Ab initio calculations at the B3LYP/6-31G(d) level of theory were carried out on selected cyclic hydrogen-bonded (H-bonded) dimers of glycine and alanine as models for beta-sheets and on the C-alpha-centered radicals derived from them. The structures mirrored the cycles found in the II-bonded network of parallel and antiparallel beta-sheet secondary structure, and were otimized both with and without enforcement of constraints on the Phi,Psi torsion angles. Transition structures for the migration of an H atom from an UC site to another C-alpha site or to an S atom were located. It was found that the presence of a H-bonded strand of a beta-sheet has little effect on the C-alpha-H bond dissociation enthalpy (BDE) of glycine but raises the BDE of other residues by a significant amount. The parallel beta-sheet structure and Phi,Psi angles lead to a significant increase in BDE, relative to the random coil structure, due to loss of captodative stabilization. The antiparallel beta-sheet structure and Phi,Psi angles do not lead to a significant increase in BDE. All residues incorporated in beta-sheet secondary structure, with the exception of glycine, are protected from oxidative damage because the C-alpha-H bond is internal to the sheet and inaccessible to oxidizing radicals. Glycine is susceptible to oxidative damage because it has a second C-alpha-H bond which is exposed. Among residues in secondary structures, only glycine is susceptible to damage by weak oxidants such as thiyl radicals and superoxide, provided it is in an antiparallel beta-sheet. Radical damage may propagate readily from one strand to another above the beta-sheet, but not within the beta-sheet. beta-Sheet structure narrows the difference between the glycyl C-alpha-H BDE and S-H BDE and facilitates interstrand H atom transfer between the glycyl C-alpha site and the S atom of cysteine.