Structure and dynamics of water and lipid molecules in charged anionic DMPG lipid bilayer membranes

Abstract

Diffusion and spectroscopy of water and lipids in fully hydrated dimyristoylphosphatidylcholine bilayer membranes J. Chem. Phys. 140, 104901 (2014) Molecular dynamics simulations have been used to investigate the influence of the valency of counter-ions on the structure of freestanding bilayer membranes of the anionic 1,2-dimyristoyl-snglycero-3-phosphoglycerol (DMPG) lipid at 310 K and 1 atm. At this temperature, the membrane is in the fluid phase with a monovalent counter-ion and in the gel phase with a divalent counter-ion. The diffusion constant of water as a function of its depth in the membrane has been determined from mean-square-displacement calculations. Also, calculated incoherent quasielastic neutron scattering functions have been compared to experimental results and used to determine an average diffusion constant for all water molecules in the system. On extrapolating the diffusion constants inferred experimentally to a temperature of 310 K, reasonable agreement with the simulations is obtained. However, the experiments do not have the sensitivity to confirm the diffusion of a small component of water bound to the lipids as found in the simulations. In addition, the orientation of the dipole moment of the water molecules has been determined as a function of their depth in the membrane. Previous indirect estimates of the electrostatic potential within phospholipid membranes imply an enormous electric field of 10 8 -10 9 V m −1 , which is likely to have great significance in controlling the conformation of translocating membrane proteins and in the transfer of ions and molecules across the membrane. We have calculated the membrane potential for DMPG bilayers and found ∼1 V (∼2 · 10 8 V m −1 ) when in the fluid phase with a monovalent counter-ion and ∼1.4 V (∼2.8 · 10 8 V m −1 ) when in the gel phase with a divalent counter-ion. The number of water molecules for a fully hydrated DMPG membrane has been estimated to be 9.7 molecules per lipid in the gel phase and 17.5 molecules in the fluid phase, considerably smaller than inferred experimentally for 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) membranes but comparable to the number inferred for 1,2-dilauroyl-sn-glycero-3-phosphoethanolamine (DLPE) membranes. Some of the properties of the DMPG membrane are compared with those of the neutral zwitterionic DMPC bilayer membrane at 303 K and 1 atm, which is the same reduced temperature with respect to the gel-to-fluid transition temperature as 310 K is for the DMPG bilayer membrane. C 2016 AIP Publishing LLC. [http://dx

On the structure and dynamics of water associated with single-supported zwitterionic and anionic membranes

“…This value is greater than ∼10 water molecules/lipid estimated to be required for the full hydration of the membrane from MD simulations. 14 We also point out that the estimated effective thickness of the confined 2 water is comparable to that of the water layer between the lower leaflet of a DMPC membrane and the SiO 2 surface [see Fig. 1(a)], i.e., a thickness of ∼1 nm as determined by neutron reflectivity measurements 21 and by interferometry.…”

“…8) and extrapolating to the simulation temperature of 310 K, we estimate D = 2.3 × 10 5 cm 2 /s compared to the value of D = 2.5 × 10 5 cm 2 /s inferred from the analysis of the broad-Lorentzian component in the simulated spectra. 14 We believe that the difference in these values is within the uncertainty imposed by the limited dynamic range of BASIS and the error in extrapolation. In the analysis of both the experiment and the simulation, we have identified the broad-Lorentzian component with bulk-like water located above the membrane.…”

“…Our MD simulations 14 have investigated the structure and dynamics of water and lipid molecules in a freestanding DMPG bilayer similar to simulations reported earlier on a DMPC membrane. 10 The DMPG simulations were conducted at a temperature of 310 K, which is the same reduced temperature (relative to the gel-to-fluid transition temperature) as used in the DMPC simulations.…”

“…9 There have been several recent MD simulations that have investigated the structure and dynamics of water hydrating single freestanding bilayers. [10][11][12][13][14] In particular, simulations of freestanding DMPC 10 and DMPG 14 membranes have proved to be of some value in interpreting our QENS measurements on single-supported membranes, 8,9 although one must be cautious in their use due to the possible effects of the substrate as we discuss below.…”

Rheological characterization of thermal phase behavior of anionic lipid DMPG dispersions

“…Transparency in the intermediate phase indicates change in surface properties associated with the reflective index of the bilayer membrane or mesoscale transformation of the structure such as a rupture of the vesicle and a ripple phase. In any case, conformation change of a head group and acyl chain in DMPG monomer due to the chain-melting transition would play a role on variable morphology [6,8,15,16]. Complex endotherm profiles with multi-peaks in the same DMPG system indicate that the chain-melting continuously proceeds, and thus a chain packing in the membrane also changes continuously [2,7].…”