The effect of aliphatic alcohols on fluid bilayers in unilamellar DOPC vesicles - A small-angle neutron scattering and molecular dynamics study

Small-angle neutron scattering and coarse-grained molecular dynamics simulations have been used to determine the structural parameters (bilayer thickness D, polar region thickness D-H, interfacial lateral area of the unit cell A(UC) and alcohol partial interfacial area A(CnOH)) of fluid dioleoylphosphatidylcholine:dioleoyl phosphatidylserine (PCPS, DOPC:DOPS = 24.7 mol:mol) bilayers in extruded unilamellar vesicles with incorporated aliphatic alcohols (CnOH, n = 8-18 is the even number of carbons in alkyl chain). External (H2O)-H-2/H2O contrast variation experiments revealed that D-H decreases as a function of alkyl chain length and CnOH:PCPS molar ratio. Using measurements at single 100% (H2O)-H-2 contrast we found that (i) D decreases with CnOH:PCPS molar ratio and increases with CnOH chain length (at 0.4 molar ratio); (ii) A(UC) significantly increases already in the presence of shortest CnOH studied (at 0.4 molar ratio), further increase is observed with longer CnOHs and at higher molar ratios: (iii) A(CnOH) of alcohol molecules in PCPS bilayer increases linearly with the alkyl chain length, A(CnOH) obtained for CnOHs with n <= 10 corresponds to A(CnOH)<= 20 angstrom(2) - a value specific for the crystalline or solid rotator phase of alkanes. All these structural modifications induced by studied CnOHs were reproduced in MD simulations. The computational results give an accurate description of the alcohol effects at the molecular level, explaining the experimental data. The anomaly in A(CnOH) is discussed via the umbrella effect described for cholesterol. (C) 2011 Elsevier B.V. All rights reserved.