Energetics and dynamics of fragmentation of protonated leucine enkephalin from time-and energy-resolved surface-induced dissociation studies

Dissociation of singly protonated leucine enkephalin (YGGFL) was studied using surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer (FT- ICR MS) specially configured for studying ion activation by collisions with surfaces. The energetics and dynamics of seven primary dissociation channels were deduced from modeling the time- and energy- resolved fragmentation efficiency curves for different fragment ions using an RRKM- based approach developed in our laboratory. The following threshold energies and activation entropies were determined in this study: E-0 = 1.20 eV and Delta S-double dagger = -20 eu(1) (MH+ -> b(5)); E-0 = 1.14 eV and Delta S-double dagger = -14.7 eu (MH+ -> b(4)); E-0 = 1.42 eV and Delta S-double dagger = -2.5 eu (MH+ -> b(3)); E-0 = 1.30 eV and Delta S-double dagger = -4.1 eu (MH+ -> a(4)); E-0 = 1.37 eV and Delta S-double dagger = -5.2 eu (MH+ -> y ions); E-0 = 1.50 eV and Delta S-double dagger = 1.6 eu (MH+ -> internal fragments); E-0 = 1.62 eV and Delta S-double dagger = 5.2 eu (MH+ -> F). Comparison with Arrhenius activation energies reported in the literature demonstrated for the first time the reversal of the order of activation energies as compared to threshold energies for dissociation.