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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

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journal contribution
posted on 31.05.2017, 00:00 by Babulal Das, Hemant Kumar Srivastava
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 picolinic acid are in zwitterionic form. Further, in our surprise, the amino acid appears to undergo change in absolute configuration during cocrystallization. Crystal TRP-2,3-PZDCA (3) is observed as a salt where the amino acid exists in cationic form, and the carboxylic acid exists in anionic form. It is observed that the complementary H-bonding between the py-N/ortho- −COOH of the coformers with the α-NH3+/α-COO group of L-tryptophan primarily drives the cocrystallization process. Density functional theory calculations support the experimental observations as lower total energy and higher interaction energy values are obtained for the successfully synthesized MCCs. The solid state fluorescence of TRP (known as intrinsic fluorescence probe) shows that 2,3-PZDCA is an effective quencher.