Protein crystallography multiple isomorphous replacement

In small-molecule crystallography the phase problem was solved by so-called direct methods (recognized by the award of a Nobel Prize in chemistry to Jerome Karle, US Naval Research Laboratory, Washington, DC, and Herbert Hauptman, the Medical Foundation, Buffalo). For larger molecules, protein aystallographers have stayed at the laboratory bench using a method pioneered by Max Perutz and John Kendrew and their co-workers to circumvent the phase problem. This method, called multiple isomorphous replacement... 

The problem of phase determination is the fundamental one in any crystal structure analysis. Classically protein crystallography has depended on the method of multiple isomorphous replacement (MIR) in structure determination. However lack of strict isomorphism between the native and derivative crystals and the existence of multiple or disordered sites limit the resolution to which useful phases may be calculated. 

In early 1948 I thought that there was an experimental solution of the phase problem of X-ray crystallography. The idea was to use a double reflection hj followed by I12 which diffracts in the direction of I13 = hi + I12. If hi is set on the sphere of reflection so that it diffracts for any orientation of the crystal about a suitably chosen rotation axis, then hi and I12 should show an interference effect. This idea, beautiful in principle, was defeated by the mosaic character of crystals and possibly also crystal boimdary effects. Our experiment in which hi is 040 of a glycine crystal failed, although some reflections which were forbidden as single diffractions were observed. Shortly thereafter (1951) Bijvoet published his experimental solution to the phase problem by multiple isomorphous replacement methods, and I thought then that his discovery opened the way to solve protein structures. However, I did not start work in this direction until about 1958, and pursued it seriously beginning in 1961. 

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