THERMAL-DECOMPOSITION KINETICS OF PROTONATED PEPTIDES AND PEPTIDE DIMERS, AND COMPARISON WITH SURFACE-INDUCED DISSOCIATION

Rate constants for the unimolecular decomposition of peptide monomer a nd dimer ions by thermal and surface-induced dissociation (SID) are me asured and compared. Rate constants for thermal dissociation are measu red in a heated wide-bore capillary flow reactor attached in front of the capillary leading into the mass spectrometer. Thermal decompositio n of the leucine enkephalin ion (YGGFLH+ is observed between 600 and 6 80 K with rate constants of 20-200 s(-1), and yields many of the same fragments as SID at 35 eV, although with different relative Intensitie s. The thermal decomposition yields the Arrhenius parameters E(a) = 38 .3 kcal/mol, log A 15.7. The decomposition of the monomer and dimer io ns are also observed by using SID on C-18 and fluorinated hydrocarbon surfaces, with rate constants of 2 X 10(4) to 40 X 10(4) s(-1). The SI D activated monomer ions are assigned equivalent temperatures of 710-8 40 K by extrapolation of the thermal activation parameters. The proton ated dimer ion (YGGFLH+ decomposes thermally at 500-540 K to yield the monomer ion. The dimer also decomposes by SID at low collision energi es 10-20 eV on both surfaces to yield the monomer ion, and at much hig her energies of 60-80 eV to yield fragments identical to the decomposi tion of the monomer. The large energy requirement for fragmentation fr om the dimer is due to energy deposition into more degrees of freedom plus the additional energy required for dissociation of the dimer to t he monomer. It is assumed that the energy deposition is linear with co llision energy up to 80 eV, and that the energy becomes randomized thr oughout the dimer, including energy flow through the hydrogen bond(s). These mechanistic assumptions are supported quantitatively by the SID energy relations between monomer and dimer fragmentation. Thermal dec omposition of the larger, multiply protonated melittin ion [M + 3H](3) occurs at substantially higher temperatures, between 810-840 K, than those required for thermal decomposition of (YGGFLH+, to yield many o f the same sequence ions as produced by SID at 135 eV on a fluorinated surface.