CHARGE-STATE SPECIFIC FACILE GAS-PHASE CLEAVAGE OF ASP-75 MET-76 PEPTIDE-BOND IN THE ALPHA-CHAIN OF HUMAN APOHEMOGLOBIN PROBED BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Herein, we present the first example of charge state specific facile gas-phase cleavage of an aspartic acid-methionine peptide linkage. This cleavage (Asp 75-Met 76) was observed in the (a)lpha-chain of human adult hemoglobin (Hb) and was probed by electrospray ionization mass spectrometry. This specific conformational and/or charge density dependent dissociation was observed primarily in the [M + 11H](11+) and [M + 12H](12+) species. A mechanism involving an intramolecular proton transfer from the protonated carboxyl side chain of Asp 75 to the neighboring Met 76 residue yielding an anhydride moiety at the C-terminal of the Asp 75 is proposed. Dramatic differences in dissociation of [M + 13H](13+) and [M + 14H](14+) species were observed.