On the Relative Stability of Singly Protonated des-Arg- and des-Arg9-Bradykinins

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 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. We found that, when the preexponential factor for reaction is very high (>1017 s-1) or very low (