Introduction to Isomorphous Replacement and Anomalous Scattering Methods Measure native intensities Prepare isomorphous heavy atom derivatives Measure derivative intensities Scale native and derivative data Determine heavy atom positions and occupancies Correlate origin and hand between derivatives Compute protein phases

An isomorphous derivative crystal is one in which the only changes in electron density between it and the native crystal are peaks at the sites of heavy atom substitution. The above definition implies no appreciable changes in packing contacts or unit cell constants and no change in space group. Does it ever happen? No. Changes usually occur in protein conformation near the heavy atom binding sites, and solvent molecules are displaced. Fortunately, these effects are often small, localized and mostly alter the low resolution data. Nevertheless, for isomorphous crystals as we have defined them, the structure factor for any reflection in the derivative crystal can be expressed as the vector sum of the corresponding structure factors for the native and “heavy atom” crystals, where the “heavy atom” crystal is a hypothetical crystal with the same cell and symmetry as the native but containing only the heavy atoms. It is instructive to consider the consequences of this relationship from both an algebraic and geometric point of view.

PHASE DETERMINATION BY SINGLE ISOMORPHOUS REPLACEMENT.