Probing the mechanism of electron capture and electron transfer dissociation using tags with variable electron affinity
Electron capture dissociation (ECD) and electron transfer dissociation (ETD) of doubly protonated electron affinity (EA)-tuned peptides were studied to further illuminate the mechanism of these processes. The model peptide FQpSEEQQQTEDELQDK, containing a phosphoserine residue, was converted to EA-tuned peptides via β-elimination and Michael addition of various thiol compounds. These include propanyl, benzyl, 4-cyanobenzyl, perfluorobenzyl, 3,5-dicyanobenzyl, 3-nitrobenzyl, and 3,5-dinitrobenzyl structural moieties, having a range of EA from −1.15 to +1.65 eV, excluding the propanyl group. Typical ECD or ETD backbone fragmentations are completely inhibited in peptides with substituent tags having EA over 1.00 eV, which are referred to as electron predators in this work. Nearly identical rates of electron capture by the dications substituted by the benzyl (EA = −1.15 eV) and 3-nitrobenzyl (EA = 1.00 eV) moieties are observed, which indicates the similarity of electron capture cross sections for the two derivatized peptides. This observation leads to the inference that electron capture kinetics are governed by the long-range electron−dication interaction and are not affected by side chain derivatives with positive EA. Once an electron is captured to high-n Rydberg states, however, through-space or through-bond electron transfer to the EA-tuning tags or low-n Rydberg states via potential curve crossing occurs in competition with transfer to the amide π* orbital. The energetics of these processes are evaluated using time-dependent density functional theory with a series of reduced model systems. The intramolecular electron transfer process is modulated by structure-dependent hydrogen bonds and is heavily affected by the presence and type of electron-withdrawing groups in the EA-tuning tag. The anion radicals formed by electron predators have high proton affinities (approximately 1400 kJ/mol for the 3-nitrobenzyl anion radical) in comparison to other basic sites in the model peptide dication, facilitating exothermic proton transfer from one of the two sites of protonation. This interrupts the normal sequence of events in ECD or ETD, leading to backbone fragmentation by forming a stable radical intermediate. The implications which these results have for previously proposed ECD and ETD mechanisms are discussed.
© 2009 American Chemical Society. Received August 18, 2008. This work was supported by the National Science Foundation through Grant CHE-0416381 and the Beckman Institute at the California Institute of Technology. The computational resource was kindly provided by the Materials and Process Simulation Center at the California Institute of Technology. C.H.S. acknowledges a fellowship from the Kwanjeong Educational Foundation. P.R. acknowledges support from the NIH/NIDCR UCLA Research Training Program (Grant T32 DE007296). The NIH/NCRR High-End Instrumentation Program supported the acquisition of the LTQ-FT mass spectrometer (Grant S10 RR023045 to J.A.L.). We thank Professor Jack Simons for discussions regarding the electron capture process, Professor Woon-Seok Yeo for help with synthesis, Professor Francis Turecek, Dr. Yousung Jung, and Dr. Jiyoung Heo for assistance with the computational analysis, and Dr. Hugh I. Kim for discussions of reaction mechanisms.
Supplemental Material - Sohn2009p2210J_Am_Chem_Soc_supp.pdf
Accepted Version - nihms-108673.pdf