Desformylgramicidin: A model channel with an extremely high water permeability

The water conductivity of desformylgramicidin exceeds the permeability of g ramicidin A by two orders of magnitude. With respect to its single channel hydraulic permeability coefficient of 1.1.10(-12) cm(3) s(-1), desformylgra micidin may serve as a model for extremely permeable aquaporin water channe l proteins (AQP4 and AQPZ). This osmotic permeability exceeds the conductiv ity that is predicted by the theory of single-file transport. It was derive d from the concentration distributions of both pore-impermeable and -permea ble cations that were simultaneously measured by double barreled microelect rodes in the immediate vicinity of a planar bilayer. From solvent drag expe riments, approximately five water molecules were found to be transported by a single-file process along with one ion through the channel. The single c hannel proton, potassium, and sodium conductivities were determined to be e qual to 17 pS (pH 2.5), 7 and 3 pS, respectively. Under any conditions, the desformyl-channel remains at least 10 times longer in its open state than gramicidin A.