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.