Working together at Harvard in the early 1970s, Karplus and Warshel developed a computer program that brought together classical and quantum physics. Warshel later joined forces with Levitt at the Weizmann Institute of Science in Israel and at the University of Cambridge in Britain to develop a program that could be used to study enzymes.
Jeremy Berg, a professor of computational and systems biology at the University of Pittsburgh, said the winning work gives scientists a way to understand complicated interactions that involve thousands to millions of atoms.
"There are thousands of laboratories around the world using these methods, both for basic biochemistry and for things like drug design," said Berg, former director of the National Institute of General Medical Sciences in Bethesda, Md.
Many drug companies use computer simulations to screen substances for their potential as medicines, an approach that lets them focus their chemistry lab work on those that look promising, he said.
James Skinner, director of the Theoretical Chemistry Institute at the University of Wisconsin-Madison, said the approach pioneered by the winners can be used to analyze such things as how drugs bind to the molecules they target in the body, or how large molecules fold.
Beyond that, such simulations can be used to design materials with specific characteristics, such as those used in airplanes, he said.
"This has led to greater understanding (of) problems that couldn't be solved experimentally," said Marinda Li Wu, president of the American Chemical Society.
Earlier this week, three Americans won the Nobel in medicine for discoveries about how key substances are moved around within cells, and the physics award went to British and Belgian scientists whose theories help explain how matter formed in the universe after the Big Bang.
The Nobel in literature will be announced on Thursday, the Nobel Peace Prize on Friday and the economics prize on Monday.
Karl Ritter reported from Stockholm. AP writers Malin Rising in Stockholm, Frank Jordans in Berlin and Rodrique Ngowi in Cambridge, Mass., contributed to this report.
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