DeltaMass fluorescein labelling of peptide N-terminal using NHS ester From DeltaMass: Average Mass: 359 Formula: C21H11O6 Monoisotopic Mass Change: 359.055 Average Mass Change: 359.315 Notes: Using the DHB matrix at low pH the carboxyl group and one of the oxygens on the flurorescein molecule are protinated so the delta mass is 2Da higher than most text book illustrations would indicate. See Bioconjugate Techniques by Greg Hermanson, Academic Press, page 305, figure 204. Text books of course just show the coupling reaction at neutral or basic pH. MOD:01134 PSI-MOD artifact modification from DeltaMass