ИСТИНА

Monolayers of dimyristoylphosphatidylserine (DMPS) at the surface of electrolyte (~100 mM KCL) were constructed and analyzed by methods of molecular dynamics (MD) to compare with experimental data of X-ray reflectometry (XR). The MD method is actively used for the modeling of lipid monolayers. including research devoted to the study of the pressure-area isotherms of lipids (1-2). Constructing simulation boxes in the same way, we performed simulations in precise all-atom CHARMM36 (2) force field using GROMACS software under different area per lipid. Analysis of the reflection curves was carried out on the assumption that the monolayer contains two sub-layers, which allowed us to reconstruct the electron density profile of the monolayer. This profile is in good agreement with the profile obtained in the MD. Moreover, the MD demonstrates that one sub-layer corresponds to the lipid tails region, and the second corresponds to the polar region that contain bound and free water molecules. According to the MD data, in the region of low surface pressure in the monolayer, a transition occurs from the state of drifting individual lipid aggregates to a two-dimensional liquid. The same phenomenon was observed in DPPC monolayers in coarse-grain MD (1). Lateral compression of DMPS monolayers reveals phase transition between liquid expanded and crystalline states with intermediate region of two states coexistence. MD visualization of the system demonstrates lipid phase transition to a crystal state as a result of hydrocarbon tails ordering, while the thickness of the polar region of DMPS remains practically unchanged. The total thickness of the monolayer increased from 20 ± 3 Ǻ to 28 ± 2 Ǻ in liquid and solid states, respectively. At the same process, hydration state of polar groups decreases at least twice. When analyzing the MD data, it turned out that the thickness of the monolayer in the liquid phase is about 25 A, in the solid 30 A. The dependence of the total potential of the system on the area on the lipid obtained from MD also qualitatively coincided with the analogous dependence obtained by the Langmuir monolayer method. The good agreement between the MD, XR and Langmuir monolayer method data allows us to correlate the Volta potential changes with certain molecular structures. References: 1) Baoukina S., et al., Langmuir, 23 (25), 2007, pp 12617–12623. 2) Dominguez, H., et al. M. L. J. Phys. Chem. B, 1999, 103(44), 9582–9588. 4) Klauda, J. B., et al. J. Phys. Chem. B, 114(23), 2010, pp. 7830–7843.