Intermolecular Headgroup Interaction and Hydration as Driving Forces for Lipid Transmembrane Asymmetry

Abstract

Variations between the inner and outer leaflets of cell membranes are crucial for cell functioning and signaling, drug-membrane interactions, and the formation of lipid domains. Transmembrane asymmetry can in principle be comprised of an asymmetric charge distribution, differences in hydration, specific headgroup/H-bonding interactions, or a difference in the number of lipids per leaflet. Here, we characterize the transmembrane asymmetry of small unilamellar liposomes consisting of zwitterionic and charged lipids in aqueous solution using vibrational sum frequency scattering and second harmonic scattering, label-free methods, specifically sensitive to lipid and water asymmetries. For single component liposomes, transmembrane asymmetry is present for the charge distribution and lipid hydration, but the leaflets are not detectably asymmetric in terms of the number of lipids per leaflet, even though geometrical packing arguments would predict so. Such a lipid transmembrane asymmetry can, however, be induced in binary lipid mixtures under conditions that enable H-bonding interactions between phosphate and amine groups. In this case, the measured asymmetry consists of a different number of lipids in the outer and inner leaflet, a difference in transmembrane headgroup hydration, and a different headgroup orientation for the interacting phosphate groups.

Second-Harmonic Scattering as a Probe of Structural Correlations in Liquids

“…This work demonstrates that second harmonic scattering experiments and atomistic simulations can be used in synergy to investigate the structure of complex liquids, solutions and biomembranes, including the intrinsic inter-molecular correlations.Nonlinear light scattering has been widely used to investigate aqueous interfaces, including suspensions of metallic or semiconducting nanoparticles, water droplets and biological membranes (for example, Refs. [2][3][4][5][6][7][8][9][10][11][12][13][14][15]). In particular, second harmonic scattering (SHS), also commonly referred to as Hyper-Rayleigh scattering (HRS; the terms SHS and HRS are used interchangeably in this work), is especially sensitive to the molecular orientation of liquids at interfaces: there is a greater contribution to radiated second harmonic light arising from molecules at the interface, where inversion symmetry is broken, compared to those in the bulk, which is on average centrosymmetric.…”

Optical label-free and model-free probe of the surface potential of nanoscale and microscopic objects in aqueous solution

“…It is thus sensitive to the orientational directionality of water molecules in the interfacial region defined as the region from the surface plane to the position where the electrostatic field has decayed to zero [18]. Angle-resolved (AR) nonresonant SHS (AR-SHS) [19] is applicable to a wide variety of hard [20][21][22][23][24] and soft particles systems [25][26][27] and can be used in very dilute solutions and small sample volumes. In absence of chemical effects, the measured intensity depends quadratically on the surface potential [28].…”

Characterization of the interface of binary mixed DOPC:DOPS liposomes in water: The impact of charge condensation

“…Non-resonant second harmonic scattering 2,3 (SHS) is an optical process used to measure the net orientational order of water molecules along the surface normal of particles, 4,5 droplets, [6][7][8] or liposomes. 9,10 Polarization-and angle-resolved (AR) SHS were recently demonstrated as a method to obtain a unique value for the surface potential of a particle in aqueous solution. 11 Key to this method is the fact that the nonlinear polarization of water at the second harmonic frequency depends linearly on the ensemble electrostatic field in the solution.…”

“…12 Polarization resolved AR-SHS can be described by a set of exact analytical expressions 13 that rely on the surface potential and one non-vanishing surface susceptibility tensor element that describes the orientational order of water at the interface. Combined AR-SHS and sum-frequency scattering (SFS) studies 10 were recently used to probe the transmembrane asymmetry in single component anionic DOPS and zwitterionic DOPC liposomes, as well as liposomes composed of a 1:1 mixture of DOPC and DOPS. DOPC and DOPS are two of the main constituents of the eukaryotic plasma membrane.…”

“…In contrast, transmembrane asymmetry in the form of a different number of lipid molecules in the inner and outer leaflets was absent for liposomes composed of DOPC, DOPS, or a 1:1 mixture of DOPC:DOPS. 10 In this work, we further quantify the surface properties of liposomes in water that are composed of a binary mixture of DOPC and DOPS. We apply AR-SHS and electrophoretic mobility measurements to dilute solutions of liposomes composed of different binary mixtures of DOPS and DOPC, spanning the full range of possible mixtures.…”