Thursday, Nov. 11, 2010
Nobel laureate Linus Pauling is probably best known for his advocacy of vitamin C for good health. But the quantum chemist, who died in 1994, also developed a definition for hydrogen bonds that’s been the textbook standard for more than 50 years.
Hydrogen bonding is a molecular phenomenon that explains many of the physical properties of liquids and compounds, including the “rungs” that hold DNA’s ladder-like double helix together and why objects float in water.
Pauling’s definition is changing, however, as chemists, using such tools as computing and spectroscopy, are discovering more about the interaction of hydrogen atoms. Utah State University chemist Steve Scheiner is one of two leaders of a 14-member team that contributed to a proposed updated definition recently published by the International Union of Pure and Applied Chemistry’s Physical and Biophysical Chemistry Division, the world authority on chemical nomenclature. The international chemical community has until the end of March 2011 to respond the proposal, which is expected to be adopted shortly thereafter.
“The new definition doesn’t so much change the existing definition; rather it broadens the meaning into a more modern, inclusive definition,” says Scheiner, professor in USU’s Department of Chemistry and Biochemistry.
Many current and former chemistry students will recall that a hydrogen bond is written as “AH - - B,” where a hydrogen atom interacts with electronegative atoms (A and B) from such elements as nitrogen, oxygen or fluorine.
“But it isn’t necessarily limited to just those elements,” says Scheiner, the 2010 recipient of USU’s D. Wynne Thorne Career Research Award, the university’s top research honor. “A or B could be chlorine, sulfur or carbon and the ‘B’ in the formula doesn’t necessarily have to be an atom; it could also be a chemical bond between two atoms.”
Using computer calculations, he says, scientists can essentially dissect hydrogen bonds and examine the forces that create them.
Spectroscopy is another tool at scientists’ disposal. In addition to infrared — IR — spectroscopy, which tells chemists about the strengths of chemical bonds, scientists use nuclear magnetic resonance spectroscopy — more commonly known as NMR — to investigate the properties of hydrogen bonds. Scheiner has shown how spectroscopy can be used to characterize a previously overlooked hydrogen bond interaction, known as the CH - - O bond. The interaction, though weak, is a critical component of protein structure.
“This kind of interaction shifts the IR peak toward the blue end of the spectrum,” he says. “It was previously thought that hydrogen-bond interactions always shifted the peak to the red.”
Scientific definitions aren’t static, Scheiner says.
“Before the turn of the 20th century, scientists often thought of space and time as fully separate from one another, as well as constant and unchanging,” he says. “But those ideas changed dramatically. It’s the same with hydrogen bonding, where we have actual experimental findings to support new ideas.”
Writer: Mary-Ann Muffoletto, 435-797-3517, email@example.com