Solvent motion near phospholipid interfaces

Abstract

The solvation dynamics near lipid/water interfaces has been determined via ultrafast time-resolved emission spectra (TRES). The lipid interfaces consisted of reverse micelles or vesicles formed with phosphatidylcholine (lecithin). These measurements represent the first determination of solvent motion on a subpicosecond timescale at a lipid interface. Because free water relaxes on a subpicosecond timescale, these measurements provide the most complete picture to date of the solvent motion at a lipid interface. Using the fluorescent probe Coumarin 343 (C343), solvation dynamics within the aqueous interior of cyclohexane/lecithin/water reverse micelles was measured as a function of hydration. Small additions of water to these reverse micelles produce a viscous gel, otherwise known as organogels or "living polymers". A single relaxation component is observed at the w = 4.8 hydration level with a response time much longer than the response of free water. At hydration levels of w ≥ 5.8, three relaxation times are observed indicating the existence of three water types that change in response to increasing hydration. To probe other lipid interfaces, a fluorescently-Iabled lipid (CLPE) was synthesized by covalently attaching C343 to the phosphatidylethanolamine (PE) headgroup. CLPE proved to be a viable solvation dynamics probe, reproducing the dynamics that were previously observed with C343 in w = 6.4 cyclohexane/lecithin/water reverse micelles. Solvent motion within the aqueous interior of reverse micelles as a function of micelle morphology is also investigated. Spherical reverse micelles formed in benzene are compared to worm-like reverse micelles formed in cyclohexane at the same hydration level. The results demonstrate that the spherical reverse micelles display significantly faster relaxation. It is proposed that spherical reverse micelles allow for the formation of a defined water core that is a prerequisite for free-water type relaxation. Finally, the solvation dynamics at a vesicle interface is determined with and without 33 mol% cholesterol present. Surprisingly, the dynamics measured at a vesicle interface are very similar to the dynamics measured within w = 6.4 cyclohexane/lecithin/water reverse micelles. Cholesterol shows little influence on the types of relaxation at the interface, but does reduce the overall solvent response indicating cholesterol tightens the interface.