4 2 4 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
NH 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 (NH 2 )GOOH 3 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 facilit} 7 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 Avater, 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. Hoffmann's 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, 
