Teaching old coefficients new tricks: new insight into the meaning of the osmotic and diffusive permeation coefficients.

In living cells specialized water channels, the glycero-aquaporins, are responsible for rapid passive transport of water through the membrane. Crystal structures showed that these protein pores have diameters comparable to the size of a single water molecule (around 0.3 nm) and lengths around 2 to 3 nm. Despite being strongly confined, water moves rapidly through these and other narrow peptide pores such as gramicidin. Understanding how water behaves in confinement has not been easy, not the least because there is no experimental technique available to unambiguously measure water flux through a single water pore that would be comparable to single channel patch clamp recordings on ion channels. Thus, if we want to understand how a physiological function such as the water flux derives from the structure, namely the geometry and chemical character of the water pathway, we can do this only with models that relate macroscopic transport coefficients to microscopic properties of the pore. Although there is no satisfactory microscopic theory for water transport across all the pore dimensions relevant for biology (1,2) it is clear that the single most important parameter for water permeation is the radius of the pore.