Multiple isomorphous replacement
is the most common approach of solving the phase problem in X-ray crystallography
. This method is conducted by soaking the crystal of a sample to be analyzed with a heavy atom
solution or co-crystallization with the heavy atom. The addition of the heavy atom should not affect the crystal formation or unit cell
dimensions in comparison to its native form, hence, they should be isomorphic
Data sets from the native and heavy-atom derivative of the sample are first collected. Then the interpretation of the Patterson difference map reveals the heavy atom's location in the unit cell. This allows both the amplitude and the phase of the atom to be determined. Since the structure factor of the heavy atom derivative (Fph) of the crystal is the vector sum of the lone heavy atom (Fh) and the native crystal (Fp) then the phase of the native Fp and Fph vectors can be solved geometrically.
At least two isomorphous derivatives must be evaluated since using only one will give two possible phases.
Some examples of heavy atoms used in protein MIR:
Two methods for providing the needed phasing information by introducing heavy atoms into isomorphous crystals: