130 



BIG MOLECULES 



carbon 6 falls almost directly above nitrogen /, and the two are hydrogen 

 bonded about 1.5 A apart. The diameter of the helix so formed is about 8 A. 

 The helix has an open core, about 2 A across,. and the R-groups, or side 

 chains to the main structure, jut out radially from the central axis of the 

 cylinder. 



Figure 6-1. The Planar -CONH — Linkage (boxed) 

 Between Amino Acids in a Protein. Lengths in angstroms. 



Since the helical shape is a property of poly alpha amino acids, it was given 

 the name "alpha-helix, " and it is now probably the most famous structure of 

 macromolecular physical chemistry. Figure 6-2 is a drawing, similar to the 

 original disclosure, which shows the main chain (bold bonds) and the posi- 

 tions of attached groups ( — R); and which indicates the positions of the hy- 

 drogen bonds, the "bones" which give the helix rigidity. 



It is now known to be the main structural component of «-keratin, hair, 

 wool, nail, muscle, and connective tissue, etc. Recently it has been traced 

 in muscle to the contractile enzyme, myosin itself. Because of the unique 

 role of myosin, some of its physical and chemical properties are expounded 

 in Chapter 10. 



One protein, of unquestioned importance, which has intrigued biological 

 investigators for years, is hemoglobin, the "oxygen carrier" of the respira- 

 tory enzyme system, first crystallized and purified by Hoppe-Seyler in 1862. 

 However, with a molecular weight of 67,000, its amino acid sequence and 

 the physical structure of the molecule have only slowly yielded to persistent 



