The protonation and deprotonation effects on electron transport properties of single butane molecule terminated with amine or carboxylic-acid groups are theoretically investigated. The numerical results show that current values of the molecular junction have a significant change as amine (or carboxylic-acid) groups are protonated (or deprotonated), presenting an obvious switching behavior. The study demonstrates that protonation of the amine groups weakens the coupling energy between molecules and electrodes, whereas deprotonation of the carboxylic-acid groups strengthens it. This Letter is used to understand the experimental observations that suggest a chemically controllable method on manipulating conductance of molecular junctions. (C) 2013 Elsevier B. V. All rights reserved.