The manipulation of charge transport through single molecules so that electronic information can be controlled is a basic challenge that is important for both fundamental understanding of the mechanisms and the potential applications in single-molecule technologies. This paper reports the influence of protonation on the gating effect in a series of molecular wires utilizing a pyridinoparacyclophane (PPC) moiety as the edge-on gate. It was found that the molecular conductance, transition voltage, and the corresponding tunnelling barriers can be reversibly switched by the protonation/deprotonation process of the nitrogen atom on the PPC pyridine ring. It was found that protonation levels off the tunnelling barrier of different molecules and converts p-type molecular wires into n-type, reversibly.