CYTOPLASM 



M5 



These bonds between macromolecules are very important, because 

 they transform the protein from the state of a corpuscular sol into 

 that of a reticular gel. But in spite of this fact it is probable that the 

 intramolecular and intermolecular forces are alike, because it makes 

 no difference whether distant parts of one polypeptide chain or 

 sections of two different chains react with each other. In both cases 

 attractive forces between side groups are involved. The places where 

 the side chains are mutually connected will again be called "junc- 

 tions" (see p. 67) and the nature of these points of attachment will 

 now be discussed in more detail. 



The theory of junctions^. The attraction between the side groups of 

 neighbouring protein mole- 

 cules may be of a number of V 

 different types. Some of these 



I 



CH3 

 ■cHj y 

 CH3 



CH3- 



o°o° o 



O O n 



.C/" 



^OH 



OH NH3 



SO4 





 -c- 



-0H° OHf- 



O O O o 



o o 



^ 



II 



CO-NH3 



Salt formation 



-COOH* 



o 

 '^ o ° 



-0- 



Ester bridge 



W 

 -S S- 



NH 

 II II 

 C-NH-C- 



Amide bridge 



Sulphur bridge 



-0- 



Ether bridge 



possibilities are shown in Fig. 

 96. Both hpophilic and hy- 

 drophilic groups may attract 

 each other. Salt-Uke or ester- 

 like bonds can be formed be- 

 tween neighbouring acidic 

 and basic or alcoholic groups, 

 and even main valency bonds 

 may be operative, forming 

 ether-, acid amide- or sulphur 

 bridges. Not all side chains 

 take part in these reactions, 

 but a certain number with 

 free end groups will combine 

 with lipids, hydrophiHc groups or water, as has already been described 

 (Fig, 94, p. 1 39). Furthermore, they form points of attraction for ions 

 of the inorganic salts which, according to their charge, will gather 

 round acidic or basic groups. It is important that the end groups of 

 many side chains remain free, for if they were all interlinked, the 

 result would be a molecular aggregate of very small reactivity. 



There exist four kinds of junctions keeping together the molecules 

 formed by polypeptide chains. In Fig. 96 these have been numbered 

 I-IV, and thev can be characterized as follows : 



^ In German: Haftpunkt-Theorie 



Fig. 96. Schematic representation of junction 



possibilities between neighbouring polypeptide 



chains; o = water molecule. 



