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(New page: "Phase problem" refers to the fact that in X-ray analysis of macromolecules, phases cannot be measured, but have to be calculated. This is a problem because both the amplitudes (which are ...) |
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"Phase problem" refers to the fact that in X-ray analysis of macromolecules, phases cannot be measured, but have to be calculated. This is a problem because both the amplitudes (which are the result of measurement) ''and'' phases are needed to compute the electron density (see K. Cowtan's [http://www.ysbl.york.ac.uk/~cowtan/fourier/fourier.html Book of Fourier]), and the procedures for calculating phases are difficult to explain and understand. | "Phase problem" refers to the fact that in X-ray analysis of macromolecules, phases cannot be measured, but have to be calculated. This is a problem because both the amplitudes (which are the result of measurement) ''and'' phases are needed to compute the electron density (see K. Cowtan's [http://www.ysbl.york.ac.uk/~cowtan/fourier/fourier.html Book of Fourier]), and the procedures for calculating phases are difficult to explain and understand. | ||
A recent introduction into the principles of phase calculation with the help of experiments (SIR/MIR/SIRAS/MIRAS/SAD/MAD) can be e.g. found in Taylor, G. (2003) The phase problem. Acta Cryst D59, 1881-1890 [http://journals.iucr.org/d/issues/2003/11/00/ba5050/index.html]. | A recent introduction into the principles of phase calculation with the help of experiments ([[experimental phasing]]; SIR/MIR/SIRAS/MIRAS/SAD/MAD) can be e.g. found in Taylor, G. (2003) The phase problem. Acta Cryst D59, 1881-1890 [http://journals.iucr.org/d/issues/2003/11/00/ba5050/index.html]. | ||
The phase problem may also be solved with the positioning of a similar molecule in the correct orientation and location in the asymmetric unit of the (crystallized) unknown structure. This approach is called [[Molecular Replacement]] (MR). | The phase problem may also be solved with the positioning of a similar molecule in the correct orientation and location in the asymmetric unit of the (crystallized) unknown structure. This approach is called [[Molecular Replacement|molecular replacement]] (MR). | ||
The solution of the phase problem with the help of [[Direct Methods]] was awarded with the Nobel Prize in 1985 ([http://www.iucr.org/people/nobel-prize]). In the context of macromolecular crystallography, this approach is used in substructure determination, and at very high resolution (beyond 1.2A). | The solution of the phase problem with the help of [[Direct Methods|direct methods]] was awarded with the Nobel Prize in 1985 ([http://www.iucr.org/people/nobel-prize]). In the context of macromolecular crystallography, this approach is used in substructure determination, and at very high resolution (beyond 1.2A). | ||