Dissociation Kinetics of Singly Protonated Leucine Enkephalin Investigated by Time-Resolved Photodissociation Tandem Mass Spectrometry

The yields of post-source decay (PSD) and time-resolved photodissociation (PD) at 193 and 266 nm were measured for singly protonated leucine enkephalin ([YGGFL + H]+), a benchmark in the study of peptide ion dissociation, by using tandem time-of-flight mass spectrometry. The peptide ion was generated by matrix-assisted laser desorption ionization (MALDI) using 2,5-dihydroxybenzoic acid as the matrix. The critical energy (E0) and entropy (ΔS‡ at 1000 K) for the dissociation were determined by Rice-Ramsperger-Kassel-Marcus fit of the experimental data. MALDI was done for a mixture of YGGFL and Y6 and the plume temperature determined by the kinetic analysis of [Y6 + H]+ data were used to improve the precision of E0 and ΔS‡ for [YGGFL + H]+. E0 and ΔS‡ thus determined (E0 = 0.67 ± 0.08 eV, ΔS‡ = −24.4 ± 3.2 eu with 1 eu = 4.184 J K−1mol−1) were significantly different from those determined by blackbody infrared radiative dissociation (BIRD) (E0 = 1.10 eV, ΔS‡ = −14.9 eu), and by surface-induced dissociation (SID) (E0 = 1.13 eV, ΔS‡ = −10.3 eu). Analysis of the present experimental data with the SID kinetics (and BIRD kinetics also) led to an unrealistic situation where not only PSD and PD but also MALDI-TOF signals could not be detected. As an explanation for the discrepancy, it was suggested that transition-state switching occurs from an energy bottleneck (SID/BIRD) to an entropy bottleneck (PSD/PD) as the internal energy increases.

Graphical Abstractk(E) for [YGGFL + H]+ dissociation was determined by time-resolved photodissociation MALDI-tandem TOF. It was different from BIRD and SID results possibly due to TS switching.Figure optionsDownload full-size imageDownload high-quality image (55 K)Download as PowerPoint slide