An electrospray ionization mass spectrometry study of copper adducts of protonated ubiquitin

A study of the addition of Cu(II) to a ubiquitin electrospray solution shows that the copper ion in the ubiquitin remains doubly charged and displaces two protons on the protonated protein molecule. This observation indicates a chelating bond between the protein and the Cu(II) species. The addition of Cu(I) also was studied and significant intensity was observed for adducts with up to four Cu(I) species attached, with each Cu(I) bonded to one basic site on the protein.