ON THE EXTENDIBILITY OF X-RAY CRYSTALLOGRAPHY TO NONCRYSTALS

This paper discusses the concept that crystallinity is not an essentia l requirement for applying the techniques of X-ray crystal structure a nalysis. Assuming this to be true, the removal of crystallinity as a p rerequisite for the techniques would allow the imaging of structures w ell beyond the present range of sizes accessible to X-ray crystallogra phy. An example of an imageable structure could be a single small biol ogical cell, containing perhaps 10(13)-10(14) Da. The proposed concept differs from the usual diffraction method of studying noncrystalline structure, i.e. small-angle scattering, in carrying out the diffractio n experiment and subsequent processing as if the structure being studi ed were in fact the asymmetric unit of a crystal: i.e.: orienting the structure in all directions in the X-ray beam needed to explore its Fo urier transform (F transform), and phasing and inverting the transform to obtain the electron-density image of the structure. The one actual difference from the crystal case is that the F transform is faint and also continuous, rather than displaying discrete intense Bragg spots. As a result, to get a readable pattern, the structure must be exposed to high levels of radiation. This last fact creates the principal lim itation of the technique. With single air-dried biological cells at ro om temperature as diffracting specimens and soft X-rays in the wavelen gth range 18-32 Angstrom, patterns to date have not been observed beyo nd resolutions of 140-300 Angstrom before radiation damage has become evident. At this resolution, the technique nevertheless would lie on t he same curve of resolution vs specimen size as do the existing major imaging techniques of X-ray crystallography, electron microscopy and l ight microscopy, falling directly between the latter two. Thus, X-ray diffractive imaging is not destroyed by the withdrawal of crystallinit y but instead is shifted to a new size range of structures, which have hitherto been somewhat inaccessible to imaging. A method for phasing the diffraction pattern, based on the ability to sample the pattern mo re finely than in the case of the crystalline specimen, is giving good results in preliminary testing. The principal need at present is for better instrumentation for collecting the diffraction data, including the additional motions needed for collecting data:in three dimensions.