Bond dissociation energies and radical stabilization energies associated with model peptide-backbone radicals

Geoffrey P F Wood, Damian Moran, Rebecca Jacob, Leo Radom*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

70 Citations (Scopus)

Abstract

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) have been calculated for a series of models that represent a glycine-containing peptide-backbone. High-level methods that have been used include W1, CBS-QB3, U-CBS-QB3, and G3X(MP2)-RAD. Simpler methods used include MP2, B3-LYP, BMK, and MPWB1K in association with the 6-311+G(3df,2p) basis set. We find that the high-level methods produce BDEs and RSEs that are in good agreement with one another. Of the simpler methods, RBMK and RMPWB1K achieve good accuracy for BDEs and RSEs for all the species that were examined. For monosubstituted carbon-centered radicals, we find that the stabilizing effect (as measured by RSEs) of carbonyl substituents (CX=O) ranges from 24.7 to 36.9 kJ mol -1, with the largest stabilization occurring for the CH= O group. Amino groups (NHY) also stabilize a monosubstituted α-carbon radical, with the calculated RSEs ranging from 44.5 to 49.5 kJ mol-1, the largest stabilization occurring for the NH2 group. In combination, NHY and CX=O substituents on a disubstituted carbon-centered radical produce a large stabilizing effect ranging from 82.0 to 125.8 kJ mol-1. This translates to a captodative (synergistic) stabilization of 12.8 to 39.4 kJ mol-1. For monosubstituted nitrogen-centered radicals, we find that the stabilizing effect of methyl and related (CH2Z) substituents ranges from 25.9 to 31.7 kJ mol-1, the largest stabilization occurring for the CH3 group. Carbonyl substituents (CX=O) destabilize a nitrogen-centered radical relative to the corresponding closed-shell molecule, with the calculated RSEs ranging from -30.8 to -22.3 kJ mol -1, the largest destabilization occurring for the CH=O group. In combination, CH2Z and CX=O substituents at a nitrogen radical center produce a destabilizing effect ranging from -19.0 to -0.2 kJ mol-1. This translates to an additional destabilization associated with disubstitution of -18.6 to -7.8 kJ mol-1.

Original languageEnglish
Pages (from-to)6318-6325
Number of pages8
JournalJournal of Physical Chemistry A
Volume109
Issue number28
DOIs
Publication statusPublished - 21 Jul 2005
Externally publishedYes

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