The challenges to reveal the molecular organization and interactions at the biological and atmospheric relevant interfaces were confronted in this dissertation by using vibrational sum frequency generation (VSFG) spectroscopy. In particular, the interfaces of biological membrane represented by model phospholipid monolayers, and the aqueous organosulfur species (dimethyl suloxide, DMSO and methanesulfonic acid, MSA) are studied.
A condensing effect is observed for the model phospholipid (dipalmitoylphosphatidylcholine, DPPC) monolayer on concentrated DMSO subphases. When the DMSO molecules interact with the phospholipid membranes, DMSO molecules squeeze aside the phospholipids, which cause them to form tightly packed domain structures as well as causing the membrane to expand. In addition, the miscibility of DMSO with water and its powerful solvation of many substances make the formed pores a transportation corridor across the membrane, which as a result accounts for the enhanced permeability of membranes upon exposure to DMSO. Similar effects were found through “in-situ” Brewster angle microscopy (BAM) on dipalmitoylphosphatidyl ethanolamine (DPPE), glycerol (DPPG), and serine (DPPS) phospholipids, indicating that the condensing effect is not dependant upon the phospholipid headgroup structure.
Novel structural features of water confined in phospholipid monolayers are revealed. At the air/D2O/monolayer interface, the dangling OD stretching mode showed a marked frequency red-shift as well as spectral structure upon increasing the monolayer surface coverage. Furthermore, the dangling OD was found to exist even when a D2O surface was fully covered by the lipid molecules. This phenomenon was observed in monolayers formed with DPPC and with palmitic acid. The frequency red-shift of the dangling OD is interpreted to be due to the perturbation imposed by the lipid hydrophobic tail groups.
In addition, phase sensitive vibrational sum frequency generation is employed to investigate the ordering of water at phospholipid/water interfaces. Interfacial water molecules are found to be oriented preferentially by the electrostatic potential imposed by the phospholipids and have, on average, their dipole pointing towards the phospholipid tails for all phospholipids studied. Zwitterionic DPPC and DPPE reveal weaker water orienting capability relative to net negative DPPA, DPPG, and DPPS. Binding of calcium cations to the lipid phosphate group reduces ordering of the water molecules.
Besides biological interfaces, a comprehensive investigation of the molecular organization at the aqueous DMSO/MSA surface has been made. Surface reorganization causes a decrease in the dangling OH fraction at the surface and further confirms the strong surface propensity of DMSO and MSA. Both DMSO molecules and MSA molecules are found to have their CH3 groups pointing outwards into the air but are differ in their tilt angle.
For surface DMSO molecules, the S=O group at surface forms strong hydrogen bond with water, which results in reorientation of interfacial water molecules with their hydrogens pointing up towards the S=O group. MSA molecules completely dissociate into hydrated ions at low concentrations (< 0.1 x). The interfacial water structure is therefore found to be affected by both the methanesulfonate anions and the hydronium ions residing at the surface.