Measure native intensitiesPrepare isomorphous heavy atom derivativesMeasure derivative intensitiesScale native and derivative dataDetermine heavy atom positions and occupanciesCorrelate origin and hand between derivativesCompute protein phases

Introduction to Isomorphous Replacement and Anomalous Scattering Methods

An isomorphous derivative crystal is one in which the onlychanges in electron density between it and the nativecrystal 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, andsolvent molecules are displaced. Fortunately, these effects are often small, localized andmostly alter the low resolution data. Nevertheless, for isomorphous crystals as we havedefined them, the structure factor for any reflection in the derivative crystal can beexpressed 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 thesame 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 algebraicand geometric point of view.

PHASE DETERMINATION BY SINGLE ISOMORPHOUS REPLACEMENT.