Three Protected Tetrapeptides

The structures of thee protected tetrapeptides, containing the Boc-Gly-Gly-L-Phe-X-OMe chain, tert-butoxy-carbonyl-glycil-glycil-phenylalanine-leucine methyl ester dihydrate, Boc-Gly-Gly-L-Phe-D-Leu-OMe, C25H38N4O7.2H2O, tert-butoxy-carbonyl-glycil-glycil-phenylalanine-methionine methyl ester dihydrate, Boc-Gly-Gly-L-Phe-D-Met-OMe, C24H36N4O7S.2H2O and tert-butoxy-carbonyl-glycil-glycil-phenylalanine-norleucine methyl ester dihydrate, Boc-Gly-Gly-D-Phe-L-Nle-OMe, C25H38N4O7.2H2O, are described. The three molecules have the same conformation of the tetrapeptide chain and display the same packing, consisting of couples of molecules linked head-to-tail by two hydrogen N-H--O) bonds;other hydrogen bonds, also involving two water molecules of crystallization, link these couples together and give rise to a planar structure.