CHEMICAL COMPOSITION OF PROTEIN MOLECULE n 



estimation of ammonia is frequently combined with the estimation of 

 the diamino acids (p. 34). 



The separation and estimation of the two main groups of amino 

 acids is generally not carried out in one experiment, but only when 

 the amount of protein available is small, as very different quantities of 

 material are required. Thus, the diamino acids can be determined in 

 25-50 grammes of protein with considerable accuracy, whereas the 

 monoamino acids can only be determined with fair accuracy when 

 250-500 grammes of protein can be used. The two processes, of 

 which the details are given under the two sections, may be combined 

 as follows : 



The protein is hydrolysed by boiling for fifteen to twenty-four hours 

 with six times the quantity of 25-30 per cent, sulphuric acid. The 

 solution is neutralised with baryta and the filtrate and washings from 

 the barium sulphate are evaporated down to a small volume. Tyrosine 

 (and cystine) crystallise out. The filtrate is diluted with water and 

 sulphuric acid added till the content of acid is 5 per cent. The 

 diamino acids are then precipitated with phosphotungstic acid (pp. 65, 

 75); from this precipitate they are obtained by decomposition with 

 baryta and separated by means of their silver compounds as described 

 in section B (p. 34). The filtrate is freed from phosphotungstic acid 

 and sulphuric acid with baryta, excess of which is removed with carbon 

 dioxide and sulphuric acid, and then treated for the other monoamino 

 acids as described in section A (p. 17). 



On the whole it is not advisable to combine the two processes, 

 since the phosphotungstic acid precipitation does not effect a perfect 

 separation of the two groups. 



