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Structure and Dynamics of Interfacial Water in an L Phase Lipid Bilayer from Molecular Dynamics Simulations

Ken Åman^*, Erik Lindahl, Olle Edholm, Pär Håkansson^* and Per-Olof Westlund^*

^* Department of Chemistry, Biophysical Chemistry, Umeå University, SE-901 87 Umeå, Sweden, and Department of Physics, Theoretical Biophysics, Royal Institute of Technology, Stockholm Center for Physics, Astronomy and Biotechnology, SE-106 91 Stockholm, Sweden

Correspondence: Address reprint requests to Per-Olof Westlund, Biophysical Chemistry, Umeå University, SE-901 87 Umeå, Sweden.

Based on molecular dynamics simulations, an analysis of structure and dynamics is performed on interfacial water at a liquid crystalline dipalmitoylphosphatidycholine/water system. Water properties relevant for understanding NMR relaxation are emphasized. The first and second rank orientational order parameters of the water O–H bonds were calculated, where the second rank order parameter is in agreement with experimental determined quadrupolar splittings. Also, two different interfacial water regions (bound water regions) are revealed with respect to different signs of the second rank order parameter. The water reorientation correlation function reveals a mixture of fast and slow decaying parts. The fast (ps) part of the correlation function is due to local anisotropic water reorientation whereas the much slower part is due to more complicated processes including lateral diffusion along the interface and chemical exchange between free and bound water molecules. The 100-ns-long molecular dynamics simulation at constant pressure (1 atm) and at a temperature of 50°C of 64 lipid molecules and 64 x 23 water molecules lack a slow water reorientation correlation component in the ns time scale. The ²H₂O powder spectrum of the dipalmitoylphosphatidycholine/water system is narrow and consequently, the NMR relaxation time T₂ is too short compared to experimental results.

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