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.