Energy partitioning in the surface-induced dissociation of linear and cyclic protonated peptides at an organic surface

Abstract

Full understanding of the surface-induced dissociation (SID) of biological ions requires the determination of the energy channeling into the surface and the scattered ion kinetic and internal energies. Parent and fragment ion kinetic energy distributions were measured for five peptide ions scattered off a hexanethiolate monolayer on Au(111). Singly protonated ions of triglycine, tetraglycine, cyclo(Pro-Gly), cyclo(His-Phe) and tentoxin were formed by electrospray ionization and scattered at 15–55 eV collision energies off the organic surface. The scattered parent ion kinetic energies were 24% of the incident ion energy for the linear peptides, 21% for the cyclic dipeptides and 17% for the four-peptide ring. These results suggest that ion size and/or structure influences the scattered kinetic energy. Using these values and assuming an average internal excitation efficiency of 17%, it is estimated that the final internal energy given to the surface is 59–66% of the initial collision energy. This energy transfer to the surface is very close to that previously estimated for a host of smaller polyatomic ions scattered from similar organic targets. However, comparison with small ion SID shows that the peptides leave the surface with a wider distribution of kinetic energies. Finally, the measured kinetic energy distributions show that the fragment ions for a given peptide leave the surface with a common velocity, suggesting that dissociation occurs away from the surface. All fragments were found to result from non-reactive, inelastic scattering off the organic surface. Copyright © 1999 John Wiley & Sons, Ltd.