Probing the Gas-Phase Folding Kinetics of Peptide Ions by IR Activated DR-ECD

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Abstract
The effect of infrared (IR) irradiation on the electron capture dissociation (ECD) fragmentation pattern of peptide ions was investigated. IR heating increases the internal energy of the precursor ion, which often amplifies secondary fragmentation, resulting in the formation of w-type ions as well as other secondary fragments. Improved sequence coverage was observed with IR irradiation before ECD, likely due to the increased conformational heterogeneity upon IR heating, rather than faster breakdown of the initially formed product ion complex, as IR heating after ECD did not have similar effect. Although the ECD fragment ion yield of peptide ions does not typically increase with IR heating, in double resonance (DR) ECD experiments, fragment ion yield may be reduced by fast resonant ejection of the charge reduced molecular species, and becomes dependent on the folding state of the precursor ion. In this work, the fragment ion yield was monitored as a function of the delay between IR irradiation and the DR-ECD event to study the gas-phase folding kinetics of the peptide ions. Furthermore, the degree of intracomplex hydrogen transfer of the ECD fragment ion pair was used to probe the folding state of the precursor ion. Both methods gave similar refolding time constants of ∼1.5 s−1, revealing that gaseous peptide ions often refold in less than a second, much faster than their protein counterparts. It was also found from the IR-DR-ECD study that the intramolecular H· transfer rate can be an order of magnitude higher than that of the separation of the long-lived c/z product ion complexes, explaining the common observation of c· and z type ions in ECD experiments.