Three protected tetrapeptides

The structures of three protected tetrapeptides, containing the Boc-Gly-Gly-Phe-X-OMe chain, tert-butoxycarbonyl-glycy-glyc-phenylalanine-leucine methyl ester dihydrate, Boc-Gly-Gly-L-Phe-D-Leu-OMe, C25H38-N4O7.2H(2)O, tert-butoxycarbonyl-glycy-glycl-phenylalanine-norleucine-methionine methyl ester dihydrate, Boc-Gly-Gly-L-Phe-D-Met-OMe, C24H36N4O7S.2H(2)O and tert-butoxycarbonyl-glycy-glycl-phenylalanine-norleucine methyl ester dihydrate, Boc-Gly-Gly-D-Phe-L-Nle-OMe, C25H38N4O7.2H(2)O, 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.