15:2/ X-ray Analyses of Proteins and Nucleic Acids 273 



rotated into one another (except when R is a proton) . A carbon atom, 

 which is sterically asymmetric in this fashion, is called optically active 

 because in solution it rotates polarized light. The two stereoisomers 

 are labeled D and L for dextro-rotary and levulo-rotary, respectively. 

 Most test tube syntheses give equal amounts of the D and L isomers, but 

 most living cells produce only one variety or the other. With very few 

 exceptions, the amino acids polymerized into proteins in the living cell 

 are all Z,-a-amino acids. 



Two amino acid molecules may react to eliminate a molecule of 

 water, thereby forming a peptide bond. Schematically, this can be 

 represented as 



OH 



NH 2 O 



Peptide Bond 



The peptide bond so formed is very stable and the molecule is called a 

 dimer or dipeptide. It is then possible to attach this molecule to other 

 amino acid molecules, forming chains, or polypeptides. When these 

 chains include 50 or more amino acids, they are called proteins. In 

 some cases, the chains may be branched or cross linked; many proteins 

 contain, in addition, a few small molecules other than amino acids. 

 Molecular weights of proteins vary from several thousand into the 

 millions. 



The number of different amino acids conceivable has no known limit. 

 A very large number have been synthesized in test tubes. Of these, 

 approximately 20 Z,-a-amino acids make up almost all of the proteins of 

 all living cells. Other amino acids occur in nature, especially in 

 bacteria, but they are the exceptions rather than the rule. The 20 

 amino acids tabulated in the table on pages 275-277 are the building 

 units which are polymerized to form complex polypeptide chains called 

 proteins. The physical form and arrangement of these polypeptide 

 chains is discussed further in Section 5 of this chapter. 



The various proteins differ from one another in the number and order 

 of the various amino acids in the polypeptide chains and in the con- 

 figuration of these chains. Although the detailed order is known for 

 only a few small proteins, pieces of many others have been studied to 

 determine the order of the amino acids. Figure 4 gives the amino acid 

 sequence for the protein, insulin. 



