## Abstract

Two crystal structures of (1<latex>$\beta $</latex>-mercaptopropionic acid) deamino-oxytocin are reported. The `dry form' in space group C2 has cell dimensions a = 27.08<latex>$\pm $</latex>0.03, b = 9.06<latex>$\pm $</latex>0.01, c = 22.98<latex>$\pm $</latex>0.02 angstrom, <latex>$\beta $</latex> = 102.06<latex>$\pm $</latex>0.03 with one deamino-oxytocin and six water molecules per asymmetric unit. The `wet form' in space group P2<latex>$_{1}$</latex> has cell dimensions a = 27.27<latex>$\pm $</latex>0.02, b = 9.04<latex>$\pm $</latex>0.01, c = 23.04<latex>$\pm $</latex>0.02 angstrom, <latex>$\beta $</latex> = 102.24<latex>$\pm $</latex>0.02, with two deamino-oxytocin and 13 water molecules per asymmetric unit. A local twofold parallel to the monoclinic axis gives a pseudo C2 packing. Initial phases of the `dry form' were calculated by the heavy-atom method from the isomorphous and anomalous difference Pattersons and anomalous difference Fourier synthesis. The structure was refined by using restrained least-squares at 1.2 angstrom resolution to a crystallographic R = 0.10. The molecular replacement method yielded the P2<latex>$_{1}$</latex> structure that was refined with geometric restraints to R < 0.09, by using all data to 1.09 angstrom resolution. Deamino-oxytocin consists of a cyclic tocin ring formed by six amino acids, closed by a disulphide bridge, S1S6, and held by two trans-annular hydrogen bonds N2O5 and N5O2 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.