4 2 4 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



2 group in his reaction was not replaced by hydrogen but by hydroxyl, 

 and so finally arrived at the formula C 6 H 4 .OH.C 2 H 3 (NH2)COOH, which is 

 in agreement with all the experimental facts, and is now universally accepted. 



When fairly pure, tyrosine crystallises in long slender needles, which occur 

 both singly and in double sheaves or in rosettes. If impure, however, it very 

 often separates in balls or nodules closely resembling those of leucine, 

 recrystallising from warm water in the crystalline form above described ; if 

 the solution containing the crystals be filtered, these felt themselves together 

 on the surface of the paper to a thin, snow-white, paper-like mass. Tyrosine 

 is much more insoluble in water than leucine (1 in 1900 of cold water), more 

 so (1 in 150) in boiling water and in dilute and concentrated mineral acids, 

 and also in alkaline solutions (ammonia, alkalies and their carbonates, and the 

 alkaline earths). Tyrosine exhibits the usual facility of amido-acids for 

 forming compounds, both with bases and acids ; the copper compound is 

 sparingly soluble in water, and is formed in dark blue needles on the addition 

 of freshly precipitated cupric hydrate to a boiling solution of tyrosine, and 

 allowing to cool. 



Tyrosine, unlike leucine, cannot be sublimed without decomposition, and on 

 dry distillation yields carbon-dioxide and a base of the composition C 8 H n NO. 



Tests for tyrosine. Tyrosine may be identified by the following tests : 



1. Its crystalline form. 



2. Scherer's test, which consists in evaporating a portion with strong nitric 

 acid in a platinum dish, leaving a transparent deep yellow residue, which 

 turns red on moistening with caustic soda solution, and then a blackish brown 

 on again evaporating. 



3. Piria's test. A drop or two of strong sulphuric acid is added to the 

 tyrosine in a watch-glass ; after half-an-hour, during which tyrosine sulphuric 

 acid forms, the acid is diluted with water, and neutralised by the addition of 

 calcium carbonate. The solution is filtered from the calcium sulphate so 

 formed, and a drop of neutral ferric chloride solution added, when a deep 

 violet colour appears, similar to that given by salicylic acid. 



4. R. Hoffmanris test. This is really identical with the Millon test for 

 proteids, and in cases where there is no group present in the proteid molecule 

 capable of yielding tyrosine, the test with Millon's reagent does not succeed, 

 e.g., in the case of gelatin and of antipeptone. The test may be carried out 

 directly in the case of tyrosine itself, by boiling a solution containing this with 

 Millon's reagent, when the solution passes through pink into deep crimson. 



Separation of leucine and tyrosine. Leucine and tyrosine may very easily 

 be separated when in solution together by means of their very different 

 solubilities. To separate them after pancreatic digestion, it is best to allow 

 digestion to proceed for several days ; at the end of this time there is no 

 coagulable proteid, or albumose, except in traces, left in the solution. This is 

 neutralised and evaporated down, when the tyrosine, on account of its sparing 

 solubility in water, is thrown out in crystalline masses, while the more soluble 

 leucine nearly all remains in solution ; on cooling, more of the tyrosine separates 

 out, and when the solution is cold it is filtered off, extracted with hot alcohol 

 to remove traces of leucine, and purified by recrystallisation from hot water, 

 or by dissolving in weak ammonia and precipitating by neutralisation. 



The filtrate containing the leucine and peptone is still further evaporated 

 until it becomes syrupy ; it is then extracted with boiling alcohol, which takes 

 up only the water and leucine. On evaporating off the alcohol, leucine is thrown 

 out of the concentrated solution, and may be purified by sublimation or by 

 repeated recrystallisation from alcohol. More tyrosine may be obtained from 

 the residue left by the boiling alcohol. 



Or the solution, after completion of digestion and careful neutralisation, 

 may at once be evaporated to a thin syrup and set aside for twenty-four hours, 



