X-RAY DIFFRACTION 



if one has a number of single crystal diagrams, taken nndcr very well- 

 defined geometric conditions, each diagram containing a large number 

 of individual spots. How^ever, if one is v^illing to combine x-ray data 

 with other evidence, coming from chemical analysis or from physico- 

 chemical investigations, it is possible to advance well-founded structural 

 proposals, provided that the investigated samples exhibit a so-called 

 twofold orientation, such as stretched or rolled thin films of cellulose, 

 rubber, or collagen (5,24). However, if one has only diagrams with 



Table II 

 A Few Spacings of Lattice Planes in the Lattice of Natural Silk 



Number of reflection in 

 Figure 1 



Sine of Bragg's angle, 6 Lattice plane spacing, d, A. 



Ai 

 A, 

 A4 

 A5 



0.084 

 0.178 

 0.251 

 0.324 



9.2 

 4.3 

 3.1 

 2.4 



axial symmetry (so-called fiber diagrams), which contain spots of the 

 number and character as shown in Figure 1, it is, in general, not possible 

 to arrive at reliable quantitative conclusions regarding the exact size 

 and shape of the elementary cell of the lattice of the crystalline con- 

 stituents. 



It seems that in some special cases, such as the protein in native 

 silk (8), the protein in native wool and hair called /3-keratin (3,26) and 

 a-keratin (2,3), and the protein in muscle called collagen (5,6) rather 

 reasonable proposals for the lattice parameters have been suggested by 

 supplementing the x-ray data with chemical and morphological data, 

 and with the aid of a certain amount of intuitive guessing. However, 

 even in these cases one cannot, at present, advance well-founded data 

 as to the exact location of the various substituents along the main poly- 

 peptide chains in the lattice, although one knows their approximate 

 arrangement. Summarizing, it may be said that the large-angle x-ray 

 study of fiber proteins to date has furnished enough data to arrive at 

 rather probable structures for the most important protein fibers if one 

 combines the x-ray data with all the other physical and chemical knowl- 

 edge of these materials. 



445 



