Tryptic y(++) fragment ion distributions are guided by Coulombic repulsion.

Ideal tryptic peptides contain only a single basic residue, located at the C-terminus. Collisional fragmentation of their doubly- or triply-protonated ions generates doubly-charged y(++) fragment ions with modest intensities. The size distribution of the y(++) fragments, when averaged over many spectra, corresponds closely to the expectations from charge-directed backbone cleavage and a Coulomb-Boltzmann distribution of mobile protons. This observation should be helpful in developing mechanistic models for y(++) formation.