... 4 Da, the decrease of the molecular mass value. The most proba-ble reason for a 2 Da decrease is the formation of anS–S bond; although this was totally unexpected and unprecedented, the top-down ... causes a 1 Da increase in any fragment ion containing that residue. The average mass gain of N-terminal and C-terminal fragment ions are plottedas positive and negative, respectively, mass increases, ... Neil Kelleher,Harold Scheraga and Klaas van Wyck for valuablediscussions, and the General Medical Institute of the National Institutes of Health, GM16609, for generousfinancial support.References1...