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
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
98 THE PROTEIN SUBSTANCES.
The determination of the molecular weight of the proteids has been
attempted by various methods which are more or less uncertain. 1
There
is no doubt that the molecular weight of the proteids is very high, but
the statements about the size vary considerably. For the true proteids
thus far investigated, values ranging from 4000—6000—10,000 have
been found.
The general reactions for the proteids are very numerous, but only
the most important will be given here. To facilitate the study of these,
they have been divided into the two following groups. It must be
remarked that the precipitation reactions are not only applicable for
the soluble true proteids but also, more or less, for other soluble proteins
in general. The color reactions are applicable to all soluble or insoluble
proteins with few exceptions, which will be mentioned later.
Precipitation Reactions of the Proteid Bodies.
1. Coagulation Test. An alkaline proteid solution does not coagulate
on boiling, and a neutral solution only partly and incompletely; the reac-
tion must therefore be acid for coagulation. The neutral liquid is first
boiled and then the proper amount of acid added carefully. A fiocculent
precipitate is formed, and with proper technique the filtrate should be
water-clear. If dilute acetic acid be used for this test, the liquid must
first be boiled and then 1, 2, or 3 drops of acid added to each 10-15 cc,
depending on the amount of proteid present, and boiled before the addi-
tion of each drop. If dilute nitric acid (25 per cent) be used, then to
10-15 cc. of the previously boiled liquid }5-20 drops of the acid must
be added. If too little nitric acid be addei, a soluble combination of the
acid and proteid is formed, which is precipitated by more acid. A pro-
teid solution containing a small amount of salts must first be treated with
about 1 per cent NaCl, since the heating test may fail, especially on using
acetic acid, in the presence of only a slight amount of proteid.
2. Precipitation by Alcohol. The solution must not be alkaline,
but must be either neutral or faintly acid. It must, at the same time,
contain sufficient quantity of neutral salts.
3. Neutral Salts, such as Na2S(>4 or NaCl, when added to saturation
precipitate certain proteids but not others. Ammonium sulphate when
dissolved to saturation in the liquid is considered as the general pre-
cipitant for proteids. In the presence of free acetic or hydrochloric
acid the above-mentioned salts, NaCl or Na2S04, in sufficient con-
centration, are also general precipitants for the proteids.
4. Precipitation by Metallic Salts such as copper sulphate, ferric
chloride, neutral and basic lead acetate (in small amounts), mercuric
1
See especially F. N. Schulz, Die Grosse des Eivveissmoleciile, Jena, 1903.
<< prev. page << föreg. sida << >> nästa sida >> next page >>
