Partitioning of small alcohols into dimyristoyl phosphatidylcholine (DMPC)membranes: Volumetric properties

Volume changes associated with the partitioning of eight different alcohols and acetone into unilamellar vesicles of dimyristoyl phosphatidylcholine i n the fluid state was investigated by newly designed, automated titration d ensitometric equipment. The change in free volume of the partitioning proce ss, expressed by changes in the specific excess volume of the solutions, wa s positive for all solutes. The magnitude of the total volume change per mo le alcohol partitioned, DeltaV(m), was calculated by combining the volumetr ic data with previously published partitioning coefficients. For aliphatic 1-alcohols, DeltaV(m) increased gradually from 3 cm(3)/mol for ethanol to s imilar to 29 cm(3)/mol for 1-hexanol. Values of DeltaV(m) for the cyclic al cohols c-hexanol and benzyl alcohol were similar to3 times smaller than for aliphatic compounds of a similar molecular weight. The composition depende nce of the volumetric excess functions could be determined with sufficient accuracy to allow estimation of the partial specific excess volume of all t hree components (water, alcohol, and lipid). These functions elucidate the contribution of each component to the measured expansion of the system. The partial volume of vesicles and alcohols increased, whereas the volume of w ater was diminished as a result of the partitioning process. This result su ggests that the volume increase of the membrane-alcohol complex is larger t han the measured volume change of the whole system (DeltaV(m)) because the latter is partially compensated by contraction of the solvent. The results support a molecular picture in which adsorption of alcohols promotes a loos ening of the molecular packing and an increase in the water content of lipi d bilayer membranes.