Conformational studies of oxytocin analogues with different ring sizes. I. Circular dichroism.

Circular dichroic spectra were measured for three analogues of deamino-oxytocin of different ring sizes where the disulfide group of oxytocin is replaced by the (CH2)n group. Their backbone rings are composed of different numbers of atoms, i.e., they are nineteen, twenty and twenty-one for [1,6-aminopimelic acid]oxytocin (n = 1), [1,6-aminosuberic acid]oxytocin (n = 2) and [1,6-aminoazelaic acid]oxytocin (n = 3), respectively. The pH dependence of the circular dichroism spectra indicates that the conformation of [1,6-aminoazelaic acid]oxytocin is different from those of others and the temperature dependency reveals that the conformation of [1,6-aminopimelic acid]oxytocin is most rigid. [1,6-Aminosuberic acid]oxytocin is biologically most active among three derivatives and their biological activities are related to the conformation and internal motions of the peptide hormone analogues.