what is isomorphous replacement with respect to silicates? explain with an example.
Answers
Answer:
Isomorphous Replacement
The isomorphous replacement method was first applied by Max Perutz and John Kendrew back in the 1960s. It requires the attachment of a heavy atom (i.e with high atomic number, e.g. Hg, Pt, Au, Pb, Ag) to the protein in the crystal. Usually, the protein crystals are soaked in a solution containing salts of the heavy atom at low concentrations or alternatively, the protein can be co-crystallised with the heavy atom compound. The heavy atom addition/substitution should neither affect the protein structure nor the crystal unit cell dimensions (isomorphism), and so soaking is usually preferred. These heavy-atom derivatised crystals should show measurable changes in the intensities of some reflections in relation to the ‘native’ crystals, which can be used to deduce the positions of the heavy-atoms. In this method, the interference effects on the intensities of the diffracted beams caused by the addition of heavy atoms to the protein provide the estimates of the protein phase angles. A good single isomorphous derivative may be sufficient for structure determination (single isomorphous replacement, SIR), but usually multiple derivatives are necessary to derive phase information (multiple isomorphous replacement, MIR).