Use of “Tailor-Made” Additives for the Study of Disorder in Crystals. Application to the Racemic Compound of Valine

Abstract

The packing disorder in racemic valine is characterized by techniques previously used to control nucleation, growth, and dissolution of crystals. R,S-valine crystals were grown and dissolved in the presence of other racemic α-amino acid additives. We inferred the presence of disorder in R,S-valine crystals from the lack of enantiomeric segregation of the additives occluded inside growing crystals, and from the non-specific etch-pit formation on the faces of dissolving crystals. Subsequent X-ray diffraction studies showed the disorder to arise from “flipping” of hydrogen-bonded bilayers across interfaces which are linked by relatively weak hydrophobic interactions. Possible mechanisms for the disorder are discussed.