Measuring the lengths of molecules.

"Small is beautiful" has become the motto of chemists investigating molecular arrays on the nanometer (10-9 meter) scale. Besides learning about the properties of molecules as they begin to form such assemblies, these tiny arrays may one day be the building blocks of minuscule electroni devices and chemical sensors (sn:10/3/87,p.214).

But in order for good things to come from these tiny molecular packages, scientists must devise tools for imaging and measuring them. In the Jan. 20 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, researchers describe a kind of "molecular ruler" for doing that. Larry L. Miller at the University of Minnesota in Minneapolis and his colleagues developed an improved method for using a scanning transmission electron microscope (STEM) to measure organic molecules' lenghts.

normally these molecules cannot be imaged with a STEM because it only detect atoms that have a dense enough electron cloud to deflect the STEM's probing electron beam. So scientists have sized up organic molecules by marking their ends with a protein called ferritin, which contains electron- rich irons atoms. However, ferritin is large and unpredictable in the way it binds to the molecules, making it difficult to measure structure smaller than about 20 nanometers. Moreover, says Miller, it wasn't entirely clear that this method was accurate, since past work was done on biological molecules that could distort when ferritin was attached.

Miller's group showed that the labeling method does indeed work by measuring rigid organic molecules of previously calculated lenghts. And instead of ferritin, the researchers attached smaller iridium clusters to the molecules' ends, which enabled them to resolve distances as small as 2 nanometers. Miller expects that with refinement, the technique will allow his group to measure separations 10 time smaller-distances just a bit larger than the typical length of molecular bonds.