Conformational analysis of tryptophan in solution using nuclear magnetic resonance methods

The conformation of the amino-acid tryptophan in solution has been examined by n.m.r. methods. Coupling constants, nuclear Overhauser effects, and lanthanide perturbations were all analysed in terms of six conformers, each having one of three values for the Cα–Cβ dihedral angle (χ1) and one of two values for the Cβ–Cγ dihedral angle (χ2). A set of populations was obtained which was consistent, within reasonable limits, with all the n.m.r. data. It was found that one conformer, that having the side-chain trans to the carboxylic acid group and the ring perpendicular to the Cα–Cβ bond, was the dominant species in solution. There are similarities between the population distribution in solution and the distribution of dihedral angles found for tryptophan in protein crystal structures.