Proenkephalin-A has been described to generate enkephalins, opoid peptides, and several derived peptides, which display various biological effects, including antinociception and immunological enhancement. Recently, we have isolated from bovine chromaffin granules a new antibacterial peptide, named enkelytin, which corresponds to the bisphosphorylated form of PEAP209-237 (Goumon, Y., Strub, J. M., Moniatte, M., Nullans, G., Poteur, L., Hubert, P., Van Dorsselaer, A., Aunis, D., and Metz-Boutigue, M. H. (1996) Eur. J. Biochem. 235, 516-525). In this paper, the three-dimensional solution structure of synthetic PEAP209-237 was investigated by NMR. These studies indicate that this peptide, which is unstructured in water, folds into an alpha-helical structure in trifluoroethanol/water (1/1). NMR data revealed two possible three-dimensional models of PEAP209-237. In both models, the proline residue Pro-227 induces a 90 degrees hinge between two alpha-helical segments (Ser-215 to Ser-221 and Glu-228 to Arg-232) leading to an overall L-shaped structure for the molecule. The negative charge of PEAP209-237 and the low amphipathy of the two alpha-helical segments imply new mechanisms to explain the antibacterial activity of enkelytin.