I. FANKUCHEN AND H. MARK 



diamond, calcite, quartz, urea, benzene, etc. Later it was possible 

 also to elucidate the molecular structure of more complicated systems 

 such as silicates, dyestufFs, and hormones; and recently the x-ray dif- 

 fraction method has been successfully applied even to such complicated 

 systems as cellulose, starch, rubber, and protein. Molecular models 

 have been worked out for these high polymeric substances which reflect 

 in a fairly satisfactory way the various chemical and physical properties 

 of the materials. 



Proteins of all kinds have for a long time been extensively 

 studied with x-rays. For recent comprehensive articles see particularly 

 references 1, 10, 13, 14, 15, 21 and 30. By 1920 diagrams of several 

 fiber proteins had been obtained by Herzog and his collaborators (8, 

 19); virus proteins were first successfully studied in 1936 by Bernal and 

 Fankuchen (7); and single protein crystals have been investigated 

 systematically by a number of authors since 1939 (11,12,27). Each of 

 these three types of proteins has challenged the x-ray method in a dif- 

 ferent way; and the investigations have both originated improvements 

 of the experimental technique and led to refinements of the theoretical 

 interpretation of the diagrams. In this essay we shall describe briefly 

 how fibrous proteins have been studied with x-rays, and will then pro- 

 ceed from the simple qualitative inspection of the diagrams to their 

 more quantitative evaluation. 



Typical X-Ray Patterns of Fiber Proteins 



Figure 1 shows the x-ray diagram of a bundle of native silk 

 fibers taken with a well-filtered and collimated beam of Cu K radiation. 

 Patterns of this kind can be obtained in about one hour or less with the 

 aid of modern commercial x-ray tubes now being produced in this 

 country by various companies. Since the taking of the diagram itself, 

 until a few years ago a difficult procedure, is now more or less a matter 

 of professional routine, all care and time can be spent on the prepara- 

 tion of the specimens and on the interpretation of the diffraction pat- 

 tern. It can be seen that the pattern of Figure 1 consists of a number of 

 diff'raction spots of moderate sharpness and varying intensities which 

 are arranged symmetrically about the axis of the incident beam. 

 The primary beam is very intense and would, therefore, produce a 

 large, black spot in the center of the diagram, which might ovei-shadow 



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