Lipid Compositions Modulate Fluidity and Stability of Bilayers: Characterization by Surface Pressure and Sum Frequency Generation Spectroscopy

Cell membranes are crucial to many biological processes. Because of their complexity, however, lipid bilayers are often used as model systems. Lipid structures influence the physical properties of bilayers, but their interplay, especially in multiple-component lipid bilayers, has not been fully explored. Here, we used the Langmuir-Blodgett method to make mono- and bilayers of 1,2-dihexadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), and 1-hexadecanoyl-2-(9Z-octadecenoyl)-sn-glycero-3-phospho-L-serine (POPS) as well as their 1:1 binary mixtures. We studied the fluidity, stability, and rigidity of these structures using sum frequency generation (SFG) spectroscopy combined with analyses of surface pressure-area isotherms, compression modulus, and stability. Our results show that single-component bilayers, both saturated and unsaturated, may not be ideal membrane mimics because of their low fluidity and/or stability. However, the binary saturated and unsaturated DPPG/POPG and DPPG/POPS systems show not only high stability and fluidity but also high resistance to changes in surface pressure, especially in the range of 25-35 mN/m, the range typical of cell membranes. Because the ratio of saturated to unsaturated lipids is highly regulated in cells, our results underline the possibility of modulating biological properties using lipid compositions. Also, our use of flat optical windows as solid substrates in SFG experiments should make the SFG method more compatible with other techniques, enabling more comprehensive future surface characterizations of bilayers.