Stereochemical control of polymorph transitions in nanoscale reactors.

Crystallization of glycine in the cylindrical nanopores of anodic aluminum oxide (AAO) revealed the formation of metastable β-glycine in pores having diameters less than 200 nm. Two-dimensional X-ray microdiffraction indicated that the [010] axis of the embedded β-glycine nanocrystals coincided with the pore direction, identical to behavior observed previously in the cylindrical nanopores of polymer monoliths. Whereas the β-glycine nanocrystals were stable indefinitely in ambient air and persisted upon heating, they transformed to the α polymorph upon standing at room temperature and 90% relative humidity (RH). The α-glycine nanocrystals were oriented with the [010] axis nearly perpendicular to the pore direction, reflecting a nearly 90° rotation of the glycine molecules during the transition. When the β-glycine nanocrystals were formed in the AAO cylinders in the presence of small amounts of racemic hydrophobic amino acid auxiliaries, which are known to bind selectively to the (010) and (010) faces on the fast-growing end of β-glycine enantiomorphs, the β → α phase transition at 90% RH was suppressed. In contrast, β-glycine nanocrystals grown in the presence of an enantiopure amino acid auxiliary, which binds to the fast-growing end of only one of the enantiomorphs, thus suppressing its formation and leaving the other enantiomorph unperturbed, transformed into the α polymorph under the same conditions. This observation confirms that binding of an amino acid to the {010} faces is stereoselective and that access of water to these faces is essential for the transition to the α polymorph.