Crystal structures of two retropeptides related to the Gly-Gly and Ala-Ala sequences Larbi El Masdouri, André Aubry, Emmanuel José Gomez, Bernard Vitoux and Michel Marraud J. Chim. Phys., 85 (1988) 583-588 Published online: 29 May 2017 DOI: 10.1051/jcp/1988850583