Cholesterol and POPC segmental order parameters in lipid membranes: solid state¹H–¹³C NMR and MD simulation studies

Abstract

The concentration of cholesterol in cell membranes affects membrane fluidity and thickness, and might regulate different processes such as the formation of lipid rafts. Since interpreting experimental data from biological membranes is rather intricate, investigations on simple models with biological relevance are necessary to understand the natural systems. We study the effect of cholesterol on the molecular structure of multi-lamellar vesicles (MLVs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a phospholipid ubiquitous in cell membranes, with compositions in the range 0-60 mol% cholesterol. Order parameters, |S(CH)|, are experimentally determined by using (1)H-(13)C solid-state nuclear magnetic resonance (NMR) spectroscopy with segmental detail for all parts of both the cholesterol and POPC molecules, namely the ring system and alkyl chain of the sterol, as well as the glycerol backbone, choline headgroup and the sn-1 and sn-2 acyl chains of POPC. With increasing cholesterol concentration the acyl chains gradually adopt a more extended conformation while the orientation and dynamics of the polar groups are rather unaffected. Additionally, we perform classical molecular dynamics simulations on virtual bilayers mimicking the POPC-cholesterol MLVs investigated by NMR. Good agreement between experiments and simulations is found for the cholesterol alignment in the bilayer and for the |S(CH)| profiles of acyl chains below 15 mol% cholesterol. Deviations occur for the choline headgroup and glycerol backbone parts of POPC, as well as for the phospholipid and cholesterol alkyl chains at higher cholesterol concentrations. The unprecedented detail of the NMR data enables a more complete comparison between simulations and experiments on POPC-cholesterol bilayers and may aid in developing more realistic model descriptions of biological membranes.

Effect of cholesterol on the molecular structure and transitions in a clinical-grade lung surfactant extract

“…At 30 • C, the |SCH| values for the fluid fraction of the clinical-type LSE sample were determined to be 0.2 to 0.25 for the first acyl-chain carbons and decrease along the acyl chain and finally reach values close to zero for ω CH3. The |SCH| profile closely resembles previously measured SCH profiles of saturated acyl chains in the L α(d) phase (29,47,48) in model phospholipid systems with simple composition. The decrease of |SCH| from the acyl-chain carbons near the glycerol backbone to the carbons at the bilayer center relates to the different probabilities for dihedral rotations along the acyl chains.…”

“…The increase in |SCH| upon the addition of cholesterol is most pronounced for segments in the crowded spectral region, most likely corresponding to the acylchain carbons near C3. There is also a strong increase for the methylene groups close to the bilayer interface, which was also observed for related model lipid systems (29,38). The measured |SCH| profiles for the sample composed of LSE + 5 wt % cholesterol falls in between the measured profiles obtained for the clinical-type LSE and the LSE + 10 wt % cholesterol.…”

“…1A. The 13 C peak assignment was based on previous studies of single lipids species (29,30). As expected, the 13 C NMR spectrum is dominated by signal from the lipid species in the major phospholipid class, PC.…”

“…The acyl chains in the L α(d) phase become disordered [i.e., adopt trans/gauche (TG) conformations], which induces the shift of the crowded spectral region from 31 to 33.5 ppm in the gel phase to around 29 to 31 ppm in the L α(d) phase. The spectra of DPPC at 46 • C is compared with previously measured spectra of POPC (29) to further ensure the appearance of an L α(d) phase. If DPPC is mixed with POPC, an L β phase and L α(d) phase coexist at temperatures below 42 • C (36).…”

Influence of the sterol aliphatic side chain on membrane properties: a molecular dynamics study

“…However, the extent of this increase varies noticeably across the different sterol species. Echoing the trends observed for other properties, a maximal ordering effect is observed for cholesterol (for which the calculated curves agree well with published experimental and simulated profiles 45 ), whereas androstenol is clearly the least efficient species at ordering the phospholipid acyl chains.…”