The use of Fournier transforms in protein crystal analysis.

Abstract

In the recent development of X-ray crystallography, methods of great power have been evolved for the determination of the structure of molecules with a large number of atoms. These have been used with complete success in the case of molecules with up to forty or so atoms, but have so far led to no conclusive results for those of the order of a thousand such as are provided in the structure of proteins. This is not simply due to their greater complexity as such. The fundamental difficulty lies in the limitations of the resolving power and what may be called the 'discrimination' of the X-ray patterns in relation to the actual structure which it is desired to analyse. The first difficulty is partly physical and experimental; it depends on the rapid fade-out of the diffraction intensities with angle. Clearly, if only planes of spacing >2⋅5 Å are observed, it is absolutely impossible to locate atoms and consequently to employ effectively the various methods of Patterson analyses or sign determina­tion which depend for their efficacy on the existence of discrete maxima in electron density. This difficulty can be, and indeed has been, overcome, as it is now possible, by technical improvements, to push the limit of observable planes down to 1⋅2 Å, which is just on the limit of resolvability of separate atoms, and there is no reason to suppose that it may not be driven still lower.