MACROMOLECULES 99 



R O H R O H R 



: i: H I I II H I I 



C C I N C C I N C 



\/;\/\l/\/l\/\l/\/\/ 



CjN C C|N C C N 



.i H I I II H I 1 II I 



O H R O H R OH_H 



HHO R H O R H O 



I II I H I H II I I H II 

 N C C|N|C C N|C 



/ \l/ \ / \l/ \ / \ /1\ / \|/ \ 



C N C C N I C C 



i I II I I H 11 I 



R H O R H O R 



• • • • 



• • • • 



The formation of hydrogen bridges is an expression of the tendency 

 possessed by hydrogen atoms to share the electrons of an oxygen atom. 



V • • • • \. • • • • 



c::o: h:o— -* c:;o:h:o — 

 / ,. .. / •• •• 



Hydrogen b onds may also be formed between hydrogen and nitrogen atoms 

 between -OH of tyrosine and free -COOH groups, and also between amides 



\ / 



N-H H 0=C 



0=C 0---H— N N-H 



\ '^ \ /^ 



H-C-CH0-CH2-C C-CH2-C— H 



/ ■ \ / \ 



H— N N— H---0 C=0 



\ I / 



n H-N 



\ 



Glutamine Asparagine 



Other bridges between polypeptides are of the disulphide type (covalent), 

 or of a salt-like nature (non-covalent, electro-valent). 



We have said that the shape of the globular proteins depends upon 

 relatively weak secondary bonds. On raising the temperature, thermal 

 agitation of the molecules may be sufficient to break these secondary bonds. 

 Mineral salts and urea, by polarizing the water molecules around their 

 molecules or their ions, cause dehydration of the globular macromolecule 

 thus modifying the electrical field of force around it and changes in shape 

 result. Acids and bases, by modifying the ionization of basic and acidic 



