PNNL

Abstract

Normal-incidence surface-induced dissociation of singly protonated des-Arg1-bradykinin (PPGFSPFR) and des-Arg9-bradykinin (RPPGFSPF) has been studied using a specially designed Fourier Transform Ion Cyclotron Resonance mass spectrometer (FT-ICR MS). We found that with a reaction time of 1 s the collision energy-resolved fragmentation efficiency curve (FEC) for des-Arg9-bradykinin is shifted to lower energies by about 4 eV relative to the FEC for des-Arg1-bradykinin. Because the Arrhenius activation energies found by Williams and co-workers in BIRD experiments are 0.82 eV and 1.2 eV for des-Arg1 and des-Arg9 bradykinin, respectively, we expected to find the reverse order in our long reaction time FTICR-SID experiments. We rationalize the difference between our data and the thermal kinetics using Tolman?s theorem to calculate threshold energies from the Arrhenius activation parameters. The threshold energies are 1.15 eV and 1.24 eV for des-Arg1 and des-Arg9 bradykinin, respectively. However, des-Arg1-bradykinin dissociates via a very tight transition state. Consequently, microcanonical rate-energy dependencies for these two peptides cross at low internal energies. Rate constants of 1 s-1 are reached at internal energies of 7.9 eV for des-Arg1-bradykinin and 6.9 eV for des-Arg9 bradykinin, in excellent agreement with the energy shift in our FECs.