Influence of the Local Chemical Environment in the Formation of Multicomponent Crystals of L-Tryptophan with N-Heterocyclic Carboxylic Acids: Unusual Formation of Double Zwitterions

Abstract

Formation of multicomponent crystals (MCCs) of L-tryptophan (TRP) with N-heterocyclic carboxylic acids such as 2-picolinic acid (PA) and its 3- or 4-substituent isomers (nicotinic acid or isonicotinic acid), pyrazinecarboxylic acid (PZCA), 2,3-pyrazinedicarboxylic acid (2,3-PZDCA), 2-quinaldic acid (QA), and its 3-subsituent isomer (3-QA) is investigated in this manuscript. The investigation results in four multicomponent solid forms of the amino acid with coformers where the electron withdrawing functional group (−COOH) is present in the ortho-substituent to N-heterocyclic rings. The isomers of PA or QA having −COOH at the meta- or para- position failed to produce new phases. These solid phases were identified by powder X-ray diffraction results, and the MCCs derived from 2-picolinic acid (1) and 2,3-pyrazinedicarboxylic acids (3) were further characterized with single crystal X-ray diffraction. The crystal structure of TRP-PA (1) reveal a rare form of cocrystal where both the amino acid and the picolini...