Chiral Discrimination in Solutions and in Langmuir Monolayers

In this paper we examine theoretically the chiral discrimination of molecules with a single chiral center. We propose a definition of the chiral discrimination parameter A in terms of the difference between the second virial coefficient of pure enantiomers and their racemic mixture. This parameter enters in the equation of state of racemic mixtures and will determine their phase diagrams. We calculate then the chiral discrimination between Dand L-alanine using a Monte Carlo simulation to average over 1 1 molecular degrees of freedom at fixed intermolecular distances using the CHARMM energy function. The discrimination is found to slightly favor homochirality and mainly comes from steric hindrance at short distances. We also perform a direct integration for rigid chiral tetrahedron-shaped molecules. Here there are only five rotational degrees of freedom. For a Lennard-Jones potential, the overall chiral discrimination is found to be predominantly heterochiral. One of our main observations is that the pair free energy, internal energy, and entropy differences between the two enantiomers may change signs as a function of the interpair distance. We find that homochirality is preferred at shorter distances whereas heterochirality is favored at larger distances. With our model molecules a strong chiral discrimination of about 43% is found. The calculation is repeated for molecules that are restricted to lie a t the water/air interface. Those model molecules can be regarded as tripodal amphiphiles creating a chiral Langmuir monolayer a t the water/air interface. Here the chiral discrimination is found to be smaller (about 8.8%) but still significantly heterochiral.