Crystal structure of methionine-enkephalin.

The crystal structure of methionine-enkephalin has been determined by X-ray crystallography. There are two independent pentapeptides in the asymmetric unit and both display extended backbone conformations with their side chains arranged alternately below and above the backbone plane. The two molecules form a hydrogen-bonded head-to-tail dimer similar in conformation to one dimeric pair of leucine-enkephalin molecules in a previously reported crystal structure.