Conformations of protonated gas-phase bradykinin ions: evidence for intramolecular hydrogen bonding.

The post-source decay of bradykinin, Lys1-bradykinin, des-Arg1-bradykinin, des-Arg9-bradykinin and [D-Phe7]-bradykinin [M + H]+ ions was examined in order to assertain the influence of secondary structure on peptide ion dissociation. Fragment ions corresponding to the elimination of H2O and HN=C=NH are observed in the product ion mass spectra of Lys1-bradykinin and des-Arg1-bradykinin but not in the spectra of bradykinin or des-Arg9-bradykinin. Cleavage reactions at the Phe-Ser and/or Ser-Pro bonds are observed for all peptide [M + H]+ ions with the exception of des-Arg9-bradykinin. The product ions arising from the processes described above are rationalized in terms of the intramolecular solvation of the protonated guanidino groups of the arginines. The strongest intramolecular interaction appears to be a proton bridge between the guanidino groups of the N- and C-terminal arginines in bradykinin. In addition, increased abundances of fragment ions in the vicinity of Ser-Pro may be attributed to intramolecular solvation of the protonated C-terminal guanidino group by the Ser-Pro portion of the molecule. This self-solvation of the ionizing proton leads to a gas-phase peptide conformation that is supported by solution-phase NMR studies at elevated temperatures and in non-polar solvents but which is different from the conformation in polar solvents.

Photodissociation of high molecular weight peptides and proteins in a two-stage linear time-of-flight mass spectrometer.

A two-stage linear time-of-flight mass spectrometer is used to investigate the requirements for performance of laser photodissociation of peptide and protein ions. Results are presented that demonstrate that desorption and dissociation laser pulses can be synchronized to irradiate ions that travel at high velocities down the drift tube of a time-of-flight mass spectrometer. For example, 193-nm photodissociation of bovine insulin and doubly charged lysozyme is demonstrated, and laser power studies suggest that dissociation is initiated by the absorption of a single 193-nm photon. These results are encouraging because they suggest that laser photodissociation of high molecular weight proteins can lead to fragmentation on time scales compatible with time-of-flight mass spectrometry.