15:2/ X-ray Analyses of Proteins and Nucleic Acids 271 



sometimes are found as small ring or chain structures called monoses, or 

 simple sugars having three to seven carbon atoms. The most common 

 number is six carbons, as exemplified by the common sugars glucose and 

 fructose. Carbohydrates are also found as compound sugars (or dimers) 

 such as sucrose, and as long polymerized chains of monoses such as 

 starches and celluloses. Several carbohydrates are indicated in Figure 3. 

 Of the various compounds found in the cell, the proteins and nucleic 

 acids have been singled out for attention in this chapter and in the three 

 following ones for a number of reasons. First, their structure and their 

 action have been studied by means of the methods of physics and 

 physical chemistry. Second, their importance both for cell life and 

 reproduction is understood. In addition, they present a complexity and 

 diversity which challenges man's abilities to investigate them, as well 

 as a simplicity and uniformity in over-all plan which cannot but fill one 

 with awe. 



2. Proteins 



To recapitulate briefly, proteins are one of the major classes of com- 

 pounds found in all living matter. Enzymes are protein catalysts 

 controlling the rates of many biological reactions. Muscular contrac- 

 tion depends on proteins, and active transport across cell membranes 

 appears to be a lipoprotein function. Many physical means have been 

 used to study protein structure; currently accepted ideas lean heavily 

 on the results obtained from studies of X-ray diffraction. Before 

 examining the X-ray data, the chemical composition of proteins will be 

 briefly considered. 



Proteins are natural high polymers built from small blocks, called 

 amino acids. These are molecules with an average molecular weight 

 of about 120, each of which has an organic acid group, 



O 



// 



— c 



\ 



OH 



and a basic amino group, — NH 2 . The acid and basic groups are 

 attached to the same carbon leading to the form, 



NH 2 O 



I // 

 R— C— C 



I \ 

 H OH 



