Molecular pair potential of chiral amino acid amphiphile in Langmuir monolayers on the basis of an atomistic model

In the present work, the pair potential of enantiomeric N-palmitoyl aspartic acid amphiphile monolayer at the air/water interface is calculated based on an atomistic model. The molecular structure and partial charges are calculated using two semi empirical (PM3, AM1) and one empirical (Gasteiger and Marcili) methods. A distance-dependent dielectric function is used to represent the interfacial dielectric constant at the aqueous subphase. The present study indicates that a pair of molecules have favorable interaction at specific ranges of mutual orientations. Other orientations are favorable but at larger separations. Favorable electrostatic interaction at a specific combination of orientation and short separations of the head groups significantly contribute to the total energy. The curvature of the domain boundary is suggested to be driven by the favorable arrangement which is dependent on the pair potential of molecules. The use of charges obtained by the PM3 and GM do not lead to a significant variation of the orientation-dependent features, while the AM1 predicts higher partial charges and interactions are stronger than the former two methods. However, orientation-dependent features remain the same. The variations in the LJ parameters and charges indicate that the conclusions made are insensitive to the choice of parameters. The mutual favorable interaction predicted by calculation agree with the handedness of curvature of domains.