Induced Changes in Solvent Structure by Phospholipid Monolayer Formation at a Liquid:Liquid Interface

摘要

Vibrational sum frequency spectroscopy has been used in conjunction with dynamic surface tension measurements to study formation of a 1,2-dilauroyl- sn-phosphatidylcholine (DLPC) monolayer at a water:carbon tetrachloride interface. Surface tension measurements show that an aqueous solution of liquid crystalline phosphocholine vesicles (4.5 micro Molar DLPC) requires several hours to form a tightly packed, fully equilibrated monolayer of DLPC monomers. Vibrational spectra of the interfacial region at different stages in the monolayer formation process indicate that the solvent environment undergoes dramatic changes as the monolayer forms. Adsorption of the initial DLPC monomers severely disrupts the interfacial solvent structure. Intensity in the water stretching region oscillates in a systematic fashion during the first two hours of monolayer formation before finally leveling out at an intensity characteristic of the fully equilibrated monolayer. Frequency shifts of the OH stretching vibration show that water molecules with their C2 axes aligned parallel to the interface experience a markedly different environment than those water molecules aligned perpendicular to the interface. This difference is attributed to the effect of the adsorbed, zwitterionic DLPC headgroups which, if aligned parallel to the interface, can stabilize in-plane water molecules.