Chiral recognition in gas-phase cyclodextrin: amino acid complexes - Is the three point interaction still valid in the gas phase?

The validity of the 'three-point interaction' model is examined in the guest exchange reaction involving complexes of cyclodextrins and amino acids. The amino acid guest is exchanged in the gas phase in the presence of a gaseous alkyl amine. The net reaction is proton transfer between the protonated amino acid and the alkyl amine. The amino acid is lost as a neutral species. This reaction is sensitive to the chirality of the amino acid. Several amino acids are examined as well as the respective methyl esters to determine the role of the three interacting groups (ammonium, carboxylic acid, and side chain) in enantioselectivity. We find that the three-point interaction model is indeed valid in the gas phase. Enantioselectivity is optimal when two points of attraction and one repulsion is present in the gas-phase complex. The results are supported by molecular modeling calculations. A mechanism for the exchange is proposed.