Quantifying Solute Partitioning in Phosphatidylcholine Membranes.

Time-resolved fluorescence measurements were used to characterize and quantify solute partitioning into 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid vesicles as a function of solute concentration and temperature. The solutes, coumarin 152 (C152) and coumarin 461 (C461), both belong to a family of 7-aminocoumarin dyes that have distinctive fluorescence lifetimes in different solvation environments. The two solutes differ in the 4-position where C152 has a trifluoromethyl group in place of C461's -CH3 group. In vesicle containing solutions, multiexponential fluorescence decays imply separate solute populations in the aqueous buffer, solvated in the vesicle headgroup region and solvated in the acyl chain bilayer interior, respectively. Fluorescence amplitudes, corrected for differences in radiative rates, are used to calculate absolute partition coefficients and average number of solutes per vesicle as a function of coumarin:lipid ratio and average number of solutes per vesicle. Results show that C152 has an ∼10-fold greater affinity than C461 for lipid bilayers, despite both solutes having similar hydrophobicities as inferred from their log(P) values. Temperature-dependent partitioning data are used to calculate enthalpies and entropies of C152 partitioning as a function of concentration. These values are used to extrapolate to the infinitely dilute limit. Above and below the lipid gel-liquid crystalline temperature, partitioning is exothermic with negative changes in entropy. In the vicinity of the transition temperature, these quantities change sign with ΔHpart becoming endothermic (+70 kJ/mol) and entropically favored (ΔSpart = +240 J/(mol·K)).