Figure 2: Nine combinations of the torsion angles θ1 and θ2 in gabapentin. (a) Gpn with chair conformation of the cyclohexane ring in which the aminomethyl group is equatorial and carboxymethyl group axial, (b) the inverted chair conformation of the cyclohexane ring with respect to (a), in which the aminomethyl group is axial and carboxymethyl group is equatorial. Sterically disallowed combinations are shaded dark. Numbers of Gpn residues crystallographically characterized are shown in circles in the respective cells. In the case of achiral structures both the combinations, (+60°, +60°) and (-60°, -60°) are indicated. (21achiral structures in (a) and 11 in (b)).