The radiation products of hydroxyproline have been investigated using density functional theory. In the resulting radicals, the choice of the apical ring atom is found to be dependent on the nature and strength of the intramolecular hydrogen bonding. The observed hyperfine couplings previously assigned to two zwitterionic conformers of the hydroxyproline primary radical anion are found to be better described by its nonzwitterionic isomers and corresponding neutral protonated isomers. Similarly, the observed hyperfine couplings for radicals formed by cleavage of the C-alpha-N bond (deamination) are in closest agreement with those calculated for their neutral forms. Theoretical proton hyperfine couplings support the experimental assignment of the radical cation formed by decarboxylation and the radicals resulting from hydrogen abstraction from the C-2 and C-3 positions. The proton hyperfine couplings are sensitive to the conformations bf the radicals, which, in turn, are highly dependent upon the extent of intramolecular hydrogen bonding.