Choose Your Bond

Two completely unrelated papers have got us thinking about chemical bonds. When we refer to chemical bonds, we generally mean covalent bonds: Atoms become "wedded," sharing electrons, and breaking them apart takes energy. By comparison, other types of bonds are weak attractions - mere flirtations, or mild sparks between hydrogen and oxygen in passing water molecules.

So why would a researcher in organic chemistry - a field based on carbon, the king of covalent bonds - be investigating the properties of non-covalent, hydrophobic bonds? The answer, of course, is that they can be used to create materials with unique properties. Let's back up a minute. Hydrophobic bonds occur between hydrophobic - or "water-hating" - molecules in the presence of water. Water, it seems, is equally averse to contact with hydrophobic molecules, and sticking closely to their own kind turns out to be the most amicable arrangement for both. And, while not exactly eternal, hydrophobic bonds turn out to be surprisingly strong. Strong enough, according to Dr. Boris Rybtchinski, to create filters for sorting nanoparticles that can even work under a bit of pressure. The beauty of using hydrophobic bonds comes after the particle solution has passed through the filter. Since the hydrophobic molecules are not covalently "wedded," their bonds are easily dissolved in a bit of ethanol - and easily reformed, as well, for repeat filtering. Just add water.

Hydrophobic bonds are better known in the world of biochemistry. Many structures and processes in the biological cell depend, at least in part, on the avoidance tactics of hydrophobic molecules and water. Lipids, for instance, are hydrophobic, and the formation of various lipid membranes in the cell is a classic example of hydrophobic bonding. Prof. Zvulun Elazar of the Institute's Biological Chemistry Department has been investigating how a double-walled lipid membrane gets assembled in a temporary organelle called an autophagosome - a sort of digesting tank for recycling cellular components. Two different proteins are involved in cobbling together smaller membrane fragments, fusing their walls and sealing off the whole organelle around the recyclable stuff. Surprisingly enough, these proteins attach the lipid fragments together using - covalent bonds! Apparently the having the strongest possible seal is more important for the autophagosome than easy disassembly.

What does it all mean? I suppose that depends on whether you're a chemist looking for inspiration in the world of biology or a biologist who is finding that it all boils down to chemistry. In any case, the timing of these two papers is, at the very least, an interesting coincidence.