Three protected tetrapeptides

The structures of three protected tetrapeptides, containing the Boc-Gly-Gly-Phe-X-OMe chain,tert-butoxycarbonyl-glycy-glycl-phenylalanine-leucine methyl ester dihydrate, Boc-Gly-Gly-L-Phe-D-Leu-OMe, C25H38N4O7·2H2O,tert-butoxycarbonyl-glycy-glycl-phenylalanine-methionine methyl ester dihydrate, Boc-Gly Gly-L-Phe-D-Met-OMe, C24H36N4O7S.2H2O andtert-butoxycarbonyl-glycy-glycl-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 Boc-Gly Gly Phe-X-OMe tetrapeptide chain and display the same packing, consisting of couples of molecules linked head-to-tail by two hydrogen (N-HO) bonds; other hydrogen bonds, also involving two water molecules of crystallization, link these couples together, and give rise to a planar structure.