Rate and extent of gas-phase hydrogen/deuterium exchange of bradykinins: evidence for peptide zwitterions in the gas phase

The gas-phase H/D exchange of bradykinin [M + H](+), [M + Na](+), [M + 2H](2+) and [M + H + Na](2+) ions; des-Arg(1)-bradykinin, des-Arg(9)-bradykinin, and bradykinin fragment 2-7 [M + H](+) ions; and O-methylbradykinin [M + H](+) and [M + 2H](2) ions with D2O have been examined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry at 9.4 T. The different peptides vary widely in exchange rate and extent of deuterium incorporation. H/D exchange was slowest and deuterium incorporation was least for bradykinin [M + H](+), [M + H + Na](2+) and bradykinin methyl ester [M + 2H](2+) ions. In contrast, H/D exchange and extent of deuteration are higher for des-Arg(1)-bradykinin, des-Arg(9)-bradykinin, and bradykinin fragment 2-7 [M + H](+) ions; and highest for bradykinin [M + Na](+) and [M + 2H](2+) and O-methylbradykinin [M + H](+). Because the most Likely site of protonation is the guanidino group of arginine, the above reactivity pattern strongly supports a zwitterion form for protonated gas-phase bradykinin. (C) 1999 Elsevier Science B.V.