Frans_M_Everaerts_Isotachophoresis_378342.pdf

320
/
c
Cu-His complex
/
-
t
B
AMINO ACIDS, PEPTIDES AND PROTEINS
A
His +
-
t
Fig.13.4. Isotachopherogram obtained with the operational system listed in Table 13.1. Species
injected: A, histidine; B, CuSO,; C, a mixture of the two. R = Increasing resistance; f = time.
With propionaldehyde, experiments could be carried out satisfactorily, although it
must be distilled several times under nitrogen. Even after distillation, propionic acid is
present in small amounts, but it was found that the amount of propionic acid did not
increase during the analysis. A similar disturbance to that discussed briefly in section
13.1.3, due to carbonate (hydrogen carbonate), can be expected; this disturbance does
not obscure the analytical results either qualitatively or quantitatively.
For optimal information, before the analyses were carried out, the pK values of the
various amino acids were determined in aqueous propionaldehyde solutions of various
concentrations, and the results are given in Table 13.6. It can be seen that the pK values
of the acidic groups decrease, because the amino groups are blocked.
The analysis of some amino acids was carried out in a solution containing 3% of
propionaldehyde, with the operational system specified in Table 13.7. The leading
electrolyte was adjusted to pH 7.2 because it was found that the pK value of ethanolamine
was 7.2 in a 3% propionaldehyde solution. Only measurements at ‘neutral pH: are possible
with an aqueous solution containing 3% of propionaldehyde. At a pH of the leading
electrolyte, which contains propionaldehyde, of above 8, a white insoluble component is
formed after some time.
In Table 13.8, some step heights of amino acids obtained in the operational system