By Jim Barlow Stable organic structures the size of small proteins with interior cavities have been formed by the hydrogen-bond-driven self assembly of "programmed" synthetic polymers known as dendrimers. Self-assembling complexes that produce cavities capable of encapsulating small molecules are not new. Nor are studies aimed at making dendrimers, which are tree-like, multibranched polymers and among the largest organic molecules known. In what is viewed as a fundamental breakthrough, however, UI chemists led by Steven C. Zimmerman mimicked nature, creating a natural-like molecular self-assembly process in which six identical dendrimers interact via hydrogen bonding, forming a larger, hexameric structure. Zimmerman documented the creation of distinct disk-shaped assemblies - measuring 10 nanometers in diameter and two nanometers thick - in the Feb. 23 issue of the journal Science. Co-authors were graduate students Fanwen Zeng, David E.C. Reichert and Sergei V. Kolotuchin. A related journal article said Zimmerman's technique allows for much better control of dendrimer architecture. Previous syntheses of dendrimers involved a series of carbon-carbon, carbon-oxygen or peptide bonding, all of which have worked in making dendrimers. However, resulting structures contained defects and took on unpredictable sizes and shapes as they got bigger. Zimmerman's use of more controllable hydrogen bonding interaction resulted in a two-layered structure in which two benzene rings in the center each were surrounded at 60 degree angles by six interconnected tetracarboxylyic acid branches. In analyzing the action of the dendrimers, the researchers unexpectedly found that the smaller, first-generation dendrimers would form linear structures, but subsequent larger generations were forced by their size to fold into desirable hexamers. "We have demonstrated the fundamental principle for creating subunits that form together predictively into a larger and more stable single unit. Hydrogen bonds are the glue that hold it together," said Zimmerman, an organic chemist and affiliate of the UI Beckman Institute for Advanced Science and Technology. "It's actually more stable than we expected it to be." Zimmerman's basic research - funded by the National Institutes of Health - primarily is of interest to chemists at this point, but eventually could be applied in future nano-sized electronic equipment or for drug-delivery systems for the human body. "If we can get the sphere large enough, we can start to encapsulate things exactly the same way viruses encapsulate DNA or RNA," he said. "It may be possible to encapsulate drug molecules within the periphery of the dendrimers that specifically recognize certain cell types, such as those with cancer, and deliver the drugs to where they are needed."