Molecular Dynamic simulations of model PC bilayers in water: charge parametrization dependence of the phospholipid hydration

GARAY, ALBERTO S.; NICASTRO, ALCIDES; RODRIGUES, DANIEL E.

Montevideo, Uruguay

Congreso; XXXVI Reunión Anual de la Sociedad Argentina de Biofísica, 6th International Conference on Biological Physics y 5th Southern Cone Biophysics Congress; 2007

IUPAP, IUPAB, SAB, SUB

<!-- @page { size: 8.5in 11in; margin: 0.79in } P { margin-bottom: 0.08in } --> We have performed Molecular Dynamics simulations of model PC lipid bilayers in water to study how the hydration properties derived from their results are affected by the charge parametrization. The different sets of atomic charges used are obtained from ab-initio quantum mechanical calculations over several molecular conformations that appear in a equilibrated bilayer. The simulations are carried out at NPZ<gama>T conditions in the liquid-crystalline-phase, and the value of the surface tension is adjusted to reproduce the Deuterium order parameter from NMR data for multilamellar vesicles (MLV). Under this conditions we obtained values of the area per lipid and the average number of gauche defects, both in good agreement with those found experimentally in MLV by several techniques. The atomic charge sets obtained from different configurations lead to important differences in the atomic profiles at the interfaces, the hydration of the polar head groups, and the structure of H-bonds that links the lipids. The atomic profiles of the polar head groups show to be mainly sensitive to the excess of electron charge of the carbonylic oxygen at the Sn-2 acyl chain. The distributions of dihedral angles close to the polar head along the simulation are also analyzed. We found that among dihedral angles of the acyl chain first portions, only the distribution of that substained by the carboxylic C and Calfa of Sn-1 presents appreciable changes between the gel and LC phases. The relation among the dihedral angles of different molecular conformations and the quantum mechanical derived charge sets are studied. We propose that in a model of the bilayer-water interface a conformational dependent atomic charge parametrization could be essential to understant the hydration of the lipid head and to interpret the IR-spectral features associated to this process.