How Bijvoet Made the Difference: The Growing Power of Anomalous Scattering

Johannes Bijvoet made pioneering contributions to the determination of noncentrosymmetric structures. He was the first to exploit the isomorphous replacement method to reveal a noncentrosymmetric structure, using isomorphous sulfate and selenate salts to determine the structure of strychnine on the basis of two projections. He recognized that anomalous scattering could be used to identify the correct enantiomorph of a noncentrosymmetric structure. In 1951, Bijvoet and his colleagues observed the intensity differences between Friedel-related pairs of X-ray reflections from sodium rubidium tartrate crystals. These observed differences showed that the convention established by Emil Fischer to discuss the configuration of bonds at asymmetric carbon atoms, especially in sugars, represents the true three-dimensional (3D) enantiomorph of these molecules. The Bijvoet differences did give significant information to resolve ambiguities in the phases determined by isomorphous replacement, but the large errors made a quantitative estimate difficult. The results were simply categorized as Bijvoet difference probably positive, insufficient information, or Bijvoet difference probably negative.