Microscopic study of chiral interactions in langmuir monolayer: monolayers of N-palmitoyl aspartic acid and N-stearoyl serine methyl ester

In the present paper, we theoretically investigate the effect of chiral interaction on the morphology of the domain in the condensed phase of amino acid amphiphiles like N-palmitoyl aspartic acid and N-stearoyl serine methyl ester. We derived relations to calculate the intermolecular potential of a pair of chiral molecules, which is dependent on the intermolecular separation and the mutual orientation of the molecules. The calculation is carried out based on an effective pair potential description of the groups attached to the chiral center. It is observed that the intermolecular interaction is most favorable at a twist between the azimuthal orientations of the same type of enantiomeric molecules and the energy difference between the twisted arrangement and the parallel arrangement (of neighboring molecules) is of the same order as the intermolecular hydrogen bond energy of the corresponding amino acid head groups. Thus, the observed curvature of the domain shape of amino acid amphiphiles is suggested to be due to the twist at the condensed phase/fluid interface where the interresidue (amino acid) hydrogen bond cycle is not as regular as within the condensed phase. Secondly, we also calculated the interaction energy of an external molecule with all molecules within a circular domain, having the same azimuthal orientation. It is observed that the interaction energy is rather anisotropic for enantiomers in the compressed state (smaller intermolecular separation). The interaction energy has a symmetric profile around the domain composed of a pair of mirror image isomers. Thus, the excess free energy at the condensed phase/fluid interface could be anisotropic for domains where the molecules have parallel arrangement throughout the lattice and this effect may act in concert with the intrinsic tendency to have an intermolecular twist due to chirality. The possibility of the relationship between the molecular twist and the handedness of the curvature of the domain is also discussed.