Structural Studies of the Interaction between Indole Derivatives and Biologically Important Aromatic Compounds. Part XXVI. X-Ray Crystal Structure of L-Tryptophan-Picric Acid Charge-Transfer Complex and Comparison with DL-Tryptophan-Picric Acid Complex.

In order to investigate the difference between the indole-picric acid interaction modes formed in the L- and DL-tryptophan-picric acid complex crystals, the crystal structure of the former complex was analysed by the X-ray diffraction method and the interaction geometry observed was compared with that of the latter complex crystal already analysed. Two crystallographically independent charge-transfer interaction pairs were formed in the crystal structure and formed discrete stacking layers consisting of tryptophan and picric acid molecules. The indole and pricric acid planes were parallely stacked with an interplanar spacing near to 3.4 Å in both pairs. This reflects the structural feature necessary for the charge-transfer interection characterized by the red coloration of complex crystal. Three kinds of indole-picric acid stacking geometries observed were similar with one another, and one of them was almost the same as that in the DL-tryptophan-picric acid complex crystal. This indicates that the observed indole-picric acid interaction mode reflects an intrinsic feature and is not significantly affected by the different chirality of the tryptophan side chain and crystal environment.