THERMODYNAMICS OF TRANSFER OF INDOCARBOCYANINES FROM GEL TO FLUID PHASES OF PHOSPHOLIPID-BILAYERS

Application of the regular solution model to thermal transition data obtained by differential scanning calorimetry has allowed the determination of partition coefficients, K-p, and the thermodynamics of transfer of a series of indocarbocyanine solutes between the gel and fluid phases of phospholipid bilayers. The indocarbocyanimes with alkyl chain lengths of 12 to 22 carbons were partitioned between dipalmitoyl- and distearoylphosphatidylcholine phases at the transition temperatures of the gel-liquid-crystal phase transition. The results indicate that as the alkyl chain length of the solute nears that of the acyl chains of the bilayer lipid, the free energy of transfer from gel to fluid is least negative, and the enthalpy of transfer is most positive. There is almost complete entropy-enthalpy compensation in the transfer process. Comparison of the partition coefficients with published values determined by a fluorescence method show good agreement when the differences in temperature of the measurements are accounted for.