Multiple isomorphous replacement

Multiple isomorphous replacement or MIR is, historically speaking, the most common approach to solving the phase problem in X-ray crystallography. For protein crystals 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 (F_ph) of the crystal is the vector sum of the lone heavy atom (F_h) and the native crystal (F_p) then the phase of the native F_p and F_ph vectors can be solved geometrically.

$\mathbf F_{ph} = \mathbf F_p + \mathbf F_h$

At least two isomorphous derivatives must be evaluated since using only one will give two possible phases.

Examples

Some examples of heavy atoms used in protein MIR:

• Hg²⁺ ions bind to thiol groups.
• Uranyl salts (UO₂ + NO₃) bind between carboxyl groups in Asp and Glu
• Lead binds to Cys residues.
• PtCl₄²⁻ (ion) bind to His

See also

Anomalous Dispersion

• Multi-wavelength anomalous dispersion (MAD)
• Single wavelength anomalous dispersion (SAD)

Isomorphous Replacement

Two methods for providing the needed phasing information by introducing heavy atoms into isomorphous crystals:

• Multiple isomorphous replacement (MIR); and
• Single isomorphous replacement with anomalous signal (SIRAS)

Other

• Patterson map

External links

• MAD phasing — an in depth tutorial with examples, illustrations, and references.

Computer programs

• The SSRL Absorption Package —
• CHOOCH —
• Shake-and-Bake (SnB) —
• SHELX —