Carbamino Group Formation with Peptides and Proteins Studied by Mass Spectrometry.

At high pH and in the presence of dissolved CO2, the N-terminus and ε-amino groups of amino acids, peptides, and proteins can form carbamino adducts with CO2, R-NH2 + CO2 .tautm. R-NHCOO- + H+. The authors report the first study of carbamino group formation by electrospray ionization (ESI) mass spectrometry (MS). Angiotensin II, bradykinin, substance P, and insulin have been studied. A careful optimization of the instrumental parameters was necessary to allow the transfer of the fragile adducts into vacuum for mass anal. Particularly, dissocn. of the adducts in the ion sampling process and pH changes in ESI must be minimized. With these precautions, levels of carbamino group formation of angiotensin II and bradykinin detd. from mass spectra agree with those expected to be in soln., calcd. from literature equil. consts. Thus, ESI MS can quant. measure ratios of carbamino adduct to total peptide concn. in soln. Values of equil. consts. for carbamino group formation with substance P (pKc = 4.77±0.18) and insulin (pKc = 4.99±0.05) are reported for the first time. [on SciFinder(R)]