Thioredoxin controls the intracellular redox potential through a disulfide/dithiol couple. Under conditions of oxidative stress, this protein functions via one-electron exchange, in which formation of the disulfide radical anion occurs. Combined quantum mechanical (QM) and molecular mechanical (MM) calculations using twoand three-level ONIOM schemes were performed on the thioredoxin (Trx) protein of Chlamydomonas reinhardtii in its oxidized-disulfide and one-electron-reduced forms. In both cases, the active site disulfide moiety was described at the MP2(fc)/6-31+G(d) level, and larger regions of varying sizes around the active site were described at the B3LYP/6-31+G(d) level. The remainder of the 112 residues and 33 water molecules of the crystal structure (PDB entry 1EP7) were described by the AMBER force field. Adiabatic electron affinities were calculated for the disulfide bond in all systems. Separate QM or QM/QM calculations were performed on the QM regions to establish the role of the remainder of the protein on the active site properties. The radical anion species becomes more stable as the number of amide groups in the vicinity increases. One-electron reduction potentials were calculated for the small molecule models, and approximated for the protein for which the values are similar to the experimental one (similar to 0 V). This high reduction potential is due to interaction with the charged end of Lys40, as indicated by mutation in silico to norleucine. The inclusion of the protonated Asp30 side chain and a water molecule in the QM region leads to an increase in the electron affinity. Proton transfer from the Asp30 side chain to the Cys39 sulfur in the radical anion species is strongly disfavored. The radical anion is more stable than the protonated form, which is consistent with experimental results.