STRUCTURE 127 



of the eye, anH of membranes — those of the erythrocyte cell wall which are 

 relatively homogeneous, and those patchy, mosaic membranes exemplified 

 by the wall of the small intestine. 



Crystalline Macromolecules 



Diffraction of X rays by the regular arrays of the electron clouds which 

 surround the atoms or ions of a crystalline substance was introduced in 

 Chapter 4. The X rays diffracted from a single crystal interfere with one 

 another in a manner which is determined solely by the position and electron 

 density of the target atoms in the crystal. If the diffracted rays are allowed 

 to fall upon a photographic plate, from the position and darkness of the spots 

 on the plate, one can (at least in principle) locate the position and electron 

 density of the diffracting atoms in the crystal. The position of the spot tells 

 the angle, 9, of constructive scatter of the X rays of wavelength, A; and the 

 Bragg interference equation, nX = 2d sin 0, relates these values, the "order" 

 of interference, n, and the wavelength, A, to the spacing, d, within the crystal 

 responsible for the scatter. The blackness of the spot gives the amplitude. 

 The superposition of those waves which give rise to the one which emerges 

 from the crystal, however, must be inferred from positions of the atoms in 

 the crystal. This is done by a trial-and-error mathematical method involving 

 superposition of infinite series, a method which will not be described here. 



It was in 1951 that Pauling and Corey made the big break-through in our 

 understanding of structure of proteins: they were able to determine from 

 X-ray diffraction patterns that synthetic polypeptides formed of alpha 

 amino acids all have a coiled, helical form. In other words, the back-bone 

 of the polypeptide chain coils around and around, to form a cylindrically 

 shaped molecular helix. This can be easily understood now, in retrospect, 

 as follows. Since all the alpha amino acids have the structural formula 



H R 



I I 

 N— C— COOH 



H H 



and since these condense through the — CONH-- linkage (Figure 6-1) in 

 the form 



H R O ! H R O 



I llil I II 



• • • -N-C-C-T-N-C-C- • • • 

 1 2\ 3 ' 4 5 1 6 



h ; h 



the atoms of the backbone of the chain, — N — C — C — , are repeated over 

 and over again. The bonds can be bent around only so far, and, in the limit. 



