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chain of cyanalcohols or cyanhydrins, as they are often called, united to
a benzene nucleus.

A cyanalcohol is a substance obtained by the union of an aldehyde
with hydrocyanic acid ; for instance —

CH3.COH+HCN=CH3.CH(CN)OH

(ethaldehyde) (hydro- (cyanethylic alcohol)
cyanic acid)

Other alcohols are formed from other aldehydes, and these are all
united to one another and to benzene to form a proteid.

Latham shows that the various products of the disintegration of
albumin can also be obtained by the condensation and intramolecular
changes . that these cyanalcohols, which are exceedingly unstable
substances, undergo. Instability and proneness to undergo intra-
molecular changes are two properties common to " living proteids " and
to cyanalcohols.

General properties and reactions of proteids. — Solubilities. —
All proteids are insoluble in alcohol and ether. Some are soluble in
water, others insoluble. Many of the latter are soluble in weak saline
solutions. Some are insoluble, others soluble in concentrated saline
solutions. It is on these varying solubilities that proteids are classified.

All proteids are soluble with the aid of heat in concentrated mineral
acids, in glacial acetic acid, and in caustic alkalis. Such treatment, how-

1 Arch./, d. ges. Physiol., Bonn, 1S82, Bd. xxix. S. 400.

- Centralhl. f. agric. Chem., Leipzig, ■1882, p. 830.

^ Chem. Ncivs, London, vol. xlviii. p. 179.

■* Brit. Med. Journ., London, 1886, vol. i. p. 629; Lancet, London, 1888, vol. ii. p. 751.



40 CHEMICAL CONSTITUENTS OF BODY AND FOOD.

ever, decomposes as well as dissolves the proteid. Proteids are also soluble
in gastric and pancreatic juices, but here again they undergo a change,
being converted into the hydrated varieties of proteid known as proteoses
and peptones. Solutions of the proteids are precipitated by a large
number of reagents, but the proteoses and peptones furnish many ex-
ceptions to this statement.

The principal precipitants of proteids are : —

1. Strong mineral acids, especially nitric, metaphosphoric, and
phosphotungstic acids.

2. Acetic acid with potassium ferrocyanide.

3. Acetic or oxalic acid, with excess of certain neutral salts, such
as sodium sulphate, sodium chloride, or magnesiiuu sulphate.

4. Salts of the heavy metals ; basic lead acetate, mercuric chloride,
silver nitrate, copper sulphate, ferric chloride or acetate, ' potassio-
mercuric iodide, sodimn tungstate, etc. The precipitates consist of
the proteid in combination with variable amounts of the metal, in
the form of albuminates. On the removal of the metal by a stream
of sulphuretted hydrogen, the proteid is recoverable in an unchanged
form.

5. Tannin ; or tannin and sodium chloride together.

6. Saturation with ammonium sulphate or sodiomagnesium sulphate,
or potassium acetate or carbonate. These precipitates are soluble on
diluting the solution of salt in which they are suspended.

7. Picric acid.

8. Salicylsulphonic acid.

9. Trichloracetic acid.

10. Alcohol, except in the presence of free alkali, w^hen the proteids
are slightly soluble in hot alcohol.

The precipitate given by the proteoses is in many cases (as with
nitric, trichloracetic, and salicylsulphonic acid, or with acetic acid and
potassimn ferrocyanide) soluljle on heating, l^ut re-appears when the
solution cools. The greater numlier of the reagents mentioned do
not precipitate peptone. It is precipitated completely Ijy alcohol,
tannin, and potassio-mercmic iodide, and incompletely Ijy phospho-
tungstic and phosphomolybdic acids.

The following are the methods used to remove all proteid from a
solution : — -

1. Brucke's metho'lA consists in the alternate addition of liydrocliloric acid
and potassio-mercuric iodide.

2. Girrjensolm's method - consists in tlie addition of sodium cldoride and
tannin. .

3. Devotds method:' — This consists in boiling an acidulated solution
of the proteid with excess of ammonium sulphate crystals ; all proteids are
precipitated by this means except peptone. Proteoses, if present, are
precipitated but not coagulated, and can be extracted from the precipitate
by water.

4. By trichloracetic acid. — This method consists in adding to the solution an
equal volume of a 10 per cent, solution of trichloracetic acid, boiling and filtering
hot. The filtrate contains the proteoses and peptone, if these are present, and
the precipitate contains the other proteids. This is by far the most rapid and

^ SUzungnh. d. h. Akad. d. JViswnscJi. , Wien, 1871.
^ N. Beperi. f. Pliarm., Miinclieii, Bd. xxii. S. 557.
'^ Ztxchr. f, pkysiol. Chcm., Stras.sburg, Bd. xv.



GENERAL PROPERTIES AND REACTIONS OF PRO TE IDS 41

simple method of separating the two cLasses of proteids.i If boiUng is omitted,
the proteoses are in part precipitated also (C. J. Martin).

5. The alcohol metliod. — The solution is rendered faintly acid with acetic acid,
and several times its volume of absolute alcohol are added. After twenty-four
hours it is boiled and filtered ; the filtrate is proteid-free.^ The action of alcohol
on proteids is peculiar ; it precipitates proteids in the cold ; and the precipitate,
if washed free from alcohol, is found to be readily soluble in suitable reagents
such as saline solution. But if the precipitate is left in contact with the alcohol
for days or weeks, the solubility of the precipitate is lost ; the precipitate has
been converted into a coagulum. This loss of solubility, however, does not
occur with proteoses and peptone, and thus this is another very good though
tedious method of separating native proteids from products of proteolysis.^

6. Salicylsulpho7iic-acid method. — This is recommended by Mc William.'*
The reagent precipitates albumins and globulins ; on heating, the precipitate is
coagulated. The same reagent precipitates proteoses ; on heating, the precipitate
dissolves, and re-appears on cooling. The reagent does not precipitate
peptones.

7. B^j hoilin<j. — In some cases the proteids are precipitated or more properly
coagulated by boiling after faintly acidulating the solution. This is the case
with the albumins and globulins, and with the proteids which are usually
found in morbid urines. For the separation of native proteids from proteoses
and peptones, the method is not to be recommended, because boiling with even
dilute acids leads to the formation of small quantities of these products of
proteolysis. The use of this method has thus produced many mistakes ; it led
Struve, Schmidt-Miilheim, and others, to the conclusion that a peptone-like
substance exists in milk and in blood ; and more recently Chabric,^ by the use of
the same method, has described a new proteose-like constituent of blood
serum, to which he has given the name "albumone." Chabrie's mistake has
been amply demonstrated by E. Brunner.*^ It should be added that Devoto's
method is not wholly free from the same objection.'^

For quantitative purposes the precipitate produced by these several
methods may be collected, washed, dried, and weighed, then incinerated,
and the ash deducted. Other methods that have been devised are
densimetric methods, in which, after removal of the proteid, the loss of
specific gravity is multiplied by a constant factor ,s and methods in which,
by Kjelclahl's process, a' nitrogen estimation is made in the precipitate
produced by some precipitant. Sebelien^ recommends tannin for this
purpose.

Precipitation hy neutral salts (German, Attssalzung). — There are a
number of organic substances which can be precipitated from their

1 Obermayer, 3fcd. Jahrb., Wien, 1888, S. 375-381 ; Starling, Journ. Physiol. ,
Cambridge and London, vol. xiv. p. 131 ; C. J. Martin, ibid., vol. xv. p. 375 ; Halliburton
and Brodie, ibid., vol. xvii. p. 169 ; Halliburton and Colls, Journ. Path, and BaderioL,
Edin. and London, 1895, vol. iii. p. 295.

- Hoppe-Seyler, "Handbucli," S. 312 ; Schmidt, Arch. f. d. ges. Physiol., Bonn, Bd.
xi. S. 10 ; Hoffman, Virchow's Archiv, 1879, November, S. 255.

^ S. Martin, Goulstonian Lectures, Brit. Med. Joiorn., London, 1892, vol. i. ; Gourlay,
Journ. Physiol., Cambridge and London, 1894, vol. xvi. p. 82.

* Brit. Med. Journ., London, 1891, vol. i. p. 837 ; 1892, i. p. 115. The reaction was
previously described by Roch, Pharm. Centr.-BL, Leipzig, 1889, S. 549.

^ ConnJt. rend. Acad. d. sc, Paris, tome cxiii. p. 557.

" Inaug. Diss., Bern, 1894.

■^ M. Matthes, Berl. klin. Wchnschr., Bd. xxxi. S. 351; Halliburton and Colls,
he. cit.

8 Bornhardt, Ztschr. f. anal. C'Acm. , Wiesbaden, 1870, S. 149; 1877, S. 124 ; Huppert
and Zalior, Ztschr. f. physiol. Chcm., Strassburg, Bd. xii. S. 467, 484.

3 Ibid., Bd. xiii. S. 135 ; Konig and Kisch, Ztschr. f. anal. Chem., Wiesbaden, Bd.
xxvii. S. 191.



42 CHEMICAL CONSTITUENTS OF BODY AND FOOD.

aqueous solutions, by the addition of certain neutral salts in large
quantities ; in some cases complete saturation is necessary. In some in-
stances, as in the precipitation of m^ates by ammonium chloride,^ or
ammonium sulphate,^ the formation of an insoluble compound with the
base of the salt used will explain the phenomenon. In other cases,
especially m the case of colloidal substances, the water-attracting power
of the salt is more probably the explanation.^ The solutions used
should not be too concentrated, or the thick precipitate obtained is
difficult of filtration.

The phenomenon is not confined to substances of a colloidal nature ;
thus, picric acid is precipitable by this means ; so are soaps, especially
potassium soaps by sodium chloride. But it is in connection with non-
diffusible substances,^ and especially with proteids, that the method is
most used.

Proteids differ from one another a good deal in the readmess by
which they are precipitated in this way. Ammonium sulphate added to
satm-ation, precipitates all proteids except peptones ^ and certain forms
of deuteroalbumose.^ Half saturation with the same salt is sufficient to
precipitate globulins,'' acid and alkali albumin and caseinogen. Speaking
generally, the globulins and nucleo-proteids are more readily precipitable
by neutral salts than the albiunins. Thus, globulins are precipitated by
magnesium sulphate and sodium chloride, whereas albumins are not, and
some globuHns, Like fibrinogen, are precipitated by half -saturation with
sodium chloride. If the operations are carried out at the temperature
of the air, the precipitated proteids are not coagulated, but are
soluble in suitable liquids ; and they then again show their characteristic
properties.^

Heat coagulation. — The albumins, globuHns, and some nucleo-proteids
are coagulated at different temperatures, by heating their solutions.
The temperature varies with the reaction of the solution,^ the quantity
and nature of the salts present^*^ (minute quantities of calcium salts
favour heat coagulation as they do ferment coagulation),^^ and
under certain circumstances, especially in an alkaline solution, with its
concentration.^^

^ F. G. Hopkins, Journ. Path, and Baderiol., Edinburgli and London, 1893, vol. i. p.
451.

^ A. Edmunds, Journ. Phyi^iol., Cambridge and London, 1S94-.5, voL xvii. p. 451.

^ 0. Nasse, Arch. f. cl. ges. Physiol., Bonn, Bd. xli. S. 504; F. Hofraeister and S.
Lewith, Arch. f. exper. Path. u. FharmakoL, Leipzig, 1888, Bd. xx. S. 247; xxv.
o. 1.

■* On the precipitation of colloid carbohydrates by salts, see Pohl, Ztschr. f. physiol.
Cham., Strassburg, Bd. xiv. S. 151 ; R. A. Young, " Proc. Physiol. Soc," 1896-97, p. xvi. in
Journ. Physiol., Cambridge and London, 1897, vol. xxi.

5 Wenz, Ztschr. f. Biol., Mlinchen, Bd. xxii. S. 1.

" Kiihne, ibid., Bd. xxiv. S. 1 and 308 ; Chittenden, Journ. Physiol., Cambridge and
London, vol. xvii. p. 48.

■^ Kauder, Arch, f, cxper. Path. v,. Pharmakol., Leipzig, Bd. xx. S. 411.

^ On the precipitation of proteids by numerous salts, see Denis, " Memoire sur le
sang," p. 39; Schrifer, Journ. Physiol., Cambridge and London, vol. iii. p.' 181;
Halliburton, ibid., vol. v. p. 177; vii. p. 321; Hanmiarsten, Arch. f. d. gcs. Physiol.,
Bonn, 1878, Bd. xvii. S. 424.

^ Traces of acid lower, of alkali raise, the temperature of coagulation ; more than
traces convert the proteid into acid or alkali-albumin respectively, and these substances do
not coagulate by heat. — Halliburton, Journ. Physiol., Cambridge and London, vol.
V. p. 165.

^" Limbourg, Ztschr. f. physiol. Chem., Strassburg, Bd. xiii. S. 450.

^^ Ringer and Sainsbury, Journ. Physiol., Cambridge and London, 1891, vol. xii. p. 170.

^- Playcraft, Brit. Med. Journ., London, 1890, vol. i. p. 167.



GENERA L PR OFER TIES AND RE A CTIONS OF PR O TEIDS. 4:



73'


C.


73°




n r^o




i i




84°




73°




^-^TO




i i





Cell globulin .


48°-50° C


Fibrinogen


56° „


Serum globulin


. 75° „


Myosinogen


56° „


Vitellin .


75° „


Crystallin


. 73° „


Haemocyanin .


68° „



The temperature of heat coagulation of some of the principal proteids
may be approximately stated as follows : —

Albumins. Globulins.



Egg albumin .
Serum albumin (a)

; (7)
Muscle albumin
Lact-albumin .



With regard to the separation of proteids by the use of the method
of fractionar heat-coagulation, the opinion has been expressed by Haycraft
that the results obtained are not trustworthy. It is probable, nevertheless,
that the method is trustworthy, since the proteids so separated can be shown
to possess other differences.^

Mechanical 'precipitation of proteieU. — By mechanical means, such as
shaking with sand, or even pom^ing from one test tube to another, a solution
of egg-white deposits threads of insoluble proteid, reminding one of fibrin
filaments, which also they resemble in their difficulty of solubility. By
prolonged shaking, 96 per cent, of the proteid present may be
deposited. Other proteids behave similarly, but as a rule less markedly,
namely, egg globulin, viteUin, the proteids of blood plasma, myosinogen,
potato pro'teid, plant vitellin, alkali albumin, and some specimens of
caseinogen (Eamsden).^

Incliffiisibility. — The proteids belong to the class of substances called
colloids by Thomas Graham; that is, they pass with difficulty or not
at all through animal membranes, or vegetable parchment, the substance
usually employed in the construction of dialysers. Proteids may thus be
separated from diftusible (crystalloid) substances, like sugar and salts.
If a mixture of albiunin and globulin, dissolved in a saline medium as
in blood serum, is placed in a dialyser, with distilled water outside, the
salts and extractives pass through the membrane into the water, and
water passes in ; the proteids remain within ; the albumin in solution, but
the globulin, which is insoluble in w^ater containing no salts, precipitated.

The term colloid does not necessarily imply that the indiftusible
substances are not capable of crystallisation; for many of the pro-
teids have now been crystallised; this is particularly the case with
the vegetable proteids (p. 52), with hsemoglobin (p. 61), with egg
albumin, and with serum albumin. F. Hofmeister^ was the first to
crystallise egg albmnin ; a solution of egg white is mixed with an equal
volmne of satiu'ated solution of ammonium sulphate, and the globulin so

^ The following are the principal papers on this question :— Halliburton on "Proteids
of Serum," Journ. Physiol., Cambridge and London, vol. v. p. 159 ; xi. 456 ; Corin and
Berard, "Egg White," Bull. Acad. roy. de med. de Belg., Bruxelles, 1888, tome xv. p. 4 ;
Colin and Ansiaux, ibid., 1891, tome xxi. p. 3 ; Haycraft and Duggan, Brit. Med. Journ.,
London, 1890, vol. i. p. 167 ; Proc. Boy. Soc. Edin., 1889, p. 351 ; Centralbl. f. Physiol.,
Leipzig, Bd. iv. S. 1 ; Fredericq, ibid., Bd. iii. S. 601 ; Chittenden and Osborne on " Corn-
Proteids," Am. Chem. Joiirn., Baltimore, vol. xiii. pp. 7 and 8 ; xiv. p. 1 ; Hewlett,
Journ. Physiol., Cambridge and London, 1892, vol. xiii. p. 512 ; Ramsden, Proc. Physiol.
Soc, London, 1892, p. 23 ; A. di Frassineto, Sperlmcntale, Firenze, 1895, tome xlix. All
the above except Haycraft and Ramsden defend the method.

^Arch.f. Physiol., Leipzig, 1894, S. 517.

^ Ztschr. f. physiol. Chem., Strassburg, Bd. xiv. S. 165; 1892, xvi. S. 187; see also
Gabriel, ibid., 1891, Bd. xv. S. 456.



44 CHEMICAL CONSTITUENTS OF BODY AND FOOD.

precipitated is filtered off. The filtrate is allowed to stand at the
temperature of the air, and as it gets concentrated minute spheroidal
glolDules of varying size, and finally minute needles, either aggregated or
separate, make their appearance (Fig. 9). On examining these crystals,
they are found to consist of egg alljimiin, with a variable (but usually
small) admixture of ammonium sulphate. Serum albumin has similarly
been obtained by G-lirber and ]\Iichel,^ in a crystalline form, from the
blood serum of horses and rabbits. More recently still, caseinogen has
been crystallised. When a solution of this substance is mixed with




Fig 9.— Clry.stals of egg albumin.

ammoniacal magnesia mixture, it proceeds after some days to deposit
sphccroliths, and ultimately aggregations of needle-like crystals. They
contain 45 per cent, of ash, and 14'0S per cent, of nitrogen. Nuclein
also yields a crystalline deposit with ammoniacal magnesia mixture
(v. Moraczewski).'^

Byrom Bramwell and Xoel Paton ■' have descrilied a case of alljum-

^ Sitzunrjsh. d. 2>hys.-med. GescUscli. :.u IVnrzhurg, 1894. Michel {ibid., No. 3, Bil.
xxix.; Ceniralbl.f. d. med. Wissensch., Ijerlin, 1896, S. l.')2) gives full details of the method
employed. The crystals are hexagonal prisms with the following percentage composi-
tion :—C, 5.3-1 ; H. 7-1 ; N, 15-9 ; S, I'O ; 0, 22-0 ; ash. only 0-22. They coagulate at
51" -53" C. (a)D= -61°.

^ Ztschr. f. physiol. Chem., Strasslmrg, Bd. xxi. S. 71.

^ Eep. Lab. Hoy. Coll. Phys., Edinburgh, 1892, vol. iv. p. 47.



GENERAL PROPERTIES AND RE A CTIONS OF PROTEIDS. 45

innria, in which the urine on standing deposited the proteid matter in a
ciystaHine form (see Fig. 10). They considered it to be of the nature of
a glol)uhn. Huppert ^ has questioned this conchision, and thinks it pro-
bable that the proteid was heteroalbumose.

It is not, therefore, upon the non-crystalline character of proteid, but
upon the enormous size of the proteid molecules, whether crystalline
or non-crystalline, that the difficulty of diffusion depends. It thus
becomes interesting to inquire into the diffusibility of the proteids of
lower molecular weight, namely, the proteoses and peptones. Peptones
are diffusible : this has long been known ; they are highly diffusible
compared to albumin, but of low diffusibihty as compared with salt.




Fig. 10. — Proteid crystals from human iiriiie.— After Byrom Bramwell and Noel Paton.

The diffusibility of the proteoses has long been inferred, but it is only
quite recently that it has been accurately made out that they are inter-
mediate in this character between peptones and albumins. The work
in this direction was done independently by Kiihne ^ and Chittenden,^
and both arrived at the same results. A cmious fact found was, that
deuteroproteose (generally regarded as intermediate between the other
proteoses and peptones) is less diffusible tlian protoproteose. But tliis

]^ ZtscJir. f. physiol. Chcm., Strassbnrg, 1896, Bd. xxii. S. 500.

- Ztschr.f. Biol., Miiucheu, Bd. xxix. S. 1.

^ Jov,rn. Physiol., Cambridge and London, vol. xiv. p. 483,



46 CHEMICAL CONSTITUENTS OF BODY AND FOOD.

is quite in accordance with Sabanejeff 's ^ cryoscopic determination of the
molecular \Yeights of these substances ; he gives the molecular weight
of protoproteose as 2467 to 2640, of deuteroproteose as 3200, and of
peptone as 400 or less. The diffusion power of the different sub-
stances investigated by Kiihne was as follows : — Heteroproteose is
the least diffusible of the proteoses; in neutral sahne solutions it is
precipitated as the salt passes out, and none goes through the dialyser ;
dissolved in ammonia it loses 5-22 per cent. Deuteroproteose comes
next (loss, 24-1 per cent.) ; then protoproteose (loss, 28-3 per cent.) ;
while peptone loses 51 to 51 '8 per cent. Each experiment lasted
twenty-four hours.

Action on 2wlarised light. — All the proteids are levorotatory. The
specific rotatory power of some of the principal members of the group
is as follows : —



Proteids.


Observer.


Value of (a)D.


Serum albumin . . . . .


( Hoppe-Seyler -
( Starke =*


-56"
-68°


Egg albumin


\ Hoppe-Seyler
( Haas-*, Starke


- 35°-5
-38°-08


Lact-albumin


Sebelieu °


- 36=-37°


Serum globulin


Haas


-59°-75


Fibrinogen


Herrman ^


-4.3°


Alkali albumin


Haas


-62°-2


Syntonin (prepared from myosin) .


Hoppe-Seyler


-72°


Casein (dissolved in MgSO^ solution) .


Hoppe-Seyler


-80'


Various proteoses ......


Kuhne and Chittenden'


- 70°-80°



Colour reactions. — These are numerous, and doubtless depend for theu'
occm-rence on the various radicles which, as we have seen, are probably
present in the proteid molecules. Many of them are given by certain of
the decomposition products of the proteids ; and by a careful comparison
of these simpler substances, conclusions have been reached concerning the
particular groups in the proteid molecule to which each reaction is due.

The majority of the colour tests are due to the presence of the
aromatic radicle ; it will, therefore, be well to preface the description of
the reactions themselves by a classification of the aromatic substances
derived from proteids by putrefaction. Salkowski ^ arranges them into
three groups ; whether all these groups exist pre-formed in the proteid
molecule, or are derived, as Maly considered, from only one aromatic
group, matters but little in the question under investigation. The
groups are as follows : —

First group — The ]3]ienol groiqi. — This includes tyrosine, the aromatic
hydroxy acids, phenol, and cresol.

Second growp — The iihenyl group. — This includes phenylacetic and
phenylpropionic acids.

1 Ber. d. deutsch. chcm. Gesellsch., Berlin, Bd. xxvi. S. 385.
^ Ztschr.f. Chem., Leipzig, 1864, S. 737.

^ Jahresb. il. d. Forfschr. d. Thier-Chcm., Wiesbaden, Bd. xi. S. 17.
■* Arch. f. d. cjes. Physiol., Bonn, Bd. xii. S. 378; Chem. Centr.-Bl., Leipzig, 1876,
S. 295, 811 ; 824.

'' Jahresb. il. d. Furtschr. d. Thier-Chem., AViesbaden, Bd. xv. S. 184.

'^ Ztschr. f. physiol. Chem., Strassburg, Bd. xi. S. 508.

"• Ztschr.f. Biol, Miinclien, Bd. xx. S. 51.

^ Ztschr.f. jihysiol. Chem., Strassburg, Bd. xii. S. 215,



GENERAL PROPERTIES AND REACTIONS OF PROTEIDS. 47

Thirdj group — The indol group, of which indol, skatol, and skatol-
carbonic acid are the most important members.

We can now proceed to the consideration of the proteid colour
reactions.

1. The xanthoproteic reaction. — This is characterised by the yellow
colour given by boiling with nitric acid, turned orange by ammonia.
0. Loew ^ considered that the yellow material was a mixture of oxynitro-,
trinitro-, and hexanitro-albumin ; but these substances are very doubtful
as chemical individuals. Salkowski found the reaction to be given by all
the members of his first and third groups of aromatic substances.
Pickering^ found that salicylic acid, and salicylsulphonic acid, cholesterin,
cholalic acid, and taurocholic acid also give the test. A large number
of other organic substances which were tested did not give the same
result. It was noticed that bodies with a benzene nucleus with one
hydrogen replaced by hydroxyl, give the xanthoproteic reaction, whereas
substances which contain a benzene nucleus without the hydroxyl, as
phenylacetic and benzoic acids, do not.

Millons reaction. — A brick-red coloration occurs when proteid matter
is boiled with Millon's reagent (a mixture of the nitrates of mercury
with excess of nitric acid); the reaction was thought by Kiihne^ to
be due to tyrosine. Salkowski also took this view, as the reaction is
given by the substances in his first group, the most prominent member
of which is tyrosine. Those in the second and third groups do not give
the test. Nasse,^ however, demonstrated that Millon's reaction is due
to benzene derivatives, in which one hydrogen atom has been replaced
by hydroxyl (hydroxybenzene nucleus) and not to tyrosine. That
Nasse's view is correct is shown by the following considerations : —
Klihne and Chittenden ^ have found that certain anti-products of diges-
tion, which yield neither leucine nor tyrosine on further digestion, or on



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