Credit: © 2007 APS

Under physiological conditions, biological lipid membranes interact with positively charged metal cations — such as sodium — to form lipid–ion networks. These interactions can influence the structure and stability of the membranes and are important in biological processes. Previous studies have described how cations interact with multiple negatively charged groups on the lipid, but imaging them directly is a challenge.

Now, Suzanne Jarvis and colleagues1 at Trinity College in Ireland have used atomic force microscopy (AFM) to image lipid-ion networks with sub-Ångstrom resolution. A model membrane composed of negative and positive head groups was deposited on mica and immersed in a physiologically relevant solution. The membrane was then imaged with an AFM cantilever driven at constant frequency and amplitude. Two main structures were found — uniformly arranged protrusions separated by 0.3 nm, which are attributed to the lipid head groups, and non-uniform subunits, which represent the average positions of mobile ions that are interacting between neighbouring head groups.

The relative occurrences of these structures depend on ion concentration and more studies are needed to fully quantify the effect of ion binding on the lipid structure. Nevertheless, this technique is a powerful tool that can be used to investigate the structure and dynamics of biological systems on very small length scales.