The one-electron addition on the disulfide bond of thioredoxin was studied by a QM/MM procedure. Three methodological aspects were considered: the presence of a MM surrounding, the choice of the QM method and the QM/MM partitioning. We show that the environment has a relatively small effect on geometry but it strongly influences electronic affinity (EA). Even with the MM part, B3LYP and HF methods are still inadequate and at least MP2 is needed for the treatment of the (2c-3e) bond. However, a relatively restrained QM part seems to be sufficient for modelling this electronic property. (c) 2006 Elsevier B.V. All rights reserved.