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 A detailed look at membrane composition. In the NanoSIMS experiments reported by Kraft et al., a focused beam of ions bombards the sample, releasing a barrage of small ions that are analyzed with mass spectrometry. By scanning over the bilayer membrane sample, a high-resolution (about 70 to 100 nm) image of its chemical composition is generated. Pic: Groves, Science 2006 Vol. 313 p.1901
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.
Related Links:
Kraft et al., "Phase Separation of Lipid Membranes Analyzed with High-Resolution Secondary Ion Mass Spectrometry"
Science 29 September 2006:
Vol. 313. no. 5795, pp. 1948 - 1951
DOI: 10.1126/science.1130279
Jay T. Groves, "Unveiling the Membrane Domains"
Science 29 September 2006:
Vol. 313. no. 5795, pp. 1901 - 1902
DOI: 10.1126/science.1133760
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Experimental Methods & Techiques 
