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The Arginine Anomaly: Arginine Radicals Are Poor Hydrogen Atom Donors in Electron Transfer Induced Dissociations
journal contribution
posted on 2006-09-27, 00:00 authored by Xiaohong Chen, František TurečekArginine amide radicals are generated by femtosecond electron transfer to protonated arginine
amide cations in the gas phase. A fraction of the arginine radicals formed (2-amino-5-dihydroguanid-1‘-yl-pentanamide, 1H) is stable on the 6.7 μs time scale and is detected after collisional reionization. The
main dissociation of 1H is loss of a guanidine molecule from the side chain followed by consecutive
dissociations of the 2-aminopentanamid-5-yl radical intermediate. Intramolecular hydrogen atom transfer
from the guanidinium group onto the amide group is not observed. These results are explained by ab initio
and density functional theory calculations of dissociation and transition state energies. Loss of guanidine
from 1H is calculated to require a transition state energy of 68 kJ mol-1, which is substantially lower than
that for hydrogen atom migration from the guanidine group. The loss of guanidine competes with the reverse
migration of the arginine α-hydrogen atom onto the guanidyl radical. RRKM calculations of dissociation
kinetics predict the loss of guanidine to account for >95% of 1H dissociations. The anomalous behavior of
protonated arginine amide upon electron transfer provides an insight into electron capture and transfer
dissociations of peptide cations containing arginine residues as charge carriers. The absence of efficient
hydrogen atom transfer from charge-reduced arginine onto sterically proximate amide group blocks one of
the current mechanisms for electron capture dissociation. Conversely, charge-reduced guanidine groups
in arginine residues may function as radical traps and induce side-chain dissociations. In light of the current
findings, backbone dissociations in arginine-containing peptides are predicted to involve excited electronic
states and proceed by the amide superbase mechanism that involves electron capture in an amide π*
orbital, which is stabilized by through-space coulomb interaction with the remote charge carriers.
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1 H dissociationsPoor Hydrogen Atom Donorstransition state energyprotonated arginine amidefemtosecond electron transferElectron Transfer Induced DissociationsArginine amide radicalsamide superbase mechanismguanidinecharge carriersarginine residuesamide group blocksIntramolecular hydrogen atom transferhydrogen atom migration1 Htransition state energiesRRKMprotonated arginine amide cationshydrogen atom transfer
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