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96 THE PROTEIN SUBSTANCES.
The precipitation of proteids and also other soluble proteins by salts
stands in close relation to their colloidal nature, and in this connec-
tion we refer to what has been said in Chapter I. The proteids do not
as a rule diffuse through animal membranes, or only to a very slight
extent, and hence have in most cases a pronounced colloidal nature in
Graham’s sense. They belong to the hydrophile colloids; their solu-
tions show properties in common with those of typical colloids and also
true solutions. Certain of them, especially the peptones and a few
proteoses, which will be discussed later, seem to occupy an intermediate
position, as their solutions are characterized by a lesser viscosity and
greater diffusibility and filtration ability, are not readily precipitable
by alcohol or coagulable by heat, and are only partially precipitable
by salts.
The solutions (or suspensions) of proteids in water, the proteid hydro-
sols, are converted by various means into proteid hydrogels. Of these
means we must specially mention the following: nocking out with salts,
precipitation with alcohol, gelatinization of a gelatin solution on cool-
ing, and coagulation by the action of enzymes or heat.
Those proteids which occur, according to the common views, pre-
formed in the animal fluids and tissues, and which have been isolated
from these by indifferent chemical means without losing their original
properties, are called native proteids. New modifications having other
properties can be obtained from the native proteids by heating, by the
action of various chemical reagents such as acids, alkalies, alcohol, and
others, as well as by proteolytic enzymes. These new proteids are called
modified (" denaturierte ") proteids, to differentiate them from the native
proteids.
The precipitation with alcohol is a reversible reaction, as the pre-
cipitate redissolves on subsequent dilution with water. The proteids
are changed by the action of alcohol, some readily and quickly, others
with difficulty and very slowly; the proteid then becomes insoluble in
water and is modified.
On heating a solution of a native proteid it is modified at a different
temperature for each different proteid. With proper reaction and other
favorable conditions, for instance in the presence of neutral salts, most
proteids can in this way be precipitated in a solid form as coagulated
proteid. The hydrosol is converted into hydrogel, but as’ a modification
takes place, this process is irreversible. The temperature at which
coagulation occurs is a variable one for the same protein under different
conditions of the experiment. The various temperatures at which
coagulation of different proteids occurs in neutral solutions containing
salt have in many cases given us good means for detecting and separating
proteids. The views in regard to the use of these means are somewhat
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