ИСТИНА

Phospholipid molecules deposited on a water surface form two-dimensional film (also known as Langmuir monolayer) oriented normally to the water substrate. Under normal conditions, increase in surface pressure Π leads to the phase transition of monolayer from two-dimensional liquid to gel (liquid crystal) structure [1]. Theoretical models of such phase transition, including molecular dymanics (MD) calculations, still holds a number of ambiguous problems, which makes it necessary to apply methods of direct probing the structure of a monolayer. Here we present the analysis of the structural state of dimyristoyl phosphatidylserine (DMPS) monolayer on a water substrate by means of X-ray reflectometry (XRR). The samples have been prepared by standard droplet method [2], the quantity of lipid has been varied in accordance with area-per-lipid parameter from 134 Å2to 41 Å2. Experimental measurements of XRR curves have been performed on a customized butterfly-type laboratory diffractometer [3] at radiation energy E = 8048.05 eV. Analysis of experimental data has been conducted by simultaneous use of the classical model-based approach [4] and of model-independent (free-form) approach based on the extrapolation of reflectivity asymptotic [5].According to the model-independent electron density distributions and structural parameters of two-layer fitting model, relative thickness LT of -C14H27 tails sub-layer changes from ≈6 Å to ≈15 Å while keeping relative electron density ρT ≈ 0.9ρH2O, indicating the transition from liquid to crystallic state, which corresponds with estimated area-per-lipid values. Inclination angle for -C14H27 tails in ordered phase found to be θ= 26±7°. Meanwhile, electron density of polar heads ρPH increases from 1.1ρH2O to 1.4ρH2O with increase in surface pressure, indicating the reduction in hydration state (from ≈ 20 to ≈ 8 H2O molecules per each lipid molecule). Comparison of experimental electron density profiles against MD simulated profiles shows a good agreement between them, with estimated surface rms roughness σ= 3.2±0.5 Å, which corresponds with predictions of capillary wave theory [6]. References: 1. Y.A. Ermakov, K. Kamaraju, K. Sengupta, S. Sukharev, Biophys. J. 98, 1018 (2010). 2. A.M. Tikhonov, JETP Lett. 92, 356 (2010). 3. V.E. Asadchikov, V.G. Babak, A.V. Buzmakov et al., Instr. Exper. Tech 48, 364 (2005). 4. F.P. Buff, R.A. Lovett, F.H. Stillinger, Phys. Rev. Lett. 15, 621 (1965). 5. I.V. Kozhevnikov, Nucl. Instrum. Meth. Phys. Res. A 508, 519 (2003). 6. A. Braslau, P. Pershan, G. Swislow et al., Phys. Rev. A 38, 2457 (1988). Acknowledgements: This work has been partially supported by the Russian Federal Agency of Scientific Organizations Agreement No 007-ГЗ/Ч3363/26), and project of RFBR #16-04-00556.