Vibrational layer eigenmodes of binary phospholipid-cholesterol bilayers at low temperatures

Raman spectra in the low-frequency spectral range-between 5 and 90cm^{-1}-were studied for multilamellar bilayers prepared with cholesterol (Chol) and phospholipids of three different types: doubly unsaturated lipids 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), monounsaturated lipids 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and fully saturated lipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The narrow peak seen below 250 K and positioned between 9 and 18cm^{-1}-depending on the system and temperature-was attributed to the vibrational eigenmode of a lipid monolayer. For the DOPC-Chol bilayer, the peak position and the peak width were found to monotonically increase and decrease, respectively, with the Chol concentration. For POPC-Chol and DMPC-Chol bilayers, these parameters revealed nonmonotonic concentration dependences, with an apparent minimum at the intermediate Chol content. The peak intensity was ascribed to interleaflet coupling. As in the literature, a coexistence of liquid-ordered and solid-ordered domains was suggested for the DMPC-Chol and POPC-Chol bilayers; the Chol concentration dependences of Raman peak parameters were discussed in line with this suggestion, under the assumption that the different composition of coexisting domains conserves upon cooling. We demonstrated that the obtained Raman data disagree with the suggested domain coexistence if the domain sizes are substantially larger than the lipid layer thickness.