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

Abstract

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.

A narrow amide I vibrational band observed by sum frequency generation spectroscopy reveals highly ordered structures of a biofilm protein at the air/water interface

“…Vibrational SFG spectroscopy is a non-invasive, label-free, and surface-selective vibrational spectroscopic method, 13 which has been used to study biomolecules at interfaces. 14, 15 Since amide I bands are characteristic of protein secondary structures, 16 we first acquired the conventional (achiral) SFG spectrum of BslA in the amide I vibrational region (1600–1700 cm −1 ) using the ssp polarization setting ( s -polarized SFG, s -polarized visible, and p -polarized infrared) (setup shown in ESI). …”

Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces

“…The surface area per lipid was estimated to be ∼44 Å 2 and ∼52 Å 2 for DPTAP and DPPG, respectively. 41 , 42 The prepared samples were equilibrated for at least 40 min. For both HD-SFG and HD-SHG measurements, the trough was rotated to avoid the lipid monolayer distortion due to heat accumulation.…”

Kinetics of peptide folding in lipid membranes

“…In addition, by using a photoresponsive structural constraint, it is possible to initiate a folding or unfolding process from a well‐selected conformational state. To improve the structural resolution of folding kinetic studies, we believe that the following approaches, many of which have already been used to study the equilibrium structures of membrane peptides and proteins, are promising: using (1) a local vibrational mode, such as that arising from an isotopically‐labeled amide unit or an unnatural amino acid side chain, , and resonance Raman scattering of a Trp probe, to gain site‐specific structural and environmental information; (2) the coupling between two site‐specific vibrators to probe tertiary interactions; (3) multiple fluorophores to simultaneously probe conformational changes of the protein of interest in different regions; (4) multi‐fluorophore FRET to reveal correlated conformational motions; and (5) various nonlinear techniques, such as two‐dimensional infrared, second harmonic generation, and sum frequency generation spectroscopy.…”