Membrane structure is inherently complex, and understanding the structure of peptide chains inserted in membranes is vitally important for the development of effective diagnosis and treatments of diseases.  We present progress toward applying Surface Enhanced Raman Spectroscopy (SERS) as a tool for studying lipid membrane structure.  Optical excitation of gold nanoparticles at their size and shape-dependent plasmon resonant frequency induces strong oscillations of the nanoparticle's free electron gas, leading to SERS � an enhancement of Raman scattering signals in a distance dependent manner at the molecular scale. This project utilizes gold nanorods, tuned to an excitation laser wavelength of 785 nm, as a substrate for lipid membranes so that their structure can be analyzed by SERS.  This project aims to displace the surfactant cetyltrimethylammonium bromide (CTAB) that stabilizes nanorods with the biologically relevant lipid dioleoylphosphatidylcholine (DOPC), and to observe the insertion of the peptide melitin into the lipid membrane using SERS for confirmation. Gold nanorods were incubated in solutions of DOPC much higher than CTAB to cause direct molecular substitution. SERS spectra clearly indicate a complete substitution based on the reduction and absence of the 760 cm-1 peak of the CTAB headgroup and the introduction of the 719 cm-1 peak of the DOPC headgroup. The continued presence of a peak at 180cm-1 indicates that the bromide ions remain bound to the nanorod surface, serving as the intermediary between the chemically similar DOPC choline group and the CTAB trimethylammonium group.  Upon addition of melittin to the lipid coated nanorods, SERS spectra show the introduction of the amide I peak at 1650cm-1 indicating the presence of the peptide.  Initial observations give us confidence that a complete and well formed lipid bilayer is present at the surface of the nanorods and the peptide melittin is close enough to provide a SERS signal.

Membrane structure from surface enhanced Raman spectroscopy

247th National Meeting (2014)

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