title: Modeling β-Scission Reactions of Peptide Backbone Alkoxy Radicals:  Backbone C-C Bond Fission.
abstract: To model the C-C β-scission reactions of backbone peptide alkoxy radicals, enthalpies and barriers for the fragmentation of four substituted alkoxy radicals have been calculated with a variety of ab initio molecular orbital theory and density functional theory procedures. The high-level methods examined include CBS-QB3, variants of the G3 family, and W1. Simpler methods include HF, MP2, QCISD, B3-LYP, BMK, and MPW1K with a range of basis sets. We find that good accuracy can be achieved with the G3(MP2)//B3-LYP and G3X(MP2)-RAD methods. Lower-cost methods producing reasonable results are single-point energy calculations with UB3-LYP/6-311+G(3df,2p), RB3-LYP/6-311+G(3df,2p), UBMK/6-311+G(3df,2p), and RBMK/6-311+G(3df,2p) on geometries optimized with UB3-LYP/6-31G(d) or UBMK/6-31G(d). Heats of formation at 0 K for the alkoxy radicals and their fragmentation products were also calculated. We predict ΔfH0 values for the alkoxy radicals of -71.4 ((•)OCH2CH [Formula: see text] O), -102.5 ((•)OCH(CH3)CH [Formula: see text] O), -176.6 ((•)OCH(CH3)C(NH2) [Formula: see text] O), and -264.6 ((•)OC(CH3)(NHCH [Formula: see text] O)CH [Formula: see text] O) kJ mol(-)(1). For the fragmentation products NH2C((•)) [Formula: see text] O and CH( [Formula: see text] O)NHC(CH3) [Formula: see text] O, we predict ΔfH0 values of -5.9 kJ mol(-)(1) and -352.8 kJ mol(-)(1).
authors.affiliation: School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and Department of Chemistry, University of Calgary, Calgary, Alberta T2N IN4, Canada.
authors.forename: Geoffrey P F
authors.initials: GP
authors.lastname: Wood
authors.affiliation: School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and Department of Chemistry, University of Calgary, Calgary, Alberta T2N IN4, Canada.
authors.forename: Arvi
authors.initials: A
authors.lastname: Rauk
authors.affiliation: School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia, and Department of Chemistry, University of Calgary, Calgary, Alberta T2N IN4, Canada.
authors.forename: Leo
authors.initials: L
authors.lastname: Radom