X-ray crystallography nailed stereochemistry of organic compound
In 1951, Johannes M. Bijvoet, a crystallographer at Utrecht University in the Netherlands, reported an X-ray crystal structure that changed the course of organic chemistry. Bijvoet’s absolute structure of sodium rubidium (+)-tartrate tetrahydrate marked the first time stereochemistry had been determined by experiment (Nature 1951, DOI: 10.1038/168271a0). Organic chemists could finally associate the molecular properties of an optically active compound with its absolute configuration.
The work revealed that German chemist Emil Fischer had guessed correctly when arbitrarily defining glucose’s d configuration as the (+) isomer. This meant that the configurations of the hundreds of organic compounds that had been determined based on Fischer’s arbitrary assignment were also correct. Organic compounds no longer had to be tediously synthesized from a molecule of known chirality for chemists to be certain of their configuration.
Bijvoet’s work also marked a key advance in X-ray crystallography. In conventional X-ray diffraction, crystallographers measure only the amplitudes of the diffraction. This information reveals interatomic distances but not phase information, which is necessary to determine absolute configuration. Bijvoet got around this problem by using a technique called anomalous X-ray scattering. The method had been used previously to determine the absolute structure of the inorganic compound zinc sulfide. Bijvoet was the first to apply it to an organic molecule.
The strategy involves using an X-ray wavelength that just excites a single heavy atom—in Bijvoet’s case, rubidium—to extract phase information. Anomalous X-ray scattering is still widely used today.
Bijvoet became interested in using X-ray crystallography for structure determination of organic molecules in the early 1940s, says Martin Lutz, a structural chemist at the Bijvoet Center for Biomolecular Research at Utrecht University. At the time, Fritz Kögl, an organic chemist there, had a theory that nonnatural d-amino acids caused cancer. He needed a method to distinguish d-amino acids from l-amino acids, Lutz notes. Bijvoet recalled the structure determination of zinc sulfide by X-ray crystallography. “He was able to transfer this inorganic knowledge to the organic world,” Lutz says.
A year after publishing the structure, Bijvoet moved his lab to a converted villa, making his private home part of the same building. His students called it the Crystal Palace. He lived there until he retired from Utrecht in 1962. Bijvoet passed away in 1980, at the age of 88.—Britt Erickson