Structural and mechanical properties of cardiolipin lipid bilayers determined using neutron spin echo, small angle neutron and X-ray scattering, and molecular dynamics simulations

Abstract

The detailed structural and mechanical properties of a tetraoleoyl cardiolipin (TOCL) bilayer were determined using neutron spin echo (NSE) spectroscopy, small angle neutron and X-ray scattering (SANS and SAXS, respectively), and molecular dynamics (MD) simulations. We used MD simulations to develop a scattering density profile (SDP) model, which was then utilized to jointly refine SANS and SAXS data. In addition to commonly reported lipid bilayer structural parameters, component distributions were obtained, including the volume probability, electron density and neutron scattering length density. Of note, the distance between electron density maxima DHH (39.4 Å) and the hydrocarbon chain thickness 2DC (29.1 Å) of TOCL bilayers were both found to be larger than the corresponding values for dioleoyl phosphatidylcholine (DOPC) bilayers. Conversely, TOCL bilayers have a smaller overall bilayer thickness DB (36.7 Å), primarily due to their smaller headgroup volume per phosphate. SDP analysis yielded a lipid area of 129.8 Å(2), indicating that the cross-sectional area per oleoyl chain in TOCL bilayers (i.e., 32.5 Å(2)) is smaller than that for DOPC bilayers. Multiple sets of MD simulations were performed with the lipid area constrained at different values. The calculated surface tension versus lipid area resulted in a lateral area compressibility modulus KA of 342 mN m(-1), which is slightly larger compared to DOPC bilayers. Model free comparison to experimental scattering data revealed the best simulated TOCL bilayer from which detailed molecular interactions were determined. Specifically, Na(+) cations were found to interact most strongly with the glycerol hydroxyl linkage, followed by the phosphate and backbone carbonyl oxygens. Inter- and intra-lipid interactions were facilitated by hydrogen bonding between the glycerol hydroxyl and phosphate oxygen, but not with the backbone carbonyl. Finally, analysis of the intermediate scattering functions from NSE spectroscopy measurements of TOCL bilayers yielded a bending modulus KC of 1.06 × 10(-19) J, which was larger than that observed in DOPC bilayers. Our results show the physicochemical properties of cardiolin bilayers that may be important in explaining their functionality in the inner mitochondrial membrane.

Determination of bending rigidity and tilt modulus of lipid membranes from real-space fluctuation analysis of molecular dynamics simulations

“…The detailed structure of tetraoleoyl cardiolipin (TOCL), a less commonly studied lipid present in the mitochondrial membrane, was recently reported from scattering experiments [52]. The area per lipid from our simulations performed under the same conditions as the experiments (i.e.…”

Lateral organization, bilayer asymmetry, and inter-leaflet coupling of biological membranes

“…More recently, AAMD simulations were used to determine the biomembrane bending modulus (K c ) based on the analysis of thermal fluctuations in lipid orientations, where the effects of lipid tilting decouple from the lipid orientation energetics associated with bending moduli 145 . AAMD simulation has also been combined with X-ray and neutron scattering techniques to study the structure and dynamics of pure lipid bilayers and the effects of lipid modifications 146,147 or the addition of various small organic molecules, such as cholesterol 146 , carotenoids 148 , and tamoxifen 149 .…”

The ionization properties of cardiolipin and its variants in model bilayers

“…For instance, surface force measurements suggest that at neutral pH, the fraction of ionized CL phosphates decreases with increasing molar concentrations of CL [54], Fourier transform infrared (FTIR) spectroscopy measurements support two widely separated pK a values [55], and liquid crystal-detected ordering transitions indicate monoanionic CL species at intermediate pH values [56]. Moreover, molecular dynamics simulations support the feasibility of intramolecular hydrogen bonding within the CL headgroup [57–60]. …”