Thermal decomposition kinetics of protonated peptides and peptide dimers, and comparison with surface-induced dissociation.

Rate constants for the unimolecular decomposition of peptide monomer and dimer ions by thermal and surface-induced dissociation (SID) are measured and compared. Rate constants for thermal dissociation are measured in a heated wide-bore capillary flow reactor attached in front of the capillary leading into the mass spectrometer. Thermal decomposition of the leucine enkephalin ion (YGGFL)H+ is observed between 600 and 680 K with rate constants of 20-200 s-', and yields many of the same fragments as SID at 35 eV, although with different relative intensities. The thermal decomposition yields the Arrhenius parameters E,, = 38.3 kcal/mol, log A = 15.7. The decomposition of the monomer and dimer ions are also observed by using SID on C18 and fluorinated hydrocarbon surfaces, with rate constants of 2 X lo4 to 40 X lo4 s-'. The SID activated monomer ions are assigned equivalent temperatures of 710-840 K by extrapolation of the thermal activation parameters. The protonated dimer ion (YGGFL)2 H+ decomposes thermally at 500-540 K to yield the monomer ion. The dimer also decomposes by SID at low collision energies 10-20 eV on both surfaces to yield the monomer ion, and at much higher energies of 60-80 eV to yield fragments identical to the decomposition of the monomer. The large energy requirement for fragmentation from the dimer is due to energy deposition into more degrees of freedom plus the additional energy required for dissociation of the dimer to the monomer. It is assumed that the energy deposition is linear with collision energy up to 80 eV, and that the energy becomes randomized throughout the dimer, including energy flow through the hydrogen bond($. These mechanistic assumptions are supported quantitatively by the SID energy relations between monomer and dimer fragmentation. Thermal decomposition of the larger, multiply protonated melittin ion [M + 3HI3+ occurs at substantially higher temperatures, between 810-840 K, than those required for thermal decomposition of (YGGFL)H+, to yield many of the same sequence ions as produced by SID at 135 eV on a fluorinated surface. The fragmentation of molecular ions or protonated molecules is essential for mass spectrometric structure determination, while the decomposition of gas-phase adducts may allow the determination of non-covalent binding energies. We previously studied the thermal decomposition kinetics of carbonium ions and protonated alcohols and ethers,'-4 and the thermal decomposition of non-covalent , hydrogen-bonded dimers is observed routinely in the reverse processes of clustering equilibria. This work extends unimolecular kinetics to larger biomolecules. A similar application was presented more recently by Busman et al. for the decomposition of melittin ions.' The present work also extends previous work of the authors in ion pyrolysis and surface-induced dissociation (SID) to compare the energetics of these two processes. Surface-induced dissociation, developed originally by Cooks and coworkers,6 has proved to be a powerful technique for the investigation of the mass spectral fragmentation of protonated pep tide^."'^ In addition to practical applications, e.g. sequencing peptides, SID has the potential to provide energetic data that can be used to describe the mechanisms of peptide fragmentation. We have recently reported, for example, that fragmentation efficiencies (% fragmentation vs. collision energy) can be correlated with the basicities of peptides and with peptide size.13 Although these qualitative trends are themselves important , it is desirable to