Characterization of water wires inside hydrophobic tubular peptide structures

Abstract

The crystallographic observation of a hydrophobic, empty channel (diameter 5.2 Å) in the peptide Boc-DPro-Aib-Leu-Aib-Val-OMe, prompted the investigation of the analog Boc-DPro-Aib-Val-Aib-Val-OMe in which the side chain at position 3 was shortened, resulting in the structure of a channel (diameter 7.5 Å) containing a one-dimensional wire of water molecules. Crystallization in the space group P65 facilitates formation of a pore lined entirely by hydrocarbon side chains. Two forms of the entrapped water wires, with O···O separations of 3.5 and 2.6 Å, are discussed. A lone hydrogen bond between the adjacent pairs of water molecules in the wire, with no strong interactions between the second water hydrogen and the hydrophobic walls of the channel, is a feature of the one-dimensional array. The structure provides the first crystallographic characterization of a water wire in a hydrophobic channel with implications in water and proton transport in membranes and carbon nanotubes.