15:5/ X-ray Analyses of Proteins and Nucleic Acids 287 



The problem of determining the structure of fibrous proteins is quite 

 different from that of crystalline proteins. If a fiber were made up of 

 little crystalline regions all lined up, then one should obtain spots 

 similar to those from a single crystal. If these were slightly disoriented, 

 the spots would become arcs. If the crystallites were completely 

 randomly disordered, the spots would become circles similar to those of a 

 powder pattern. Early investigators took many pictures of X-ray 

 diffraction patterns of fibrous materials but no one understood the 

 results. 



Around 1930, Astbury studied many protein and nucleic acid fibers. 

 He showed that the proteins gave rise to two types of X-ray diffraction 

 patterns called a and /3. He recognized the a-configuration as a folded 

 or more dense structure, and the /S pattern as due to a stretched structure. 

 To these, he assigned the forms 



I ^R 



R 



Astbury's a form Astbury's /? form 



The double-bonded oxygen is slightly negative, whereas the hydrogen on 

 the nitrogen is somewhat positive. Alternate rows of polypeptide chains 

 were postulated to be held in place by the attraction between these two, 

 called hydrogen bonding. 



Astbury showed that wool and hair changed from an a to a /S form on 

 stretching. He postulated that in muscular contraction, the proteins 

 changed from a j8 to an a form and this was generally accepted until 

 about 1945. His models represented tremendous strides in under- 

 standing the structure of proteins but left many questions unanswered. 

 His attempts to fit DNA and RNA patterns to his models were unsuccess- 

 ful. Many spots in the diffraction pattern and the absence of other spots 

 could not be explained in terms of his models. Thus, they represented 

 a good first guess. 



The current interpretations of X-ray diffraction patterns of fibers 

 date back to about 1947. At that time, angles and configurations of 

 many of the amino acids were investigated. Pauling and Corey, in 

 1951, showed that if they drew a polypeptide chain with the known bond 

 angles on a piece of paper and twisted it into a helix, the various turns 

 could hydrogen bond to one another. Astbury and others had tried 

 helical models but always with an even number of amino acids per turn; 



