Circular Dichroism of Oxytocin and Several Oxytocin Analogues

The circular dichroic spectra of oxytocin and four analogues of oxytocin are presented at several pH values in the wave length interval, 220 to 320 mµ. The analogues are 2-isoleucine- and 2-glycine-oxytocin, in which the tyrosine residues in Position 2 have been replaced by isoleucine and glycine, respectively, as well as deamino-oxytocin and deamino-2-isoleucine-oxytocin, in which the α-NH2 groups of oxytocin and 2-isoleucine-oxytocin, respectively, have been replaced by hydrogen. Measurements of lysine-vasopressin are also reported. All of these compounds show positive circular dichroism bands associated with disulfide absorption near 250 mµ, corresponding to disulfide dihedral angles close to 90°. The intensity of this circular dichroism band is sharply diminished in deamino-2-isoleucine-oxytocin, and the band is only barely discernible in deamino-oxytocin. Neutralization of the amino group—when one is present— also diminishes the intensity of the band, but not as dramatically as replacement altogether by hydrogen. At longer wave lengths, near 280 mµ, oxytocin and its analogues exhibit a band which, at least in one case, can only be due to disulfide and which, in two other cases, may be contributed to by tyrosine. A large tyrosine band is also observed in oxytocin and lysine-vasopressin at shorter wave lengths, but this band is absent in deamino-oxytocin. These results are discussed in terms of interactions among residues within these peptides and of possible conformational differences among the derivatives examined.