Friday, June 25, 2010
Structure of Ultra-Small, Chiral Gold Nanoparticles Revealed
X-rays can reveal broken bones or a hidden weapon in a piece of luggage. For graduate student Huifeng Qian, X-rays have revealed that a gold nanoparticle he developed holds great promise as a chiral catalyst—a tool highly sought-after by the pharmaceutical industry.
Many drugs on the market today contain at least one molecule that is chiral—it exists in a right-handed configuration and a left-handed configuration. Often only one of the configurations, or isomers, is effective in the body. In some cases, the other isomer may be harmful. A striking example is the drug thalidomide, which consisted of two isomers: one of which helped pregnant women control nausea while the other caused damage to the developing fetus.
In an effort to create safer, more effective drugs, drug manufacturers are looking for ways to produce purer substances that contain only the left-or right-handed isomer. Qian, a second-year chemistry graduate student in the laboratory of Assistant Professor Rongchao Jin, has created a gold nanoparticle that has the potential to catalyze chemical reactions that will produce one isomer rather than the other. The ultra-small gold nanoparticles being developed in Jin's laboratory have unique electronic and surface properties that make them very promising as a new generation of chiral catalysts.
Using elegant chemical methods, Qian created nanoparticles comprised of precisely 38 gold atoms and measuring a mere 1.4 nanometers in size. His method allowed him to precisely control the number of atoms in each gold nanoparticle, resulting in uniformly-sized nanoparticles instead of the mixture of sizes typically created during nanoparticle preparation. Because the nanoparticles were of uniform size and shape, Qian was able to grow them into high-quality crystals. He and collaborator William Eckenhoff at Duquesne University then used single crystal X-ray crystallography to reveal the structure of the 38-gold-atom (Au38) nanoparticles.
To Qian and Jin's surprise, X-ray crystallography revealed that the Au38 nanoparticle is chiral. The unit cell is made up of one pair of left-handed and right-handed isomers. The core (Au23) of the Au38 nanoparticles is rod-shaped, and it is protected by three monomeric staples (RS-Au-SR) and six dimeric staples (RS-Au-S(R)-Au-SR); the dimeric staples are arranged in a rotating, staggered fashion that resembles two propellers assembled on the Au23 rod. This unique structure imparts chirality to the nanoparticle.
"We were really excited to discover that the Au38 nanoparticle is chiral," said Jin. "We are very interested in learning whether we can use Au38 as a catalyst to create chirally pure molecules." His laboratory will be testing Au38's catalytic capabilities over the next several months.
By: Amy Pavlak