Solid–liquid equilibria in the aqueous chiral homophenylalanine system and solid-state characterisation of the racemic species

Abstract

In this work, solid–liquid equilibria of the ternary system D-HPA/L-HPA/water were investigated at 25 and 55 °C. The chiral system is of the racemic compound forming type and characterised by very low solubilities below 0.2 wt% at 55 °C. The solid state behaviour of DL-HPA was analysed employing differential scanning calorimetry and hot stage microscopy showing a decomposition behaviour at elevated temperatures i.e. no melting point. Exemplary for the chiral system, milled samples of L-HPA were studied by powder X-ray diffractometry confirming the tendency of the crystals to preferred orientation. For the first time, single crystal structures of L-HPA and DL-HPA were determined, and deposited in the CCDC (2463661–2463662). No further solid-state forms were encountered in the temperature and solvent condition range studied. Comparison to the chiral phenylalanine system revealed similar thermal behaviour and solubility relationships except the strong difference in absolute solubilities.