E0010

The Case for Reconsidering Anomalous Scattering Effects. David A. Langs, Hauptman-Woodward Medical Research Institute, 73 High Street, Buffalo, NY 14203.

Last fall I was asked to prepare some lectures on the use of Fourier methods in X-ray crystallography for our graduate course in structural biology. When I presented these talks I took great pains to show the students, with the aid of appropriate figure diagrams, how the equations for the computation of structure factors from the electron density

F_h = __v _(r) exp (2_i h.r) dV (1)

and the electron density from the structure factors

_(r) = __h F_h exp (_2_i h.r ) (2)

were derived. To my surprise, however, I was later forced to conclude that these equations were not consistent with the convention that was used to derive the positive _f” imaginary scattering component of an atom in the presence of anomalous scattering radiation. It appeared to me that the signs in the exponential terms of equations (1) and (2) had to be reversed in order to be consistent with this positive _f” convention. If this is true, it follows that the absolute configurations of all molecules determined from previous anomalous scattering experiments would have to be changed to the opposite chiral hand, and the Fischer convention would be opposite to the configuration determined by the Bijvoet experiment.

Details concerning the confusion on this subject will be presented.

Research support from NIH grant GM-46733 is gratefully acknowledged.