Membrane Structure Studies by Means of Small-Angle Neutron Scattering (SANS)


The basic model for membrane structure - a lipid bilayer with imbedded proteins - was formulated 35 years
ago, however the detailed structure is still under active investigation using a variety of physical, chemical and
computational techniques. Every biologically active cell is encapsulated by a plasma membrane with most cells also
equipped with an extensive intracellular membrane system. The plasma membrane is an important boundary between the
cytoplasm of the cell and the external environment, and selectively isolates the cell from that environment. Passive
diffusion and/or active transport mechanisms are provided for water, ions, substrates etc. which are vital for cell
metabolism and viability. Membranes also facilitate excretion of substances either as useful cellular products or as waste.
Despite their complexity and diverse function, plasma membranes from quite different cells have surprisingly similar
compositions. A typical membrane structure consists of a phospholipid bilayer with a number of proteins scattered
throughout, along with carbohydrates (glycoproteins), glycolipids and sterols. The plasma membranes of most
eukaryotic cells contain approximately equal weights of lipid and protein, which corresponds to about 100 lipid
molecules per protein molecule. Clearly, lipids are a major constituent and the study of their structure and function in
isolation provides valuable insight into the more complex intact multicomponent membrane. The membrane bound
protein is the other major constituent and is a very active area of research for a number of reasons including the fact that
over 60% of modern drugs act on their receptor sites. The interaction between the protein and the supporting lipid
bilayer is clearly of major importance. Neutron scattering is a powerful technique for exploring the structure of
membranes, either as reconstituted membranes formed from well characterised lipids, or as intact membranes isolated
from selected biological systems. A brief summary of membrane structure will be followed by an outline of the neutron
scattering techniques used to understand membrane structure and dynamics. The emphasis will be on the small angle
neutron scattering technique since there is a very powerful instrument at Serpong, however brief mention of other
techniques will be included to demonstrate how a multidisciplinary approach is usually required
Keywords: neutron scattering, membrane structure, biology, SANS. R. B. Knottl,2;1Bragg Institute, ANSTO, Private Mail Bag, Menai NSW 2234, Australia
2CSIRO Minerals, Box 312, Clayton South VIC 3169, Australia