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POLYPEPTIDES. 89
(brompropionyl glycine). On subsequent treatment with ammonia the halogen
(Br) is replaced by XH2 and the dipeptide alanylglycine
CH3CHXH2CO.XHCH2COOH+XHJ3r
is obtained. By the second action of brompropionylchloride and then treatment
with NH3 we introduce a new alanyl group and the tripeptide alanyl-alanyl
glycine is prepared. By the action of a halogen derivative of an acid radical
another amino-acid residue can be introduced, and the chain of amino groups
can be thus extended.
The prolongation of the chain on the other side, namely, at the carboxyl,
Fischer has accomplished by chlorination of the amino-acids by special treatment
with phosphorus pentachloride. The carboxyl is thus transformed into COO,
while the acid at the same time fixes a molecule of HC1, for example CH3CHXH2HCI
COC1
Just as in the case of the carboxyl group of an amino-acid, so also can a poly-
peptide or its halogen acyl combination be chlorinated and then combined with
a new amino-acid, or a new peptide. As an example, Fischer, from a-brom-
isocapronyldiglycyl glycine, first prepared a-bromisocapronyldiglycylgh’cyl chlo-
ride, and then with diglycylglycine he obtained the heptapeptide leucyl-
pentaglycylglycine,
C4H9 CH(XH2 )CO.(NHCH,CO) 5.XHCH2COOH.
For the various combinations of the optically active amino-acids to poly-
peptides it was important to possess methods of preparation of these amino-acids,
and for this purpose Fischer in many cases used the so-called Walden’s reversion.
This consists in that one optically active amino-acid, for example the /-form, is
transformed into the corresponding halogen fatty acid by the action of nitrosyl
bromide, yielding the optical antipode the d-form. By the action of ammonia
the c/-amino-acid is now obtained which in the above-mentioned manner can be
retransformed into the /-form. Thus from (/-leucine we first obtain /-bromiso-
caproic acid and then by the action of ammonia /-leucine and in the preparation
of the polypeptides the same occurs. Thus, for example, if by reversion (/-leucine
is changed first into /-bromisoeapronyl chloride, if this last is combined with
/-leucine, then we obtain the dipeptide /-lcucyl-/-leucine. On combination with
diglycylglycine the tetrapeptide /-leucyl-diglycyl glycine is produced. Walden’s
reversion does not take place with all amino-acids; other methods can also be
used to obtain the optical antipodes, such as the preparation of the alkaloidal
salts of the benzoyl or formyl combinations of the racemic amino-acids.
The /3-naphthalinsulpho combination of the polypeptides and peptones may
serve, as Fischer, Abderhalden and Funk l
have shown, in explaining the
structure of these bodies. By the action of ^-naphthaline sulphochloride the
NHg groups existing at the beginning of the chain in the amino-acids react there-
with and on subsequent total hydrolysis this naphthaline-sulpho combination
remains unsplit. Thus for instance we can differentiate between glycylalanine
and alanylglycine because after hydrolysis in the first case we obtain naphthalin-
sulphoglycine and alanine and in the second naphthalin-sulphoalanine and
glycocoll (glycine). Tyrosine may, depending upon whether the XH2 as well
as the OH groups are free or not or if only one is available, yield di- or mononaph-
thalinsulpho-derivatives and in this way we can also draw conclusions as to the
structure of tyrosine containing peptides.
The previously mentioned deamidation method of van Slyke (page 781 where
oxyacids are formed by the action of HX02 upon the XH, groups can also give
1
E. Fischer and Abderhalden, Ber. d. d. Chem. Gesellsch., 40; Abderhalden and
C. Funk, Zeitschr. f. physiol. Chem., 64.
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