Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (21), 4082 - 4085
Nanowheel with Antitumor Potential
Computer simulations predict wheel-shaped molecules
with unusually bound carbon atoms
Organic chemistry is two hundred years old; over 14 million carbon compounds have been characterized during these two centuries. Now we may be on the threshold of a true revolution, which could point the way to a new, extended chemistry of carbon. The basis of traditional organic chemistry has been that carbon atoms form a maximum of four bonds, which are arranged tetrahedrally. It now seems that this does not come close to exhausting the bonding capacity of this key element. According to computer calculations, it should be possible not only to have five or six bonds to carbon atoms surrounded by a planar ring but also to increase the bonding to carbon atoms still further by joining two of these rings together.
Paul von Ragué Schleyer and Zhi-Xiang Wang predict the existence of "wheel-shaped" molecules with such hypercoordinated carbon atoms. According to them, the "tread" of the "wheel" consists of two parallel, interconnected, planar rings of five or six boron atoms. The axle between the wheels is made up of two central carbon atoms, one in each ring, which are bound together. The bonds between the central carbon atoms and the surrounding boron atoms can be described as a double set of "spokes".
Closely related molecules in which the central carbon atoms are not bound to one another can also be favorable energetically. Conversions of the wheels by breaking their central axles result in spherical structures, which have a single free electron on each carbon atom - a biradical.
This family of "virtual" molecules may not be merely of academic interest: "Our hypothetical biradicals seem to have distinct potential as pharmaceuticals," explains Schleyer. "A very similar biradical electron configuration plays an important role in the antitumor activity of the enediyne antibiotics, which include dynemycin and neocarcinostatin." Within the body of the patient they are converted into the aforementioned biradicals, which destroy DNA and bring about the desired cancer cell death.
The predicted biradicals may even allow a second strategy in cancer therapy: they contain boron atoms, which must be put into tumors for a certain type of radiation treatment. Once in the tumor, the boron atoms capture the radiation and convert it into a different, DNA-damaging form of radiation on the spot.