gamma-Aminobutyric acid: a novel tetragonal phase.

In the tetragonal phase of gamma-aminobutyric acid, C4H9NO2, the single type of molecule adopts a partially folded zwitterionic form. Whereas in the previously characterized monoclinic phase the partially folded zwitterionic molecules exhibit a gauche conformation with respect to the C2-C3 bond, in this phase the molecules exhibit a trans conformation. The altered pattern of intramolecular N...O distances may be of significance with respect to the neurotransmission behavior of this substance. In addition to three strong hydrogen bonds involving the three H atoms bound to the N atom, as in the monoclinic phase, there is a fourth weaker one which results in a two-center bifurcated bond. There is also evidence suggestive of an intramolecular bridging hydrogen bond involving the N atom, an O atom and a methylene H atom, as previously proposed as a stabilizing factor for the gauche conformation observed in the monoclinic phase.