Label-free, Spectroscopic Detection of Leaflet Composition Asymmetry in Vesicles

Abstract: Vibrational sum frequency scattering (SFS) has been used to study sub-micron, catanionic vesicles in solution. The vesicles were synthesized from a binary mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylsulfate (SDS) surfactants in deuterated water, which spontaneously assemble into thermodynamically stable vesicles. The stability of these vesicles is attributed to a surfactant concentration asymmetry between the inner and outer bilayer leaflets. This concentration asymmetry should be observable by SFS due to local inversion symmetry-breaking. Signal corresponding to the symmetric sulfate stretch mode of the SDS head group is observed at 1044 cm, indicating that there is indeed asymmetry in the local structure of the leaflets. The results indicate that it should be possible to measure the interfacial structure of liposomes in aqueous solution and study in-situ processes like the binding of sugars and proteins that are important for many processes in biophysical chemistry.