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Our idea for developing a simple and reasonable lipid model to study |
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the ripple pahse of lipid bilayers is based on two facts: one is that |
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the most essential feature of lipid molecules is their amphiphilic |
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structure with polar head groups and non-polar tails. Another fact is |
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that dominant numbers of lipid molecules are very rigid in ripple |
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phase which allows the details of the lipid molecules neglectable. In |
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our model, lipid molecules are represented by rigid bodies made of one |
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head sphere with a point dipole sitting on it and one ellipsoid tail, |
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the direction of the dipole is fixed to be perpendicular to the |
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tail. The breadth and length of tail are $\sigma_0$, $3\sigma_0$. The |
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diameter of heads varies from $1.20\sigma_0$ to $1.41\sigma_0$. The |
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model of the solvent in our simulations is inspired by the idea of |
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``DPD'' water. Every four water molecules are reprsented by one |
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sphere. |
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Spheres interact each other with Lennard-Jones potential, ellipsoids |
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interact each other with Gay-Berne potential, dipoles interact each |
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other with typical dipole potential, spheres interact ellipsoids with |
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LJ-GB potential. All potentials are truncated at $25 \AA$ and shifted |
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at $22 \AA$. |
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To make the simulations less expensive and to observe long-time range |
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behavior of the lipid membranes, all simulaitons were started from two |
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sepetated monolayers in the vaccum with $x-y$ anisotropic pressure |
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coupling, length of $z$ axis of the simulations was fixed to prevent |
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the shrinkage of the simulation boxes due to the free volume outside |
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of the bilayer, and a constant surface tension was applied to enable |
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the fluctuation of the surface. Periodic boundaries were used. There |
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were $480-720$ lipid molecules in simulations according to different |
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size of the heads. All the simulations were stablized for $100ns$ at |
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$300K$. The resulted structures were solvated in the water (about |
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$6$DPD water/lipid molecule) as the initial configurations for another |
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$30ns$ relaxation. All simulations with water were carried out at |
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constant pressure ($P=1bar$) by $3D$ anisotropic coupling, and |
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constant surface tension ($\gamma=0.015$). Time step was |
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$50fs$. Simulations were performed by using OOPSE package. |
