The Crystal and Molecular Structure of L-Azetidine-2-carboxylic Acid. A Naturally Occurring Homolog of Proline

The crystal structure of L-azetidine-2-carboxylic acid (AzCOOH) has been determined by application of superposition methods using an E²-1 Patterson synthesis. The data were collected on a four-angle Picker automatic diffractometer using θ-2θ scan. The space group is P2₁2₁2₁, with four molecules in a unit cell of dimensions a = 7.457, b = 9.810, and c = 6.328 Å. The molecule is in the zwitterionic form. Both hydrogen atoms on the nitrogen atom act as hydrogen-bonding donors to the oxygen atoms in neighboring molecules. The single hydrogen atom attached to the α carbon atom is a hydrogen-bond donor to a nearby carboxylate oxygen atom. The azetidine ring is buckled by 11 ° from a plane so that the carboxylate group lies farther from C(4), the distal ring carbon, than would be the case for a planar ring. The conformation of AzCOOH resembles those of proline and hydroxyproline, but its substitution for either of these amino acids in a polypeptide could bring about conformation changes sufficient to significantly alter the tertiary structure of the polypeptide.