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.