Observation of the Preferential Enrichment Phenomenon for Essential α-Amino Acids with a Racemic Crystal Structure

The preferential enrichment phenomenon has been observed for two neutral α-amino acids, leucine and alanine, by applying kinetic crystallization conditions using high concentrations and appropriate solvents. The mechanism of preferential enrichment of leucine and alanine is proposed on the basis of (i) the difference in solubility between the racemic and enantiomerically pure samples, (ii) the observation of polymorphic transition during crystallization by in situ attenuated total reflection infrared spectroscopy, (iii) the characterization of deposited crystals by X-ray crystallographic analysis, and (iv) the optical microscopic or atomic force microscopy observation of the crystal shape or surface, respectively.