Angewandte Chemie International Edition
Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (15), 2730 - 2734
Into the Breach
Nanofissures, molecular stirrers, and relays
Components for artificial photosynthesis?
Tiny cavities in enzymes and receptors are the places where chemical reactions occur. Such "nanofissures", measuring only a few millionths of a millimeter, can also be generated in man-made membranes and could be used for catalytic reactions. In the course of their investigations, Jürgen-Hinrich Fuhrhop's research group in Berlin made a surprising and unprecedented observation: water-soluble substances containing certain structural elements of the sugar glucose are held fast within artificial membrane fissures for weeks, even if they are in contact with water that does not contain any of the substance in question. The glucose "family" of molecules consists of various five- and six-membered rings that all contain a hydrophilic (water-loving) and a hydrophobic (water-hating) edge either above and below or to the left or right of the rigid plane of the ring. The hydrophobic edges lodge tightly against the likewise hydrophobic walls of the membrane fissure, coating them completely. The hydrophilic edges bind to water molecules. The unusual thing is that the latter edge exactly fits the hexagonal structure adopted by water in the frozen state. All of the water molecules within the nanofissure are thus fixed in the structure of ice. For weeks, the glucose molecules, whose most effective representatives include cellobiose, tyrosine, and ascorbic acid, cannot leave the water-filled gap in the membrane, and at the same time, salts cannot enter the fissure either. "This phenomenon could play a role in the recognition of sugar-specific messenger-molecules on cell surfaces," suspects Fuhrhop.
Adding 10 % ethanol to the water destroys the "ice structure" within the membrane gap. As does a narrow organic ion (dimethylviologen), which can be pulled into the gap by an applied electric potential, and can be moved around by cyclical changes in the potential. Within a few minutes, the molecule "stirs" everything out of the fissure. These phenomena also have analogues in nature: reversible membrane-rupture by alcohol and "stirrers" driven by the cellular energy source, ATP.
Fuhrhop hopes to be able to develop an artificial photosynthesis system with his membrane fissures. The immobile tyrosine and ascorbic acid molecules at the edge of the nanofissure present themselves as "relays" for electron transfer - an important step in photosynthesis - between molecules at the bottom of the membrane gap and those at the edge. The molecular stirrer could sweep "used" molecules out of the nanofissure, ensuring a constant supply.