Árpád Somogyi

Surface-induced dissociation : An effective tool to probe structure, energetics and fragmentation mechanisms of protonated peptides

The utility of surface-induced dissociation (SID) to probe the structure, energetics and fragmentation mechanisms of protonated peptides is discussed and demonstrated. High internal energy deposition provided by low-energy (eV range) ion-surface collisions yields extensive fragmentation of protonated peptides, allowing relatively uncomplicated and rapid sequence analysis of oligopeptides. SID of multiply protonated peptides is illustrated for peptides with molecular mass of up to approximately 5000 u. It is also illustrated that SID combined with electrospray ionization (ESI) provides a distinctive experimental technique to determine the energetics and mechanisms of peptide fragmentation. The relative position of ESI/SID fragmentation efficiency curves (plots of percentage fragmentation vs. laboratory collision energy) for peptides can be utilized to estimate relative energetics of peptide fragmentation and even to predict proton localization sites. The observed trends support the essential role of the mobile proton model in understanding peptide fragmentation by low-energy tandem mass spectrometry.

The effect of protonation site on bond strengths in simple peptides: Application of Ab initio and modified neglect of differential overlap bond orders and modified neglect of differential overlap energy partitioning

A comparative study of ab initio 6–31G* and semiempirical modified neglect of differential overlap (MNDO) bond orders and MNDO diatomic energy contributions for the description of bond strengths in neutral and protonated glycine, diglycine, triglycine, and dialanine is presented. Good correlations were found between 6–31G* and MNDO bond orders and between MNDO bond orders and diatomic energy contributions. Although bond orders and diatomic energy contributions are inherently different quantities, both predict the changes in bond strengths due to protonation to be qualitatively the same. The theoretically predicted differences in bond strengths for different protonated forms clearly indicate that in peptide fragmentation schemes one should consider even those protonated forms whose formation is not preferred energetically.

Average Activation Energies of Low-energy Fragmentation Processes of Protonated Peptides Determined by a New Approach

An attempt was made to estimate the average activation energies of low-energy fragmentation processes of protonated oligopeptides by combining RRKM theory and the results of electrospray ionization/surface induced dissociation (ESI/SID). The average internal energy was assumed to be deposited by three processes: thermal energy gained in the heated capillary of the electrospray source, energy gain in the capillary-skimmer region of the electrospray source, and energy deposition by collision with the surface. The latter fraction was calculated based on the position of the ESI/SID fragmentation of efficiency curves and the ratio of kinetic to internal energy conversion in SID. Using the average internal energy estimated from the experimental results, the average activation energies were evaluated by applying RRKM theory. The application of this approach for protonated leucine enkephalin resulted in an average activation energy of 36 +/- 5 kcal/mol for the lowest energy decompositions. The approach has also been applied to several other peptides in the mass range of 200-1200 Da, yielding average activation energies in the range of 35-47 kcal/mol.