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

Full understanding of the surface-induced dissociation (SID) of biological ions requires the determination of the energy channeling into the surface a nd 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(lll). Singly protonated ions of trigl ycine, tetraglycine, cyclo(Pro-Gly), cyclo(His-Phe) and tentoxin were forme d by electrospray ionization and scattered at 15-55 eV collision energies o ff the organic surface. The scattered parent ion kinetic energies were 24% of the incident ion energy for the linear peptides, 21% for the cyclic dipe ptides and 17% for the four-peptide ring. These results suggest that ion si ze and/or structure influences the scattered kinetic energy. Using these va lues and assuming an average internal excitation efficiency of 17%, it is e stimated that the final internal energy given to tbe surface is 59-66% of t he initial collision energy. This energy transfer to the surface is very cl ose to that previously estimated for a host of smaller polyatomic ions scat tered from similar organic targets. However, comparison with small ion SID shows that the peptides leave the surface with a wider distribution of kine tic energies. Finally, the measured kinetic energy distributions show that the fragment ions for a given peptide leave the surface with a common veloc ity, suggesting that dissociation occurs away from the surface. All fragmen ts were found to result from non-reactive, inelastic scattering off the org anic surface. Copyright (C) 1999 John Wiley & Sons, Ltd.