Electron-ion collisions were studied for various protonated peptide monocations with disulfide bonds, using an electrostatic storage-ring equipped with a merged-electron-beam device. Resonant neutral particle emissions at the energies of 6-7 eV were observed, as well as a rise towards zero-energy, which are typical electron-capture dissociation profiles. The presence of disulfide (S-S) bonds tends to enhance the resonant bump heights. Chemical nature of the amino-acid residues adjacent to cysteines appears to correlate with the bump strength. Molecular-dynamical simulations help clarify the role of molecular vibration modes in the electron-capture dissociation process.