Screening of multicomponent crystals of l-tryptophan with three isomers of pyridinedicarboxylic acids

• Multicomponent crystals of l-tryptophan with pyridinedicarboxylic acids are reported.
• Thermal and X-ray diffraction techniques are used to characterize these materials.
• Crystal structure and weak interactions of l-tryptophan/2,6-PDA are discussed.
• Binding ability of l-tryptophan towards these acids is monitored.
• Solid state fluorescence properties of the crystals are studied.

Multicomponent crystallization of l-tryptophan with three different isomers of pyridinedicarboxylic acids, namely, 2,3-pyridinedicarboxylic acid (2,3-PDA), 2,5-pyridinedicarboxylic acid (2,5-PDA) and 2,6-pyridinedicarboxylic acid (2,6-PDA), were screened using conventional solution cocrystallization technique. Whereas the new phases derived from the amino acid with 2,3-PDA or 2,5-PDA were analyzed using powder X-ray diffraction and thermal analysis techniques, the crystalline phase synthesized from 2,6-substituted isomer was further characterized by single crystal X-ray diffraction. Structural analysis revealed that the amino acid exists in the zwitterionic form interacting with the neutral 2,6-PDA by strong intermolecular hydrogen bonding. The components in the co-crystal self-assemble leading to a three dimensional hydrogen bonded closed packed network structure. Isothermal calorimetric titration showed that among the three isomers, 2,6-PDA showed relatively strong binding interaction towards the amino acid in water at 298 K. All the crystals exhibit marginal quenching of fluorescence properties of l-tryptophan in the solid state.