Full resolution (JPEG) - On this page / på denna sida - II. The Protein Substances
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POLYPEPTIDES. 87
leueylhistidine: tripeptides: di-glycylglycine, alanylglycyltyrosine, leu-
cyltryptophyl.ulutaniic acid; tetra peptides: glycyfglutamyldiglycine, dileu-
cylglycylglycine; pentapeptidee: tetraglycylglycine and leucyltriglycylgly-
cine; hexar and heptapeptides: leucyltetraglycylglycine and leucylpentagly-
cylglycine. The most complex polypeptide thus far prepared is an
octadecapeptide with 15 glycocoll and 3 leucine residues namely: Meucyl-
triglycyl-/-leucyltriglycyl-/-leucyl-octaglycylglycine =
XHoCH(C4H9 )( X ).[XHCHL
»C
,
(
)] 3 .XHCH(C4H9 )CO.
[NHCH2CO]3.NHCH(C4H9)CO.[NHCH2CO]8.NHCH2COOH.
with the supposition that the amino-acids are here also combined together
in the imide binding.
The large number of amino-acids isolated from the proteins make
a large number of bindings possible. The number of possible combina-
tions is still further increased by the fact that all the amino-acids with
the exception of glycocoll contain at least one asymmetric carbon atom,
and this leads to the possible formation of stereochemically different
peptides. Thus in order to give a simple example, from two optically
active amino-acids, four different isomeric forms of clipeptides may occur,
namely (if we designate the optical antipodes by d- and /-) dd, 11, dl and Id.
Of these forms two can form a racemic dipeptide, thus c/-alanyl-c?-leucine +
/-alanyl-Meucine and rf-alanyl-Meucine +/-alanyl-f/-leucine. As the pro-
teins are optically active and on hydrolysis yield chiefly optically active
amino-acids, those polypeptides which can be built up from the natural
amino-acids of the proteins are of special importance in the study of the
constitution of the proteins.
Most of the artificial polypeptides are constructed from monamino-
mono-carboxylic acids, but polypeptides have also been prepared which
contain diamino-acids or aniino-dicarboxylic acids, and in this way the
number of possible polypeptides becomes still greater. With an aminodi-
carboxylic acid such as aspartic acid, other amino-acids can be bound
with one carboxyl group or with both, but also, if we start with aspara-
gine, they can be anchored with the amide group. If we start from the
acid amides we can also obtain a peptide which still contains the CONH2
group and on total hydrolysis yields NH3, like most proteins. A poly-
peptide of this kind is the tripeptide, glycl-/-asparaginyl-Meucine prepared
by E. Fischer and Koenigs.
NH2CH2CO.NHCHCO.NHCH(C4H9)COOH
CH2CONH2
In consideration of the form of binding of the sulphur in the proteins
it is interesting to consider the preparation of thiopolypeptids as performed
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