The accumulation of proline (Pro) and overproduction of reactive oxygen species (ROS) by plants exposed to stress is well-documented. In vitro assays show that enzyme inactivation by hydroxyl radicals (•OH) can be avoided in the presence of Pro, suggesting this amino acid might act as a •OH scavenger. Although production of hydroxyproline (Hyp) has been hypothesized in connection with such antioxidant activity, no evidence on the detailed mechanism of scavenging has been reported. To elucidate whether and how Hyp might be produced, we used density functional theory calculations coupled to a polarizable continuum model to explore 27 reaction channels including H-abstraction by •OH and •OH/H 2O addition. The structure and energetics of stable species and transition states for each reaction channel were characterized at the PCM-(U)M06/6-31G(d,p) level in aqueous solution. Evidence is found for a main pathway in which Pro scavenges •OH by successive H-abstractions (?Gâ§§,298 = 4.1 and 7.5 kcal mol-1) to yield 3,4-?-Pro. A companion pathway with low barriers yielding ?1-pyrroline-5-carboxylate (P5C) is also supported, linking with 5-Hyp through hydration. However, this connection remains unlikely in stressed plants because P5C would be efficiently recycled to Pro (contributing to its accumulation) by P5C reductase, hypothesis coined here as the "Pro-Pro cycle". © 2013 American Chemical Society.