CHIRAL DISCRIMINATION IN SOLUTIONS AND IN LANGMUIR MONOLAYERS

In this paper we examine theoretically the chiral discrimination of mo lecules with a single chiral center. We propose a definition of the ch iral discrimination parameter DELTA in terms of the difference between the second virial coefficient of pure enantiomers and their racemic m ixture. This parameter enters in the equation of state of racemic mixt ures and will determine their phase diagrams. We calculate then the ch iral discrimination between D- and L-alanine using a Monte Carlo simul ation to average over 11 molecular degrees of freedom at fixed intermo lecular 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 fo r rigid chiral tetrahedron-shaped molecules. Here there are only five rotational degrees of freedom. For a Lennard-Jones potential, the over all chiral discrimination is found to be predominantly heterochiral. O ne of our main observations is that the pair free energy, internal ene rgy, and entropy differences between the two enantiomers may change si gns as a function of the interpair distance. We find that homochiralit y is preferred at shorter distances whereas heterochirality is favored at larger distances. With our model molecules a strong chiral discrim ination of about 43% is found. The calculation is repeated for molecul es that are restricted to lie at the water/air interface. Those model molecules can be regarded as tripodal amphiphiles creating a chiral La ngmuir monolayer at the water/air interface. Here the chiral discrimin ation is found to be smaller (about 8.8%) but still significantly hete rochiral.