The structure of lipid bilayers adsorbed on activated carboxy-terminated monolayers investigated by sum frequency generation spectroscopy

The formation and structure of isotopically asymmetric supported bilayer membranes (SBMs) has been investigated using sum frequency generation (SFG) vibrational spectroscopy supplemented by reflection absorption infrared spectroscopy (RAIRS). The bilayers were composed of a proximal and distal leaflet of the phospholipid dipalmitoyl phosphatidylethanolamine (DPPE) supported on a gold surface. The proximal leaflet was chemically tethered to the gold via an 11-mercapto-undecanoic acid (MUA) self-assembled monolayer (SAM) that had been chemically modified to produce an activated succinimidyl ester headgroup using N-hydroxysuccinimide (NHS) and N-(3-dimethylaminopropyl)- N′-ethylcarbodiimide (EDC). The activation of the MUA and the tethering of the DPPE were monitored and confirmed using SFG and RAIRS. The distal leaflet of the bilayer was added using either vesicle fusion (VF) or Langmuir-Blodgett (LB) deposition. To gain insight into the structure of each layer of the SBM perdeuterated DPPE (d-DPPE) and MUA (d-MUA) were used to create SBMs with a layer that was isotopically distinguishable from the rest. The polar orientation and conformational ordering of the lipids was determined using SFG. It was found that the tethering of the proximal lipid leaflet resulted in an increase in the conformational order of the MUA SAM. Furthermore, by careful analysis and comparison of spectra recorded in both the C-H (2800-3000 cm^-1) and C-D (2000-2300 cm^-1) stretching regions it was concluded that a better ordered and more biologically relevant lipid bilayer was formed when the distal leaflet was added using LB deposition. On the other hand the SFG spectra of the SBMs in which the distal leaflet was added by VF showed little evidence of conformational ordering on the time scale of minutes, suggesting the presence of an incomplete monolayer or of multilayer formation.