Structural reorganizations in lipid bilayer systems: effect of hydration and sterol addition on Raman spectra of dipalmitoylphosphatidylcholine multilayers

Abstract

Vibrational Raman spectroscopy was used to investigate the conformational behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers perturbed by cholesterol and water, 2 membrane components whose lipid interactions involve different regions of the bilayer matrix. Upon the addition of cholesterol, an intrinsic membrane constituent, to an anhydrous bilayer in concentrations varying from 7 to 30 mol %, modifications in lateral chain interactions were observed by monitoring spectral changes in the methylene C-H stretching and the CH2 deformation regions. The perturbation in the 1460-cm-1 region was not spectroscopically observed until after the addition of 7 mol % of the sterol. Although chain-chain interactions are altered, no additional trans/gauche isomerization is developed along the hydrocarbon chains. Water, a peripheral bilayer component, was added to the multilayer assembly in the hydration range of 0.3 to .apprx. 4 molecules of water/lipid molecule. Vibrational spectra characteristic of motions in the head-group, interfacial and acyl chain regions of the lipid bilayer were observed. Hydration confers a mobility to the head-group, glycerol and carbonyl moieties. Shifts in the CN symmetric and PO2- antisymmetric stretching modes, occurring on the addition of approximately 4 molecules of water, indicate a conformational rearrangement within the polar head group. After .apprx. 4 water molecules are added to the DPPC system, the spectral features of the gel system [70% (wt/wt) water] indicate that no further head-group changes nor increases in either acyl chain trans/gauche or lattice disorder arise on further hydration.