Full resolution (JPEG) - On this page / på denna sida - II. The Protein Substances - I. Simple Proteins - A. True Albuminous Bodies - Precipitation Reactions of the Proteid Bodies - Color Reactions for Proteid Bodies
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COLOR REACTIONS. 99
chloride and others. On this is based the use of proteids as antidotes
in poisoning with metallic salts.
5. Precipitation by Mineral Acids at Ordinary Temperatures. The
proteids are precipitated by the three ordinary mineral acids in proper
amounts, hut not by orthophosphoric acid. If nitric acid be placed in a
test-tube and the proteid solution be allowed to flow gently thereon, a
white opaque ring of precipitated proteid will form where the two liquids
meet (Heller’s albumin test).
6. Precipitation by the so-called Alkaloid Reagents. To these belong
the precipitation by metaphosphoric acid and by hydroferrocyanic acid,
which is carried out by the aid of potassium ferrocyanide in a liquid
containing acetic acid; precipitation by phosphotungstic acid or phos-
phomolybdic acid in the presence of free mineral acids; precipitation
by potassium-mercuric iodide or potassium-bismuth iodide in solutions
acidified with hydrochloric acid; precipitation by tannic acid in acetic
acid solutions. The absence of neutral salts or the presence of free
mineral acids may prevent the appearance of the precipitate, but after
the addition of a sufficient quantity of sodium acetate the precipitate
will in both cases appear; precipitation by picric acid in solutions acid-
ified by organic acids. Proteids are also precipitated by trichloracetic
acid in 2-5 per cent solutions, by phenol, salicyl sulphonic acid, nucleic
acid, taurocholic acid and by chondroitin sulphuric acid in acid solutions.
Color Reactions for Proteid Bodies.
1. MUlon’s Reaction. 1
A solution of mercury in nitric acid contain-
ing some nitrous acid gives a precipitate with proteid solutions which
at the ordinary temperature is slowly, but at the boiling-point more
quickly, colored red; and the solution may also be colored a feeble
or bright red. Solid albuminous bodies, when treated by this reagent,
give the same coloration. This reaction is due to the tyrosine and is
also given by other monohydroxyl benzene derivatives. According to
O. Nasse 2
it is best to use a solution of mercuric acetate which is treated
with a few drops of a 1 per cent solution of potassium or sodium nitrite;
previous to use a few drops of acetic acid are added.
2. Xanthoproteic Reaction. With strong nitric acid the albuminous
bodies give, on heating to boiling, yellow flakes or a yellow solution.
1
The reagent is prepared in the following way: 1 pt. mercury is dissolved in 2 pts.
nitric acid (of sp.gr. 1.42), first cold and then warmed. After complete solution of
the mercury add 1 volume of the solution to 2 volumes of water. Allow this to stand
a few hours and decant the supernatant liquid.
2
See O. Nasse, Sitzungsb. d. Naturforsch. Gesellsch. zu Halle, 1879, and Pfluger’s
Arch., 83; see also Vaubel and Blum, Journ. f. prakt. Chem. (N. F.), 57.
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