Proteins integrated or transiently bound to membranes are subject to strong electric fields. However, the role of these fields on tuning the reactivity of redox proteins is still an open issue. High electric fields are also present in protein-based electrochemical devices. In this work we report DFT calculations of the electric field effects on ligand binding and on the redox potentials of porphyrin models representing the active sites of typical heme proteins such as cytochrome c, cytochrome c oxidase, cytochrome c peroxidase and cytochrome P450. (c) 2006 Elsevier B.V. All rights reserved.