All rights reserved EuroPEAN JourNAL of MASS SPEctroMEtry oxytocin secreted in the posterior pituitary is a well characterized peptide hormone that elicits contraction of the uterus, smoothes muscles during lactation and feels an affinity with the brain.1 oxytocin consists of nine amino acids (cysteine1-tyrosine2-Isoluecine3-Glutamine4-Asparagine5-cysteine6-Proline7-Leucine8-Glycine9) and an amidated c-terminus.2 the synthesis of an octapeptide amide with the hormonal activity was reported by Vigneaud et al. 3 A cyclic structure involving the formation of a cys1-cys6 disulfide bridge of oxytocin has been proposed.2,4 two b-turn structures were also reported on the basis of the results of NMr spectroscopy experiments involving oxytocin. [5][6][7] A first b-turn structure was proposed in a cyclic cys1-tyr2-Ile3-Gln4-Asn5-cys6 pentapeptide involving the sequence tyr2-Ile3-Gln4-Asn5, and a second b-turn structure was proposed in the c-terminal sequence cys6-Pro7-Leu8-Gly9.the structure of oxytocin complexed with a receptor 8-9 and the biological activities of [metal ion---oxytocin] complex [10][11][12][13][14][15][16][17] have been studied by many research groups.