Reinvestigation of L-tryptophan picrate: Establishment of the existence of the L-tryptophan L-tryptophanium dimeric cation

A crystal structure redetermination of the L-tryptophan picrate crystal previously studied by Ishida et al. (Chem. Pharm. Bull. 41 (1993) 433–438) showed that it comprises L-tryptophan L-tryptophanium dimeric cation, one picrate anion and picric acid. The O⋯O distance of the O–H⋯O hydrogen bond in the dimeric cation is equal to 2.470(6) Å. The infrared spectrum of the crystal was registered and analyzed. The infrared spectrum of the crystals contains a broad absorption band centered at ca. 1170 cm−1, which is assigned to the stretching vibration of the O–H bond.

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► Crystals of L-tryptophan picrate were grown and reinvestigated.
► The structure comprises L-tryptophan L-tryptophanium dimeric cation.
► The presence of the dimeric cation is reflected in the IR spectrum.
► Crystals grown from water and ethanol differ from those grown from acetone.