^2 FRANCIS O. SCHMITT 



will help to crystallize our concepts. At the same time there should be 

 heavy concentration on a few of the most favorable proteins which 

 can be isolated in relative purity and which manifest a high degree of 

 structural regularity. Lacking more direct and more sensitive methods, 

 only thus may we approach the desired goal. 



Internal Architecture of Fibrous Proteins 



Astbury distinguishes two classes of fibrous proteins: the keratin- 

 myosin-fibrin class, which gives the characteristic alpha and beta wide- 

 angle x-ray patterns, and the collagen class. Astbury's general point of 

 view is well summarized in his essay, 'The Form of Biological Mole- 

 cules," included in the set of essays presented to D'Arcy Thompson. The 

 correlations which he has drawn have greatly helped to weave a com- 

 mon thread through the maze of assorted information which existed 

 previously. The data do not yet warrant conclusions of detailed structure 

 which are necessarily unique, but, as Astbury says, it must be "some- 

 thing like that." Until specimens are prepared which have sufficient 

 regularity of structure to give a greater wealth of diffraction data, it is 

 unlikely that the details of the folded alpha configuration will be 



deducible. 



In certain of the fibrous proteins a large repeating fiber-axis spacing 

 has been demonstrated by x-ray diffraction and electron microscopy. 

 The value of this spacing and the intensities of the various diffraction 

 orders are characteristic of each protein. Yet the true significance of 

 this remarkable characteristic remains unknown. Astbury has attempted 

 to interpret long spacings in terms of the amino acid composition along 

 stoichiometric lines. The long spacing is regarded as the product of the 

 number of amino acid residues in the molecule and the length, along 

 the fiber axis, of each residue as indicated by the meridional diffrac- 

 tions of the wide-angle pattern. However, the data are not yet suf- 

 ficiently complete to warrant this procedure except as a rough guide to 

 thought. Moreover, Bear has demonstrated in the case of collagen that 

 physical and chemical alteration may cause the long and short fiber-axis 

 spacings to vary somewhat independently of each other. 



While the wide-angle x-ray patterns reflect similarities of structure, 

 the small-angle pattern is characteristic of each particular proteim 

 Fairly well developed small-angle patterns have been obtamed for a and 

 /? keratin, collagen and paramyosin. Myosin patterns have also been 

 obtained (Bear), but as yet these lack detail. No x-ray long-spacings 

 have been reported for fibrin, though Wolpers has observed a periodic 



