Molecular and component volumes of saturated n-alkanols in DOPCÂ +Â DOPS bilayers

The volumetric properties of fluid bilayers consisting of dioleoylphosphatidylcholine (DOPC, 96 wt%) and dioleoylphosphatidylserine (DOPS, 4 wt%) with incorporated saturated n-alkanols (CnOH, n = 10-16 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry. The mixing of DOPC and DOPS was found to be ideal and the molecular volumes of pure lipids VDOPC and VDOPS are additive in mixed bilayers. The increase of VDOPS with temperature was steeper than that of VDOPC as quantified by significantly higher coefficient of isobaric thermal expansivity γ. This difference is supposed to be related to the high thermal expansivity of the serine headgroup. The molecular volumes of lipids and CnOH (VCnOH) in fluid DOPC + DOPS bilayers are constant and additive within error limits for n = 12 and 16 and for the CnOH mole fractions in the lipid bilayer xAL ≤ 0.5 and xAL ≤ 0.3, respectively. In homologous series of CnOHs incorporated into DOPC + DOPS bilayer, the VCnOH volume increases linearly with the CnOH chain length n at xAL = 0.29. From this dependence obtained at several temperatures, the component volume of the CnOH hydroxyl group VOH was calculated and found to decrease between 20 °C and 51 °C while the component volume of the CnOH methylene group VCH2 increases with temperature. The excess molecular volume, ΔV, for the transfer of CnOH into the DOPC + DOPS bilayers display a biphasic dependence on the alkyl length, n, with a minimum for n = 12. When calculated with respect to the pure CnOH, ΔV is negative for homologs with n > 8.