Full resolution (JPEG) - On this page / på denna sida - II. The Protein Substances - I. Simple Proteins - A. True Albuminous Bodies - Color Reactions for Proteid Bodies
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100 THE PROTEIN SUBSTANCES.
After making alkaline with ammonia or alkalies the color becomes orange-
yellow, due to the nitroderivatives of the benzene and indol groups.
3. Adomkieivicz’s Reaction. If a little proteid is added to a mixture
of 1 vol. concentrated sulphuric acid and 2 vols, glacial acetic acid a
reddish-violet color is obtained slowly at ordinary temperatures, but
more quickly on heating. According to Hopkins and Cole x
this reaction
takes place only on using glacial acetic acid containing glyoxylic acid.
According to them it is better to use a solution of glyoxylic acid, which
can be readily prepared by adding sodium amalgam to a concentrated
solution of oxalic acid and filtering after the discharge of the gas. A
dilute aqueous solution of the acid or some of the solid acid is added to the
proteid solution and sulphuric acid allowed to flow down the side of the
test-tube, when the reddish-violet color will appear at the point of con-
tact of the two liquids or on shaking the mixture. This color reaction,
which is generally called the Adamkiewicz-Hopkins reaction depends
upon the tryptophane and therefore gelatin (which does not contain
•any tryptophane) does not give this reaction.
As further color reactions we will mention: 4. Biuret Test. If a
proteid solution be first treated with caustic potash or soda and if then
a dilute copper-sulphate solution be added drop by drop, first a reddish
then a reddish-violet, and lastly a violet-blue, color is obtained. 5. Pro-
teids are soluble on heating with concentrated hydrochloric acid, produc-
ing a violet color, and when they are previously boiled with alcohol and
then washed with ether (Liebermann 2
) they give a beautiful blue
solution. This blue color is due, according to Cole,3
to a contamination
of the ether with glyoxylic acid, which reacts with the tryptophane
groups split off by the hydrochloric acid. The violet color obtained
with proteins not purified with ether is also considered as a tryptophane
reaction with the furfurol (oxymethylfurfurol) formed from the hexose
containing protein by the action of the concentrated hydrochloric
acid. Reaction 6 with concentrated sulphuric acid and sugar (in small
quantities) is explained in the same way. The beautiful red coloration
is connected with the formation of furfurol from the sugar. 7. With
7>-dimethylaminobenzaldehyde and concentrated sulphuric acid thepro-
teids give a beautiful reddish-violet or deep-violet coloration (0. Neu-
bai"er and E. Rohde 4
). Other aldehydes also give color reactions
by virtue of the tryptophane group in proteins. Other reactions are 8;
Arnold’s reaction 5
is a purple-violet coloration which the proteins give
1
Proceed. Roy. Soc, 08.
2
Centralbl. f. d. ined. Wissensch., 1887.
3
Journ. of Physiol., 30.
4
Zeitsche. f. physiol. Chem., 44.
Arnold, Zeitschr. f. physiol. Chem., 70.
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