Multiple isomorphous replacement

Multiple isomorphous replacement (MIR) is historically the most common approach to solving the phase problem in X-ray crystallography studies of proteins. 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 (or ion) to the structure 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 heavy-atom contribution 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.

Development

Single Isomorphous Replacement (SIR)

Early demonstrations of isomorphous replacement in crystallography come from James M. Cork,[1] John Monteath Robertson,[2] and others. An early demonstration of isomorphous replacement in crystallography came in 1927 with a paper reporting the x-ray crystal structures of a series of alum compounds from Cork.[1] The alum compounds studied had the general formula A.B.(SO4)2.12H2O, where A was a monovalent metallic ion (NH4+, K+, Rb+, Cs+, or Tl+), B was a trivalent metallic ion (Al3+, Cr3+, or Fe3+) and S was usually sulfur, but could also be selenium or tellurium. Because the alum crystals were largely isomorphous when the heavy atoms were changed out, they could be phased by isomorphous replacement. Fourier analysis was used to find the heavy atom positions.

The first demonstration of isomorphous replacement in protein crystallography was in 1954 with a paper from David W. Green, Vernon Ingram, and Max Perutz.[3]

Examples

Some examples of heavy atoms used in protein MIR:

See also

Anomalous dispersion

Other

References
  1. Cork, J.M. (October 1927). "LX. The crystal structure of some of the alums". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 4 (23): 688–698. doi:10.1080/14786441008564371. ISSN 1941-5982.
  2. Robertson, J Monteath (1937-01-01). "X-ray analysis and application of fourier series methods to molecular structures". Reports on Progress in Physics. 4 (1): 332–367. doi:10.1088/0034-4885/4/1/324. ISSN 0034-4885. S2CID 250871380.
  3. Green, D. W.; Ingram, Vernon Martin; Perutz, Max Ferdinand; Bragg, William Lawrence (1954-09-14). "The structure of haemoglobin - IV. Sign determination by the isomorphous replacement method". Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences. 225 (1162): 287–307. doi:10.1098/rspa.1954.0203. S2CID 96889917.

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

Computer programs

Tutorials and examples
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