PNNL

Abstract

The transport of water to and from the interior of cells is essential for cellular physiological function. Water molecules diffuse through cell membranes by two general pathways.1,2 The first, the lipid pathway, is universal in all cells in the human body. Water simply diffuses through the fluid lipid bilayer by a dynamic process with relatively slow velocity.1 The water transport process can thus be facilitated by the presence of specific channels in the cell membrane such as aquaporins.2,3 These are a family of proteins that associate as tetramers to form cylindrical transmembrane filters that are 2 to 3 nm long, and about 0.3 nm wide at their narrowest point. In each of these pathways, the simple notion of water transport through any matrix implies the presence of suitably sized channels defined and constricted by van der Walls surfaces.4 Recent work with crystals composed of calixarene molecules has revealed that small and mobile molecular species, such as water, can diffuse into the hydrophilic pockets of the host lattice without evidence of channels. 5 This finding raised questions about conventional conception of crystal porosity that assumed transport of guests in solid media requires suitably sized channels. Herein, we report the transport phenomena of water and CO2 molecules within seemingly nonporous crystals of the antibiotic clarithromycin (6-O-methylerythromycin A, 1), despite a lack of channels in the crystalline state.