Ab initio and density functional theory calculations are used to monitor the process wherein a OH center dot radical is allowed to approach the various CH groups of a Leu dipeptide, with its CH2CH(CH3)(2) side chain. After forming an encounter complex, the OH center dot abstracts the pertinent H atom, and the resulting HOH is then dissociated from the complex. The energy barriers for H center dot abstraction from the beta, gamma, and delta CH groups are all less than 8 kcal/mol, but a significantly higher barrier is computed for the (CH)-H-alpha removal. This higher barrier is the result of the strong H-bonds formed in the encounter complex between the OH center dot and the NH and C=O groups of the peptide units that surround the C-alpha atom. This low-energy complex represents a kinetic trap which raises the energy needed to surmount the ensuing H center dot transfer barrier.