Post-translational mechanisms of protein oxidation as a result of reactive oxygen species (ROS) can occur under physiological conditions to yield selective side-chain and backbone modifications including abstractions, donations, additions, substitutions, and fragmentation. In order to characterize the selectivity of radical-mediated fragmentation, quantum mechanical investigations using ab initio and density functional methods were employed to evaluate site, conformation, and pathway trends of small trialanine peptides resembling a beta-strand and a beta-turn. Comparisons of reaction enthalpies show that the diamide pathway is more energetically favorable than the a-amidation pathway and that both pathways are site and conformationally selective. These findings readily contribute to the understanding of oxidative stress in biochemical processes.