EMBEDDING 69 



1 



NH NH 



O O 



HC.CH2.C HC.CHo.C 



// 



NH, I OH 



C=0 C=:0 



I I 



Asparagine as part of Aspartic acid as part of 



a protein chain a protein chain 



as those between carboxyl in one chain and basic groups in 

 another,^^ are also broken by the alkaline bath. When the 

 material is warmed, the loosened chains move apart from one 

 another, and are shortened by hydrolytic transverse breakage of 

 the peptide links. ^^^ Chains varying in molecular weight from 

 about 15,000 to 45,000 are thus set free in association with water 

 as a sol. The molecules are neither fully extended into rods nor 

 compacted into globular form, but each is about 50 times as long 

 as broad. 



When the filtered sol is cooled, these molecules attach them- 

 selves to one another, probably by hydrogen bonding between 

 peptide groups in previously separate molecules.^® The bond is 



C=0 HCR 



I I 



A hydrogen bond {. . .) between two peptide groups 



in protein chains 



very susceptible to heat. Over a wide range of concentration, 

 gelatine associates with water to form a solid (gel) below 20' C, 

 a fluid (sol) above 35' C, and a substance showing anomalous 

 viscosity (neither a solid nor a true fluid) between certain tem- 

 peratures intermediate between 20° C and 35° C. There are said 

 to be some free molecules even in the gel.'^- 



