Surface-Enhanced Raman Scattering as a Tool for Biomembrane Structure

Gold nanostructures focus light to the molecular length scale at their surface. The large optical intensity leads to surface-enhanced Raman scattering (SERS) from nearby molecules. SERS spectra contain information on molecular position and orientation relative to the surface but are difficult to interpret quantitatively. Here we describe a ratiometric analysis method that combines SERS and unenhanced Raman spectra with theoretical calculations of the optical field and molecular polarizability. The analysis was also applied to fluid phase phospholipid bilayers that contain tryptophan on the surface of gold nanorods. The lipid double bond was found to be oriented normal to the bilayer and 13 Å from the nitrogen atom. Tryptophan was found to sit near the glycerol headgroup region with its indole ring 43° from the bilayer normal. The potentiometric membrane probe di-4-ANEPPS has also been detected and its orientation characterized in the lipid bilayer. This new method can determine specific interfacial structure under ambient conditions, with microscopic quantities of material, and without molecular labels.