Characterization of azo coupling adducts of benzenediazonium ions with aromatic amino acids in peptides and proteins

A synthetic peptide, VLSPADKTNWGHEYRMF(cmC)QIG, was reacted with 4-chlorobenzenediazonium hexafluorophosphate as a model for reactions of aromatic diazonium ions with proteins. At a ratio of diazonium ion to peptide of 0.8:1, three products could be seen by reversed-phase HPLC. Electrospray mass spectrometric analysis of the isolated products revealed that two of the products had the same mass of 2648 Da, being 138 Da higher than the parent peptide and corresponding to the addition of a 4-chlorobenzenediazo group. The third isolated product had a mass of 2787 Da which corresponded to the addition of two 4-chlorobenzenediazo groups (276 Da). Digestion of the monoadducted intact peptides with trypsin or endoproteinase Glu-C and HPLC separation of adduct oligopeptides followed by sequencing with electrospray ionization tandem mass spectrometry showed unambiguously that histidine and tyrosine residues were the major sites of modification. Incubation of human serum albumin with 4-chlorobenzenediazonium hexafluorophosphate at molar ratios of 1:1, 1:2, and 1:10 resulted in adduct formation as detected by shifts in the HPLC retention time of the protein and also by an increase in mass as determined by electrospray mass spectrometry.