Patent ID: 8592216

Claim:
A method of ionizing and fragmenting a peptide having an amide backbone and one or more carboxylic acid groups comprising the steps of: a) providing the peptide; b) reacting at least a portion of the carboxylic acid groups of the peptide with a tagging reagent having a functional group having a greater gas-phase basicity than the amide backbone of the peptide, wherein the tagging reagent has the formula: wherein, R 1 and R 2 , independently of one another, are a hydrogen or a binding functional group which allows the binding functional group or a nitrogen attached to the binding functional group to react with a carboxylic acid; R 3 is selected from the group consisting of branched and unbranched alkylene groups having 1 to 10 carbon atoms, which are optionally substituted; and R 4 and R 5 , independently of one another, are selected from the group consisting of hydrogen, and branched and unbranched alkyl groups having 1 to 10 carbon atoms, which are optionally substituted, wherein the tagging reagent and carboxylic acid groups react to link the functional group to the peptide to generate a labeled peptide, and wherein the functional group is not a nucleophile when the tagging reagent reacts with the carboxylic acid groups; c) ionizing the labeled peptide using electrospray ionization, thereby generating an ionized peptide, wherein the functional group of the labeled peptide increases the charge state of the ionized peptide compared to an unlabeled form of the peptide; and d) fragmenting the ionized peptide via electron transfer dissociation (ETD) or electron capture dissociation (ECD).