The conformation of deamino-oxytocin: X-ray analysis of the ‘dry’ and ‘wet’ forms

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Abstract
Two crystal structures of (1(3-mercaptopropionic acid) deamino-oxytocin are reported. The ‘dry form’ in space group C 2 has cell dimensions a = 27.08 + 0.03, b = 9.06 + 0.01, c = 22.98 + 0.02 Å, /? = 102.06 + 0.03 with one deamino-oxytocin and six water molecules per asymmetric unit. The ‘wet form’ in space group has cell dimensions a = 27.27 + 0.02, b = 9.04 + 0.01, c = 23.04 + 0.02 Å, B = 102.24 + 0.02, with two deamino-oxytocin and 13 water molecules per asymmetric unit. A local twofold parallel to the monoclinic axis gives a pseudo C 2 packing. Initial phases of the ‘dry form ’ were calculated by the heavy-atom method from the isomorphous and anomalous difference Pattersons and anomalous difference Fourier synthesis. The structure was refined by using restrained least-squares at 1.2 Å resolution to a crystallographic R = 0.10. The molecular replacement method yielded the P 2 1 structure that was refined with geometric restraints to R 0.09, by using all data to 1.09 A resolution. Deamino-oxytocin consists of a cyclic tocin ring formed by six amino acids, closed by a disulphide bridge, S1— S6, and held by two trans-annular hydrogen bonds N2— O5 and N5— O2 with a type II turn at residues 3 and 4. A flexible tripeptide tail has a loosely hydrogen-bonded type I beta-turn between N9 and O6. The sulphur of cysteine at position 1 is disordered in all the molecules leading to alternative hands of disulphide. The conformational flexibility of Ile 3, Asn 5, Pro 7 side chains and the disulphide bridge is consistent with previous models of oxytocin in which flexibility is necessary for biological activity.