Density functional theory calculations were carried out employing a set of truncated model systems for the EcClC Cl transport protein and mutants. Geometry optimizations implied a possible additional binding site near the residue R147. Analysis based on the Lowdin and Natural charge models showed that significant (about 30%) charges on the bound Cl ions were smeared out among the solvation shells and the gating residue E148, whose delocalized pi bonds provide good accommodations for the transferred charges. The extensive anion-pi charge transfer along the Cl translocation path might help to smooth/level the energy profiles for Cl transport. (C) 2010 Elsevier B.V. All rights reserved.