Phase problem

"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 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 (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 [1].

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 solution of the phase problem with the help of direct methods was awarded with the Nobel Prize in 1985 ([2]). In the context of macromolecular crystallography, this approach is used in substructure determination, and at very high resolution (beyond 1.2A).