Analysis by X-rays of Ultimate Structures of Living Materials

Abstract

SINCE the discovery in 1912 by von Laue and associates that crystals are three-dimensional diffraction gratings for x-rays, by virtue of the regular arrangement of the atoms and molecules upon lattice planes having spacings commensurate with x-ray wave lengths, a great chemical science of ultimate architectural structures of materials has risen to an eminent and well recognized place. Sir William H. Bragg and his son, Professor W. L. Bragg, began building upon the von Laue discovery and to this day retain their great leadership in the field. They demonstrated at once that a simple relationship governed the diffraction phenomenon and connected x-ray wave length, crystal interplanar spacing, and the angle of incidence, namely, the Bragg law n λ = 2d sin θ where n is an integer (the order of reflection) , the x-ray wave length, d the spacing of a set of parallel planes in a crystalline substance, and θ the angle of incidence, or 2θ the angle of diffraction, of a pencil of rays. In a comparatively short space of time hundreds of chemical compounds, inorganic and organic, metals, alloys, colloids, and materials of every imaginable variety have been subjected to x-ray diffraction analysis, with the result that indispensable information has been obtained upon what may be termed Nature's building plan, which is interpreted directly from the diffraction pattern. It is essential for such an interpretation, of course, that the material under investigation should have a spacial regular arrangement such as is readily inferred from a crystal of rock salt (Fig. 1) but scarcely from a cotton fiber or living nerve. And yet liquids and glasses, which are to be classed as amorphous in the usual sense, actually yield valuable data from the two or three broad halos which they produce as a pattern. A reasonable and intriguing extension of the diffraction technic is, of course, to the great range of complex, highly polymerized materials which are formed in living processes. If it is found that these materials are sufficiently well organized in the sense of ordered arrangement of giant molecules, so that characteristic diffraction patterns are produced, then it follows that x-rays may be expected to make a third great contribution to the field of medicine, in addition to diagnostic radiography and therapy, namely, a “supermicroscopic” analysis of biologically significant materials. It is the purpose of this paper to present very briefly an account of present knowledge of results on such materials. It will become immediately apparent that only the barest beginning has been made and yet the great promise of this application of x-rays is unmistakably indicated.