Quantitative Analysis of Supported Membrane

Composition using the NanoSIMS
Mary L. Krafta, Simon Foster Fishela, Carine Galli Marxera,
Peter K. Weberb, Ian D. Hutcheonb, Steven G. Boxer'*
aDepartment of Chemistry, Stanford University, Stanford, CA 94305-5080
bLawrence Livermore National Laboratory, Livermore, CA 94551
*Corresponding author. Tel: 650-723-4482; fax: -4817; sboxer@stanford.edu
Abstract
We have improved methods reported earlier [1] for sample preparation, imaging and
quantifying components in supported lipid bilayers using high-resolution secondary ion mass
spectrometry performed with the NanoSIMS 50. By selectively incorporating a unique stable isotope
into each component of interest, a component-specific image is generated from the location and
intensity of the unique secondary ion signals exclusively produced by each molecule. Homogeneous
supported lipid bilayers that systematically varied in their isotopic enrichment levels were freeze-dried
and analyzed with the NanoSIMS 50. The molecule-specific secondary ion signal intensities had an
excellent linear correlation to the isotopically labeled lipid content. Statistically indistinguishable
calibration curves were obtained using different sample sets analyzed months apart. Fluid bilayers can
be patterned using lithographic methods and the composition of each corralled region varied
systematically by simple microfluidic methods. The resulting composition variations can be imaged
and quantified. This approach opens the possibility of imaging and quantifying the composition of
microdomains within membranes, including protein components, without using bulky labels and with
very high lateral resolution and sensitivity.
Keywords: lipid, bilayer, SIMS, NanoSIMS, mixture, gradient, compositional analysis
1.    Introduction
1.1   Motivation
The lipid bilayer is the basic assemblage common to all biological membranes. In addition to
providing the principle structural motif for biological membranes, lipids are also actively involved in
numerous functions, including recognition and processing of lipid head groups and side chains, and
membrane fusion during viral entry, exo- and endocytosis. However, the majority of active functions
associated with membranes involve integral membrane and membrane-anchored proteins.
The classic fluid-mosaic model suggests that all membrane components are freely mixing.
During the past few years there has been an explosion of interest in the lateral associations and
organization of membrane components that expand this concept with important functional
consequences. Different lipid and protein components within membranes exhibit lateral (2D) mobility,
in some cases over the entire membrane surface, whereas in others, lateral mobility is confined to
domains whose origin is not well understood. The concept of microdomains within a membrane,