Structure of octreotide, a somatostatin analogue

Octreotide, a synthetic somatostatin analogue, is an octapeptide with one disulfide bridge. Crystals of octreotide are orthorhombic, space group P212121, a = 18.458 (5), b = 30.009 (7), c = 39.705 (27) Å, with three molecules of octapeptide, one ordered oxalate dianion and 52 water molecules in the asymmetric unit. Complete protonation of the NH2 groups (as assumed in the refinement) would require three oxalate dianions in the asymmetric unit for charge neutrality; a chemical analysis indicated that four are present. In either case they are so disordered that they cannot be distinguished from the water molecules. The 18 951 unique reflections (Rsym = 0.026) used for structure solution and refinement were recorded with the EMBL imaging-plate scanner using synchrotron radiation. The structure was solved by Patterson interpretation, locating the three disulfide bridges, followed by tangent phase expansion and E-Fourier recycling. The anisotropic refinement against all F2 data between 1.04 and 10.0 Å resolution by blocked restrained full-matrix least-squares techniques converged to a conventional R index based on F of 0.084 [I > 2a(I) and 10.0 > d > 1.04 Å] and wR2, the weighted R-index on F2, of 0.246 (for all data). One peptide molecule adopts a flat β-sheet structure; the other two possess different irregular backbone conformations, but are similar to each other. All three molecules have a distorted type II′ β-turn around the d-Trp-Lys region, but exhibit different side-chain conformations. The crystal structure is stabilized by a network of inter- and intramolecular hydrogen bonds.