On the Extendibility of X-ray Crystallography to Noncrystals

Abstract

This paper discusses the concept that crystallinity is not an essential requirement for applying the techniques of • X-ray crystal structure analysis. Assuming this to be true, the removal of crystallinity as a prerequisite for the techniques would allow the imaging of structures well beyond the present range of sizes accessible to X-ray crystallography. An example of an imageable structure could be a single small biological cell, containing perhaps 1013-1014 Da. The proposed concept differs from the usual diffraction method of studying noncrystalline structure, i.e. small-angle scattering, in carrying out the diffraction experiment and subsequent processing as if the structure being studied 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 Fourier 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 limitation of the technique. With single airdried biological cells at room temperature as diffracting specimens and soft X-rays in the wavelength range 18-32 A, patterns to date have not been observed beyond resolutions of 140--300 A before radiation damage has become evident. At this resolution, the technique nevertheless would lie on the same curve of resolution vs ~specimen size as do the existing majot" imaging techniques of X-ray crystallography, electron microscopy and light microscopy, falling directly between the latter two. Thus, X-ray diffractive imaging is not destroyed by the withdrawal of crystallinity but instead is shifted to a new size range of stmc3ures, which have hitherto been somewhat inaccessible to imaging. A method for phasing the diffraction pattern, based on the ability to sample the pattern more finely than in the case of the crystalline specimen, is giving good results in preliminary testing.

Biological imaging by soft x-ray diffraction microscopy

“…It provides a path to high resolution without the limitations imposed by an x-ray optical system. The idea to image a noncrystalline object by phasing and inverting its diffraction pattern goes back to a suggestion by Sayre (1,2) and was first demonstrated with x-rays by Miao et al (3). In this article, we report the imaging of the complex-valued exit wavefront (both phase and magnitude) of a whole freeze-dried and unstained yeast cell.…”

Femtosecond diffractive imaging with a soft-X-ray free-electron laser

“…An intense 25 fs, 4 × 10 13 W cm −2 pulse, containing 10 12 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nanostructured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling [5][6][7][8][9] , shows no measurable damage, and is reconstructed at the diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one 10 .…”

Two and Three Dimensional Extreme Ultraviolet Holographic Imaging with a Nanometer Spatial Resolution

“…Other experiments utilized synchrotron light to image biological samples, nano structures, and magnetic domains (Jacobsen et al, 1990;Lindas et al, 1996;McNulty et al, 1992). Lensless diffractive imaging, based on iterative phase retrieval, following the proposal by Sayre, (Sayre et al, 1998) have demonstrated SXR imaging with 50 nm spatial resolution utilizing = 1.5 nm source (Elsebitt et al, 2004). The first experimental demonstration of lens-less diffractive imaging using coherent soft X-rays generated by a tabletop SXR source allowed for image acquisition with spatial resolution of 214 nm (Sandberg et al, 2007) later improved to 72 nm (Sandberg et al, 2008).…”