Temperature dependent solvation in phospholipid membranes

Abstract

Experiments described in this dissertation were designed to systematically investigate solute partitioning in phospholipid bilayers as a function of phospholipid identity, solute identity, membrane phase, and membrane composition. Experiments use time-resolved fluorescence, steady-state fluorescence, dynamic light scattering, and differential scanning calorimetry to experimentally quantify solute partitioning in three specific regions of a model membrane, as well as track how solutes migrate into and out of lipid bilayers as a function of temperature. Phosphatidylcholine vesicles were comprised of DLPC (12:0 PC), DMPC (14:0 PC), and DPPC (16:0 PC). In all three lipid systems, coumarin 152 (C152) showed partitioning behavior that was qualitatively similar but quantitatively different. Partitioning into a gel phase membrane was slightly exothermic and slightly entropically unfavorable. Partitioning of C152 near the lipid membrane melting temperature was entropically driven and endothermic. Well above the melting temperature, exsolvation of C152 from the membrane back into the aqueous buffer was enthalpically driven but entropically unfavorable. Regardless of solution temperature, relatively little (<20%) C152 partitioned into the hydrophobic core of the membrane. The magnitudes of the thermodynamic forces driving C152 partitioning systematically increased with alkyl chain length (DLPC < DMPC < DPPC). C152 and C461 differ solely in the 4-position where C152 has a trifluoro methyl group in place of C461's -CH3 group. Fluorescence amplitudes were used to calculate absolute partition coefficients and average number of solutes per DPPC vesicle. C152 shows a ~10-fold greater affinity than C461 for lipid bilayers, despite both solutes having similar log P values. Differential scanning calorimetry traces of vesicles composed of binary mixtures of lipids show moderate miscibility between DLPC and DMPC and low miscibility between DMPC and DMPE. Time-resolved fluorescence decays indicate C152 partitioning into mixed PC membranes is nearly ideal; that is, even if mixed PC vesicles do form single lipid domains, C152 partitioning behavior is largely unaffected. Time-resolved fluorescence decays show C152 partitioning behavior into PC/PE membranes is distinctly non-ideal, but the cause of this non-ideal behavior requires further studies.