Our ab initio QM calculations provide insight on proton transfer at interfaces with dense packing of proton-binding surface groups, a key phenomenon in biophysics and materials science. From potential energy surfaces, we determined lowest energy path and activation energy of an elementary proton transition. Concerted tilting-rotation modes of charged surface groups facilitate the lateral proton exchange. Frequency spectra of these reactive modes indicate that local fluctuations of surface groups couple only weakly to relaxations of the remaining interface. (c) 2008 Elsevier B. V. All rights reserved.