54 FRANCIS O. SCHMITT 



The structure and composition of virus nucleoproteins, particularly 

 tobacco mosaic virus, has been closely studied. In this field the x-ray 

 work of Bernal and Fankuchen and the chemical studies of Stanley and 

 his associates are classical, but the published data do not yet permit a 

 detailed description of the structure of the protein and nucleic acid 

 constituents. 



The Avery experiment, in which nucleic acid extracted from pneu- 

 mococci has been likened to a free gene, has further heightened the 

 interest in these substances. It is urgent, therefore, that the very meager 

 information about the structure of nucleic acids and nucleoproteins be 

 extended by further x-ray examination. Since, to the author's knowl- 

 edge, no electron microscope studies have thus far been reported, the 

 possibilities in this direction cannot be assessed. 



The Continuous Polypeptide Chain Theory 



versus the 

 Corpuscular Theory of Protein Structure 



From the classical work of Emil Fischer it was natural to consider 

 the polypeptide chain as the unit of protein structure. This view was 

 adopted by those using x-ray methods, especially by Astbury. By stretch- 

 ing an alpha keratin fiber one was literally pulling the folded polypeptide 

 chains into the straight, beta configuration. In addition it was supposed 

 that these continuous polypeptide chains occur in well ordered semi- 

 crystalline regions and in poorly ordered amorphous regions; poly- 

 peptide chains coursing through both types of regions produce an anas- 

 tomotic system. 



Recent experiments of Bear and his associates, in which x-ray patterns 

 are obtained from specimens which are tilted at various angles in the 

 x-ray beam, support the view that collagen is essentially a one-dimen- 

 sional structure. By this is meant that the linear elements have a thick- 

 ness of only a few polypeptide chains and have indefinite extension 

 longitudinally. Electron microscope evidence suggests that these units 

 associate with each other laterally so that their axial periodicities are 

 in phase, thus causing the entire fibril to appear cross-striated. It is even 

 possible to dissociate these units by treating rat tail collagen with acetic 

 acid, and to cause them to join up again, on elevation of the pH, so that 

 cross-striated fibrils are again formed. In the case of collagen, therefore, 

 the polypeptide chain theory seems most attractive at the present time. 



Myosin, paramyosin and fibrin show wide-angle x-ray patterns 



