Optical properties of isolated hormone oxytocin dianions: ionization, reduction, and copper complexation effects.

Trapped oxytocin (OT) peptide dianions and copper-oxytocin complex dianions are subjected to electron detachment when irradiated by a UV light. Electron photodetachment experiments as a function of the laser wavelength were performed on the native disulfide-containing ring oxytocin, the reduced dithiol oxytocin, and the Cu(II)-oxytocin complex. The electron detachment yield was used to monitor the excited electronic spectrum of the trapped ions. The spectra can be used as a fingerprint of the ionization state of the tyrosine chromophore under different structural environments. In gas-phase oxytocin dianion OT-2H, the tyrosine is deprotonated while it is neutral for the reduced form of oxytocin OT(SH)-2H. Optical spectra for the copper complex dianion OT-4H+Cu are in favor of a neutral tyrosine in the complex. A structure with the metal cation chelated to four deprotonated amide groups is proposed for this complex.