98 UNITY AND DIVERSITY IN BIOCHEMISTRY 



The flexibility required for this bunched state, as Neurath has suggested, 

 may be obtained by the presence of an amino acid such as glycine, which is 

 without side chains to prevent free rotation. Or, in Pauling's opinion, 

 proline may be a point of flexibility since, when part of the peptide structure, 

 it does not possess an NH group, does not form hydrogen bonds 

 N-H . . . O, and retains full liberty of bending. 



H HoC CHo 



---N I I 



\ CH CH2 



CO N 



H I 



N CO 



/\/ 

 - - - CO CH 



I 

 R 



Diagram showing the possibility of a change in direction of the 

 peptide chain at a proline residue. 



id) Denaturation 



The globular proteins can undergo denaturation, a process which is 

 often irreversible, but is not well defined ; it may result from the action of 

 many and diverse agents (urea, alcohol, detergents, ultrasonic vibrations, 

 etc.) and is revealed by a decrease in solubility, an increase in viscosity, the 

 appearance of free — SH groups, etc. Denaturation is characterized by a 

 very high temperature coefficient, which implies a high degree of order in 

 the molecule of native protein. 



On the other hand, the enthalpy ( — zJH, heat liberated at constant 

 pressure) is low, indicating little change at the level of covalent bonds. 

 Denaturation appears to be a sort of collapse of a highly ordered and specific 

 arrangement. The result is a poorly ordered mixture of polypeptides. 



All these observations lead us to consider a globular protein as a highly 

 ordered three-dimensional structure of polypeptide layers of the a-k-m-e-f 

 configuration (probably containing 3-6 residues per turn) bunched into a 

 globular mass and maintained thus by linkages between side-chains and 

 by relatively weak hydrogen bonds. In the region of a polar group having 

 a spare hydrogen atom, there is an attraction for the negative charges of 

 neighbouring molecules. In two neighbouring peptide chains, a peptide 

 hydrogen may form a bridge with a pair of electrons on an oxygen atom of 

 the other chain. 



