Crystal structure of l-tryptophan–fumaric acid–water (1/1/1)

In the title compound, C11H12N2O2·C4H4O4·H2O, the l-tryptophan molecule crystallized as a zwitterion, together with a neutral fumaric acid molecule and a water solvent molecule. In the crystal, the three components are linked by a series of N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds, forming slabs lying parallel to (001). The slabs are connected by O—H⋯O hydrogen bonds, involving inversion-related fumaric acid groups, leading to the formation of a three-dimensional structure.


S1. Comment
Natural aromatic amino acids, particularly tryptophan, have near UV absorption and emission properties which are utilized extensively in solution phase investigations of structure-function relationships (Demchenko, 1986). Fumaric acid is known to exist in two different polymorphic forms viz. cis and trans ( Reis & Schneider, 1928;Yardley, 1925;Bednowitz & Post, 1966).
In conjunction with our ongoing work on non-linear optical organic crystals among the 20 naturally occurring amino acids, we have directed our interest to tryptophan (Trp), one of the essential amino acids for humans. The non-linear optical properties of organic molecules and crystals have been reviewed by Zyss (Chemla & Zyss, 1987;Zyss & Ledoux, 1994;Zyss & Nicoud, 1996). For similar properties and most the common confirmations of L-tryptophan have been reported (Bye et al., 1973;Bakke & Mostad, 1980). Compared with other amino acids, there are less than 30 tryptophan structures listed in the Cambridge Structural Database (Groom & Allen, 2014), due to the difficulty of obtaining good optical quality crystals; as noted by (Hubschle et al., 2002) who studied the crystal structure of L-tryptophan formic acid solvate. We successfully obtained good quality hard golden-yellow single crystals of L-tryptophan fumaric acid monohydrate, and we report herein on its synthesis and crystal structure.
In the title compound, Fig. 1, L-tryptophan is zwitterionic, as are most amino acids in the solid state, and fumaric acid is neutral. The bond lengths and angles in L-trypophan and fumaric acid are similar to those reported previously (Gorbitz, 2006;Gorbitz et al., 2012;Goswami et al., 1999).
In the crystal, the three components are linked by a series of O-H···O, N-H···O and C-H···O hydrogen bonds forming slabs lying parallel to (001); Table 1 and Fig. 2. The slabs are connected by O-H···O hydrogen bonds, involving inversion related fumaric acid groups, leading to the formation of a three-dimensional structure; Table 1 and Fig. 3.

S2. Synthesis and crystallization
An aqueous solution of L-tryptophan and fumaric acid in a 1:1 stoichiometric ratio was stirred at room temperature for 6 h. The resulting yellow solution was filtered and kept in a Petri dish. Yellow prismatic-shaped hard crystals suitable for X-ray analysis were obtained over a period of 5 days.

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in  The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 40% probability level.

Figure 2
The crystal packing of the title compound, viewed along the a axis. The hydrogen bonds are shown as dashed lines (see Table 1 for details).