Effects of peptide chain length on the gas-phase proton transfer properties of doubly-protonated ions from bradykinin and its N-terminal fragment peptides

Abstract

Proton transfer properties were studied for doubly-protonated ions from bradykinin and seven smaller peptides whose sequences have C-terminal residues successively removed from bradykinin. Ions were generated by electrospray ionization (ESI) and their deprotonation reactions were investigated in a Fourier transform ion cyclotron resonance mass spectrometer. Sustained off-resonance irradiation (SORI)-collision-induced dissociation (CID) was used to obtain information on sites of protonation, while molecular dynamics calculations provided information on peptide ion conformations and Coulomb energies. As the number of amino acid residues decreases the peptide ions undergo proton transfer more readily. Apparent gas-phase acidities (GA apps), for the doubly-protonated ions decreased as the peptide size decreased, with values spanning the range of 225.8±4.2 kcal/mol for the nonapeptide bradykinin to 196.6±4.4 kcal/mol for the tripeptide Arg 1Pro 2Pro 3. The magnitude of the drop in GA app as a residue was removed varied from 10.9 kcal/mol for removal of the highly basic Arg 9 to 0 kcal/mol for removal of Phe 8 (whose phenylalanine ring juts away from the peptide backbone and does not participate in hydrogen bonding). Hydrogen bonding in the modeled structures decreased as the peptide chain length decreased. The lowest energy structure for doubly-protonated bradykinin contained nine hydrogen bonds, while only one hydrogen bond was found for the tripeptide ions. In addition, the tripeptide ions had an extended structure, while the larger peptide ions were compact. There is no single reason that the peptide ions become more acidic (i.e., more reactive to proton transfer) as the peptide chain length is decreased. Depending upon the situation, factors that play a role in the proton transfer reactivity include intrinsic basicity of the protonation sites, intramolecular hydrogen bonding (particularly involving the protonated residue), Coulombic repulsion of charge sites, and conformational considerations such as accessibility/shielding of the protonation site.