnetic resonance (NMR) [3,4] has proven to be one of the most useful methods for investigating the dynamical state of the membrane. Work has con- centrated on describing the dynamical behaviour of the lipid molecules and the interaction of these molecules with other membrane components such as various proteins [5,6] and cholesteroi [7,8]. To do this, it is essential to probe various positions in the lipid molecules. Because of the overabundance of hydrogen atoms, proton NMR spectra have been difficult to interpret [9,10] and give little information about the motion of a single segment of the lipid molecule. 3’P NMR [l I] has been successfully used to probe the head group region of phospholipids. Selective incorporation of ‘II [12], 13C [ 131 and tgF [14] into lipid molecules has been the most fruitful method of probing the molecular dynamics. However, considerable time and expertise are necessary for the synthesis of the labelled molecules required for a complete investigation of the lipid dynamics. The use of naturally abundant NMR probes is there- fore advantageous whenever possible. One naturally