The mobility of single-file water molecules is governed by the number of H-bonds they may form with channel-lining residues

Abstract

Channel geometry governs the unitary osmotic water channel permeability, <i>p</i>_f, according to classical hydrodynamics. Yet, <i>p</i>_f varies by several orders of magnitude for membrane channels with a constriction zone that is one water molecule in width and four to eight molecules in length. We show that both the <i>p</i>_f of those channels and the diffusion coefficient of the single-file waters within them are determined by the number <i>N</i>_H of residues in the channel wall that may form a hydrogen bond with the single-file waters. The logarithmic dependence of water diffusivity on <i>N</i>_H is in line with the multiplicity of binding options at higher <i>N</i>_H densities. We obtained high-precision <i>p</i>_f values by (i) having measured the abundance of the reconstituted aquaporins in the vesicular membrane via fluorescence correlation spectroscopy and via high-speed atomic force microscopy, and (ii) having acquired the vesicular water efflux from scattered light intensities via our new adaptation of the Rayleigh-Gans-Debye equation.