The Structure and Conformation of Tryptophan in the Crystal of the Pure Racemic Compound and the Hydrogen Oxalate.

Abstract

The crystal structure of pure DL-trytophan was determined by X-ray crystallographic methods using 522 reflections observed by counter methods. The crystals are monoclinic, space group P21/c with unit cell dimensions a = 18.986(8) .ANG., b = 5.768(2) .ANG., c = 9.379(3) .ANG. and .beta. = 101.84(3).degree.. The structure was refined to a conventional R-factor of 0.085; estimated SD are 0.02 .ANG. in distances and 1.6.degree. in angles when H atoms are not involved. The conformation differs from that earlier reported for the tryptophan molecule. The cationic form of tryptophan in the hydrogen oxalate was also studied using 1519 observed reflections. The crystals are orthorhombic, space group P212121 with unit cell dimensions a = 5.8773(9) .ANG., b = 7.722(1) .ANG., c = 28.302(3) .ANG.. The structure was refined to R = 0.049; estimated SD are 0.005 .ANG. and 0.4.degree. in interatomic distances and angles, respectively, when H atoms are not involved. The general geometry of the tryptophan molecule and the interactions between the indole moieties in such crystal structures are discussed.