Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

276 AMINO-ACIDS
twice with hot absolute alcohol. When the filtrate cools the
hydrochloride of the glycine ester separates in crystalline form.
After twelve hours the crystals are collected by nitration at the
pump and are recrystallised from the smallest possible amount of
alcohol-—frequently some ammonium chloride remains undissolved,
and hence care must be taken not to use too much alcohol. In
this way the salt is obtained completely pure. Melting point 144°.
The thoroughly dried crude product, of which the yield amounts
to 50-60 g., can be used for the preparation of ethyl diazoacetate.
The yield can be increased by concentrating the mother liquor or
by adding ether.
In addition to this, the simplest method of synthesising a-aminoacids
(a method which is less satisfactory for the preparation of higher
members of the series), there are two other processes, both starting from
aldehydes. Strecker obtained the nitrile of the amino-acid, Chap. V. 7,
p. 229, by addition of ammonium cyanide to the next lower aldehyde,
and Erlenmeyer jun. condensed hippuric acid with the aldehyde containing
two carbon atoms less than the required amino-acid.
In this latter process an ajS-unsaturated a-benzoylamino-acid is
formed and is converted into the a-amino-acid by hydrogenation and
subsequent removal of the benzoyl group by hydrolysis.
R.C0 + H2C.C00H ^ R.CH=C.C00H ^ R.CH2.CH.C00H
H | •* I •* |
NH.COC6HS NH.COC6HS NH.COC6H5
R.CH2.CH.C00H + C6H5.COOH
y I
NH2
Recall the importance of the amino-acids as units from which the
proteins are built up. What amino-acids have hitherto been isolated
from proteins by digestion and by acid hydrolysis ?
By the method of E. Fischer it is possible to carry out a separation,
in some degree quantitative, of the individual a-amino-acids. They are
esterified as described above, and the esters are then separated from one
another by fractional distillation of the mixture in vacuo.
It is supposed that in the protein molecule the individual amino-
acids are united through amide linkages. For the synthesis of the so-
called peptides E. Fischer condensed a molecule of the chloride of an
amino-acid with a molecule of amino-acid, e.g.
H2N.CH2.COC1 + H2N.CH.C00H H2N.CH2.CO.NH.CH.COOH
CH3
CH3
Glycylalanme