MACRO MOLECULES 



103 



acids and ammonia. The latter derives from the amide groups of glutamine 

 and asparagine, but it may also be an artefact arising from the breakdown 

 of certain amino acids during the hydrolysis. Since the average molecular 

 weight of the amino acids is 110, then complete hydrolysis of a protein of 

 molecular weight 36,000 will produce around 300 molecules of amino 

 acids of 20 different kinds. It is evident that the analysis of this hydrolysate 

 is a formidable problem whose solution was a notable achievement. The 

 methods used are numerous : specific precipitation of some groups of 

 amino acids (for example by phosphotungstic acid in the case of cystine, 

 arginine, histidine and lysine); precipitation of the amino acid directly 



pH-* 

 2 3 4 5 6 7 8 9 10 U !2 13 



[---R-HF* 

 -COsH(a,p,Y) 



NH 



J 



NH 

 -'^fH3(a) 



-*NHa (8) 

 -^>0H 



-SH 



NH. 



^C=*NH, 

 -NH 



[-..R]ln->l*+H* 



-cor 



N 



J 



NH 

 NH.(a) 



-NH-le) 



-s- 



NHj 



\irNH 



-NH 



Fig. 13 (Springall) — Dissociation constants of the different groups — R-H. The dashed 

 areas indicate the pH range in which dissociation takes place when the pH increases. 



(proline by ammonium rhodanilate) ; colorimetric methods; isotope dilu- 

 tion methods; enzymatic methods (for example, measurement of COg 

 liberated by a specific decarboxylase); microbiological methods — depend- 

 ing on the fact that for a certain strain a certain amino acid is indispensable 

 for growth, the latter may be measured (for example, in the lactic acid 

 bacteria by the production of acid) and is proportional to the concentration 

 of the amino acid; chromatographic methods, etc. 



Table IX shows the amino acid composition of a collection of proteins 

 whose analysis is complete or almost complete. As can be seen, such a table 

 does not permit any useful conclusion to be drawn from it. Thus attention is 

 focussed on the sequence of the amino acids in the protein polypeptide chains. 



