The composition of lipid membranes, similar to those that surround living cells, can now be mapped at the nanometer scale. The work, by researchers at Stanford University, the Lawrence Livermore National Laboratory and UC Davis, is published in the Sept. 29 issue of the journal Science.
All living cells are wrapped in a double-layered membrane of fatty lipid molecules. Components of the membrane can move sideways and organize into patches or other structures. This organization can affect, for example, important cell functions and vulnerability to viruses.
But it is very difficult to study these structures because they are so small, measured in tens of nanometers, said Marjorie Longo, professor of chemical engineering and materials science at UC Davis. A nanometer is a billionth of a meter, or about a thirty-millionth of an inch. Scientists want to address questions such as how dynamic or active the membrane is and how small the lipid patches are, she said. An atomic force microscope, which uses a fine needle to probe surfaces, can give a contour map of the surface but without chemical information.
The research group, led by Steven Boxer of Stanford University, used a highly focused beam of charged particles to scan the surface of artificial lipid membranes containing lipid patches developed in Longo’s lab. Components of the membrane were previously labeled with heavy isotopes of carbon and nitrogen, mounted on silicon wafers and flash-freeze-dried to preserve structure.
Fragments blasted away by the beam were caught and analyzed, reconstructing the chemical composition of the surface. The process is called Secondary Ion Mass Spectrometry or SIMS. The NanoSIMS instrument, located at the Livermore lab, is one of a handful of its kind in the world, Longo said.
A comparison of AFM and NanoSIMS on the same sample showed that both methods saw the same structures, and NanoSIMS yielded extra information about chemical composition.
Ultimately, the researchers aim to look at actual cell membranes. The work grew out of collaborations between Stanford, Livermore and UC Davis through the Center for Polymer Interfaces and Macromolecular Assemblies, and was funded by the National Science Foundation, National Institutes of Health and the U.S. Department of Energy.