The limiting behavior of water hydrating a phospholipid monolayer: A computer simulation study

Abstract

We report molecular dynamics simulations of water hydrating a lipid (dimyristoylphosphatidylcholine) monolayer under conditions chosen to eliminate simulation artifacts. These simulations provide a description of the behavior of the membrane–water interface that agrees with recent experimental studies. In particular, we find that the hydrating water orients to contribute the positive end of its dipole to the substantially positive electrostatic potential of the membrane interior, consistent with interpretations of recent experiments. In addition, recent experiments show that this water reorients rapidly on the NMR time scale. Our results concur, however the relatively rapid water motion does not preclude the preferential ordering that we observe. The limiting behavior of the system shows three hydration shells about the lipid PC headgroups and significant hydrogen bonding of water to the phosphate groups. The choline group experiences different environments, and the structure of the first hydration shell clearly corresponds to a clathrate. The motion of the hydrating water was found to be slower than that of bulk water, and the computed residence times for water about the lipids (20 ps about choline, 10 ps about phosphate) were in excellent agreement with results of NMR experiments. This further shows that water resides in a clathrate shell longer than in a shell about ions. In addition, we show that the structure and dynamics of water hydrating the lipids are very sensitive to the treatment of the long-range interactions. In particular, the radial structure sharpens considerably, a third hydration shell about the phosphate was observed only with large cutoffs, and hydrogen bonding of water to the lipids increased by 25%. The water moved more slowly than bulk when large cutoffs were employed but moved faster than bulk water when small cutoffs were used and the residence times for water about the lipids were twofold–fivefold larger using large cutoffs. In general it was found that the lipids significantly influence water out to several hydration shells, and that water hydrating the lipids behaves differently than bulk water.

Insights into the behavioral difference of water in the presence of GM1

“…We have found a faster decay in the region 0-4 Å than in the region 4-8 Å from the phosphorus atom, implying strong association of water in the later region. In case of pure POPC, and POPC-Cholesterol systems, from the distance 8 Å and onwards, the estimated residence times, obtained by exponential fits (Table S2 [31,70]. In accordance with our previous analyses, it is again proved that in case of GM1, up to a longer distance ($16 Å) from the phospholipid surface the water molecules have an order of magnitude of slower dynamics as they behave as interfacial due to the presence of GM1.…”

“…Beside experiments, computer simulation also aids to understand the interfacial properties of water molecules [25,26]. The orientation and polarization of the interfacial water [27][28][29][30][31][32], water conductance through carbon nanotube [33], the hydrogen bonding structure of water and of the lipid headgroups has also been assessed [32,34,35] using MD simulations. Theory of solvation of small and large apolar species in water was also developed [36].…”

Structure of the adsorption layer of various ionic and non-ionic surfactants at the free water surface, as seen from computer simulation and ITIM analysis

“…In fact, computer simulation studies can well complement experimental investigations since, given that a reliable enough model of the real system is used, they can provide a three dimensional insight of atomistic resolution into the structure and dynamics of the system to be studied. Thus, the adsorption layer of various cationic [1,2,18,19,25,30], anionic [1,2,20,21,24,[28][29][30][31], catanionic [25], zwitterionic [15][16][17], and non-ionic surfactants [1,2,[22][23][24]26,30] at the surface of their aqueous solutions have been studied several times. However, there is still a need for systematic comparisons of various structural features, e.g., headgroup type or tail length of the surfactants on the properties of their adsorption layers.…”

“…With the recent development of both various surface sensitive experimental methods and of fast and easily accessible computers, the molecular level properties of surfactant adsorption layers have been studied with increasing intensity in the past two decades both experimentally [3][4][5][6][7][8][9][10][11][12][13][14] and by computer simulation methods [1,2,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In fact, computer simulation studies can well complement experimental investigations since, given that a reliable enough model of the real system is used, they can provide a three dimensional insight of atomistic resolution into the structure and dynamics of the system to be studied.…”

Short grafted chains: Monte Carlo simulations of a model for monolayers of amphiphiles

“…On the one hand, phenomenological descriptions of the different condensed phases in terms of Landau expansions in the characteristic order parameters [4,5] have offered valuable insight into the nature and the interrelations of different phase transitions on a very general level. On the other hand, Molecular dynamics simulations of atomically realistic models have complemented experiments and provided structural information on quantities, which are hard to access experimentally [7][8][9][10][11]. These two approaches are in a sense antipodal: Whereas phenomenological treatments focus on universal properties and make little or no contact to the microscopic structure of the systems, atomically realistic models seek to imitate nature as faithfully as possible, and to reach quantitative agreement.…”