Activation of manganese-dependent class Ib ribonucleotide reductase by hydrogen peroxide was modeled using B3LYP* hybrid density functional theory. Class Ib ribonucleotide reductase R2 subunit (R2F) does not react with molecular oxygen. Instead R2F is proposed to react with H2O2 or HO2-, provided by the unusual flavodoxin protein Nrdl, to generate the observed manganese(III) manganese(III) tyrosyl-radical state. On the basis of the calculations, an energetically feasible reaction mechanism is suggested for activation by H2O2, which proceeds through two reductive half-reactions. In the first reductive half-reaction, H2O2 is cleaved with a barrier of 13.1 kcal mol(-1) [Mn(II)Mn(II) -> Mn(III)Mn(III)], and in the second reductive half-reaction, H2O2 is cleaved with a barrier of 17.0 kcal mol(-1) [Mn(III)Mn(III) -> Mn(IV)Mn(IV)]. Tyrosyl-radical formation from both the Mn(IV)Mn(IV) state and a Mn(III)Mn(IV) state, where an electron and proton have been taken up, is both kinetically and thermodynamically accessible. Hence, chemically, H2O2 is a possible oxidant for the manganese-dependent R2F. The selectivity between the second reductive half-reaction and a competing oxidative reaction, as in manganese catalase, may be the time scale for the availability of H2O2. The role of Nrdl may be to provide H2O2 on the correct time scale.