"conjugated proteins" in particular, their structure is very complex.
The molecular weight of some of the proteins has been determined
directly, in the case of those particular ones which can be prepared in
proper form for the usual determination of molecular weight by the
osmotic pressure method; and has been computed for various others, from
the percentage of sulfur found on analysis, or (in the case of the
haemoglobin of the blood) from the proportion by weight of oxygen
absorbed. From these determinations and computations, the following
formulas for certain typical proteins have been calculated: for zein
(from Indian corn), C_{736}H_{1161}N_{184}O_{208}S_{3}; for gliadin
(from wheat), C_{685}H_{1068}N_{196}O_{211}S_{5}; for casein (from
milk), C_{708}H_{1130}N_{180}O_{224}S_{4}P_{4}; for egg-albumin,
C_{696}H_{1125}N_{175}O_{220}S_{8}. These few examples will serve to
illustrate the enormous size and complexity of the protein molecule. The
conjugated proteins are still more complex than the simple proteins
whose formulas are here presented.

Fortunately for the purposes of the study of the chemistry of the proteins,
however, it has been found that most of the common plant proteins, known as
the "simple proteins," can easily be hydrolyzed into their constituent unit
groups, which are the comparatively simple amino-acids, whose composition
and properties are well understood. A study of the results of the
hydrolysis of some twenty common plant proteins has shown that it is rarely
possible to recover the amino-acids in sufficient quantities to account for
a full 100 per cent of the material used, the actual percentage of
amino-acids recovered usually totaling from 60 to 80 per cent. The
remaining material is supposed to be also composed of amino-acids which are
linked together in some arrangement which is not broken apart by any method
of hydrolysis which has yet been devised. This view is borne out by the
fact that substances which exhibit all the characteristic properties of
proteins have been artificially synthetized, by using only amino-acid
compounds. Animal proteins often show a much larger proportion of
unhydrolyzable material than do plant proteins.

AMINO-ACIDS AND PEPTID UNITS

The products of hydrolysis of the common simple proteins are all
amino-acids. These are ordinary organic acids with one (or more) of the
hydrogen atoms of the alkyl group replaced by a --NH_{2} (or sometimes