SELECTIVE BINDING OF CROWN-ETHERS TO PROTONATED PEPTIDES CAN BE USED TO PROBE MECHANISMS OF H D EXCHANGE AND COLLISION-INDUCED DISSOCIATIONREACTIONS IN THE GAS-PHASE/

Selective binding of crown ethers to model protonated peptides is util ized to study the site selectivity and mechanisms of gas-phase hydroge n/deuterium exchange reactions with ND3 in an external ion source FT-I CR mass spectrometer. Mechanisms for H/D exchange reactions in the gas phase can be classified into two different types: Type I involving di rect participation of the labile protons at the charge site and Type I I in which the charge site at most plays only an ancillary role in the process (e.g., salt bridge formation). Localization of the labile pro ton at the charge site by crown ether attachment inhibits Type I proce sses, as evidenced by a dramatic reduction in the rates of H/D exchang e. For example, crown ether attachment to protonated ethylenediamine a nd 1,4-diaminobutane inhibits H/D exchange reactions, while the free p rotonated species undergo rapid exchange of all five labile hydrogens. Type II processes are still observed with the crown ether adducts. Bo th the amide and the carboxyl hydrogens of peptides exchange via a Typ e II process for which a salt bridge mechanism has been proposed. In t he salt bridge mechanism, the charge site may play an important role b y stabilizing a charge separated ion pair. Immobilization of the labil e proton by crown ether attachment does not eliminate this stabilizati on. Charge localization by crown ether attachment also affects the dis sociation processes of protonated peptides, inhibiting charge directed mechanisms where endothermic proton transfer from the most basic grou p to a less basic site is a prerequisite for fragmentation. Collisiona l activation of the crown ether complex with protonated GGDPG and GGI results in no backbone cleavage in the peptide, while the free protona ted peptides lead to cleavage at the C-terminus side of aspartic acid and the second glycine, respectively.