Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

Abstract

Although single-lipid bilayers are usually considered models of eukaryotic plasma membranes, their research drops drastically when it comes to exclusively anionic lipid membranes. Being a major anionic phospholipid in the inner leaflet of eukaryote membranes, phosphatidylserine-constituted lipid membranes were occasionally explored in the form of multilamellar liposomes (MLV), but their inherent instability caused a serious lack of efforts undertaken on large unilamellar liposomes (LUVs) as more realistic model membrane systems. In order to compensate the existing shortcomings, we performed a comprehensive calorimetric, spectroscopic and MD simulation study of time-varying structural features of LUV made from 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS), whereas the corresponding MLV were examined as a reference. A substantial uncertainty of UV/Vis data of LUV from which only T_m was unambiguously determined (53.9 ± 0.8 °C), along with rather high uncertainty on the high-temperature range of DPPS melting profile obtained from DSC (≈50-59 °C), presumably reflect distinguished surface structural features in LUV. The FTIR signatures of glycerol moiety and those originated from carboxyl group serve as a strong support that in LUV, unlike in MLV, highly curved surfaces occur continuously, whereas the details on the attenuation of surface features in MLV were unraveled by molecular dynamics.

Keywords: 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine sodium salt (DPPS); MD simulations; interbilayer water; multilamellar and large unilamellar vesicles (MLV and LUV); spectroscopic and calorimetric study; surface curvature fluctuations.

Figure 1

Structural formulas and pK_a values of particular titrable functional groups of 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (DPPS) and 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) [18,20].

Figure 2

Temperature-dependent UV/Vis spectra of DPPS in the following forms: (a) MLV (40 °C: magenta solid curve, 65 °C: purple solid curve, spectral profile of the principal component: magenta dotted curve) and (b) LUV (40 °C: green dotted curve, 65 °C: olive solid curve, spectral profile of the principal component: green dotted curve). Concentration profiles of the (first) principal components that project temperature-dependent UV/Vis spectra and DSC curves of DPPS: (c) MLV (DSC: violet curve; UV/Vis: purple curve for spectral projection and magenta curve for double sigmoidal fit) and (d) LUV (DSC: olive curve; UV/Vis green curve for spectral projection and dark yellow curve for single sigmoidal fit). Phase transition temperatures determined from DSC (intersected lines for _o and dashed lines for _m) and UV/Vis experiments (dotted lines) are highlighted with corresponding colors on graphs. * and yellow rectangles refer to the L_c → L_β (in MLV only) and T_m,H of DPPS (in both MLV and LUV).

Figure 3

Normalized FTIR spectra of DPPS in the forms of MLV (35 °C: magenta, 65 °C: purple) and LUV (35 °C: green, 65 °C: olive) in the following spectral ranges: (a) 2980–2820 cm⁻¹ (ν_(a)sCH₂), (b) 1780–1530 cm⁻¹ (νC=O, ν_asCOO⁻), (c) 1505–1395 cm⁻¹ (γCH₂, ν_sCOO⁻, δCOH) and (d) 1255–1190 cm⁻¹ (ν_asPO₂⁻, ν_a(s)C−O). * Temperature-dependent displacement of the bands are highlighted with rectangular filled pattern.

Figure 4

(a) angular distribution functions for water molecules in the first hydration shell of DPPS and DPPG lipids; (b) tilt of water molecules as a function of distance from the membrane; (c) the sum over all solvent molecules of cosθ as a function of distance from the membrane.