In biological water channel aquaporins (AQPs),it is believed that the bipolar orientation of the single-file water molecules inside the channel blocks proton permeation but not water transport. In this paper,the water permeation and particularly the water-selective behaviour across a single-walled carbon nanotube (SWNT) with two partial charges adjacent to the wall of the SWNT are studied by molecular dynamics simulations,in which the distance between the two partial charges is varied from 0.14 nm to 0.5 nm and the charges each have a quantity of 0.5 e. The two partial charges are used to mimic the charge distribution of the conserved non-pseudoautosomal (NPA) (asparagine/proline/alanine) regions in AQPs. Compared with across the nanochannel in a system with one +1 e charge,the water permeation across the nanochannel is greatly enhanced in a system with two +0.5 e charges when charges are close to the nanotube,i.e. the two partial charges permit more rapid water diffusion and maintain better bipolar order along the water file when the distance between the two charges and the wall of SWNT is smaller than about 0.05 nm. The bipolar orientation of the single-file water molecules is crucial for the exclusion of proton transfer. These findings may serve as guidelines for the future nanodevices by using charges to transport water and have biological implications because membrane water channels share a similar single-file water chain and positive charged region at centre and provide an insight into why two residues are necessitated in the central region of water channel protein.
