CHEMICAL COMPOSITION OF PROTEIN MOLECULE 43 



9. Oxyproline. The amount of this substance in the protein is 

 greater than the quantities which have been isolated. The method for 

 its isolation is so laborious that data are only available for a few proteins. 



10. Tyrosine and Cystine. The data for tyrosine most probably 

 represent the content of the protein in this unit very closely. 



The data for cystine are unsatisfactory ; in many cases the data 

 are calculated from the sulphur content of the protein ; in the other cases 

 they are those from pure isolated cystine. More satisfactory data are 

 given by Van Slyke's amino method (p. 78). 



11. Tryptophane. A large amount of protein is required for the 

 isolation and estimation of tryptophane ; it is on this account most prob- 

 ably that so few data exist. The isolation is easier the larger the 

 quantity of material. Tryptophane might be isolated first in most cases 

 and the remainder of the products isolated subsequently. 



Osborne, allowing for all losses, calculates that from 41-82 per 

 cent can actually be recovered. In the cases of zein and gliadin 86 and 

 68 per cent, respectively are known ; in the case of vignin only 5 per 

 cent, more has to be accounted for. The losses from the esters are 

 computed to be 50 per cent, of the alanine, serine, aspartic acid ; 

 30 per cent of the valine, proline, glutamic acid, phenylalanine ; 20 

 per cent of the leucine : only 50 per cent, of the cystine and trypto- 

 phane may be accounted for. 



A method which may prove of some service in separating these 

 various mixtures was described by Engeland in 1907 and 1910. The 

 amino acids are methylated in alkaline solution with methyl iodide and 

 converted into their betaines : 



CH 2 . NH 2 CH 2 . N~ CH 3 



+ 3 CH.I + 3 NaOH = 3 NaI + 3 H 2 O + | ' | \CH 3 . 

 COOH CO O 



These products are separated* by means of their double salts with 

 mercuric chloride, gold chloride and platinum chloride. The details 

 have still to be worked out. 



