DNA isn't just for storing genetic codes any more. Since DNAcan polymerize -- linking many molecules together into largerstructures -- scientists have been using it as a nanoscale buildingmaterial, constructing geometric shapes and even working mechanicaldevices.
Now Cornell University researchers have made DNAbuckyballs -- tiny geodesic spheres that could be used for drugdelivery and as containers for chemical reactions.
The term"buckyballs" has been used up to now for tiny spherical assemblies ofcarbon atoms known as Buckminsterfullerenes or just fullerenes. Underthe right conditions, carbon atoms can link up into hexagons andpentagons, which in turn assemble into spherical shapes (technicallytruncated icosahedrons) resembling the geodesic domes designed by thearchitect-engineer Buckminster Fuller. Instead of carbon, the Cornellresearchers are making buckyballs out of a specially prepared, branchedDNA-polystyrene hybrid. The hybrid molecules spontaneouslyself-assemble into hollow balls about 400 nanometers (nm) in diameter.The DNA/polystyrene "rods" forming the structure are each about 15 nmlong. (While still on the nanoscale, the DNA spheres are much largerthan carbon buckyballs, which are typically around 7 nm in diameter.)
About70 percent of the volume of the DNA buckyball is hollow, and the openspaces in the structure allow water to enter. Dan Luo, Cornellassistant professor of biological and environmental engineering inwhose lab the DNA structures were made, suggests that drugs could beencapsulated in buckyballs to be carried into cells, where naturalenzymes would break down the DNA, releasing the drug. They might alsobe used as cages to study chemical reactions on the nanoscale, he says.
Thenanoscale, hollow buckyballs are also the first structures assembledfrom "dendrimerlike DNA." If three strands of artificial DNA arecreated such that portions of each strand are complementary to portionsof another, the three strands will bind to each other over thecomplementary portions, creating a Y-shaped molecule. By joiningseveral Y's in the same way, Luo's research group created moleculeswith several arms, a sort of tree shape (dendri- means tree in Greek).Then they attached polystyrene molecules to the dendrimerlike DNAforming a hybrid molecule called an amphiphile -- a molecule that bothlikes and hates water. DNA is hydrophillic -- attracted to water --while polystyrene is hydrophobic -- water repels it.
Theresearchers expected the amphiphiles to assemble in water into somesort of solid structure arranged so that DNA would have a maximuminteraction with water and polystyrene would avoid water as much aspossible. Other researchers have used other amphiphiles to makespheres, rods and other solids. The hollow buckyballs were anintriguing and serendipitous surprise. A model suggests that onebuckyball consists of about 19,000 amphiphiles, with their water-lovingDNA mostly on the outside of the rods that form the structure. Howthese tens of thousands of molecules were able to self-organize to formsuch an intricate and complex structure is still an open question, theresearchers say. They are seeking collaborators to solve the puzzle.
Luoand Ph.D. graduate students Soong Ho Um, Sang Yeon Kwon and Jong BumLee described DNA buckyballs in an invited talk titled "Self-assemblyof nanobuckyballs from dendrimer-like-DNA-polystyrene amphiphiles"Sunday, Aug. 28, at the 2005 annual meeting of the American ChemicalSociety in Washington, D.C. They reminded the audience that althoughthe geometry of solid truncated icosahedrons was first described byArchimedes on paper more than 2,000 years ago, the skeletal,hollow-faced version of buckyballs had not been envisioned untilLeonardo da Vinci's illustrations in 1494.
Luo added that DNAbuckyballs may turn out to have unusual electronic, photonic andmechanical properties, and that because DNA is easily labeled andmanipulated, his research group's work offers a way to study in detailthe self-assembly process -- a process very important to the futuredevelopment of nanotechnology.
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