ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 6, pp. 1077–1080. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © F.V. Pishchugin, I.T. Tuleberdiev, 2013, published in Zhurnal Obshchei Khimii, 2013, Vol. 83, No. 6, pp. 946–949.
Kinetics and Mechanism of Transaldimination
of Amino Acids and Aromatic Amines with Pyridoxal
F. V. Pishchugin and I. T. Tuleberdiev
Institute of Chemistry and Chemical Technology, Kyrgyz Republic National Academy of Sciences,
pr. Chui 267, Bishkek, 720071 Kyrgyzstan
e-mail: pishugin@rambler.ru
Received May 28, 2012
Abstract―Kinetics and mechanism of transaldimination of amino acids and aromatic amines with pyridoxal
have been studied by means of UV spectroscopy and polarimetry. It has been shown that aminal intermediates
are formed in reaction of the Schiff’s bases with p-aminobenzoic acid and β-alanine. The structure of aminal
and Schiff’s base is determined by the spatial arrangement of the amino acid and aromatic fragments with
respect to the pyridine ring plane. The presence of ОН and СН2–ОН groups in the o-positions in pyridoxal
structure turns amino groups by 90° with respect to the pyridine ring. The scheme of Schiff’s bases
transaldimination by amino acids and biological amines has been developed according to stereospecific,
energy, and geometric factors.
DOI: 10.1134/S1070363213060121
In our previous studies [1–4] it was demonstrated
that the reaction of pyridoxal with amino acids
proceeded through 3 kinetically different stages:
(1) NH2 group addition to the pyridoxal carbonyl
group with the formation of the amino alcohol (very
fast stage); (2) amino alcohol dehydration with the
formation of the Schiff’s base (slow stage); (3) Schiff’s
base rearrangement into a quinoid structure by α-
hydrogen elimination, with subsequent hydrolysis
giving pyridoxamine and keto acids.
fast and are not always unambiguous. Due to this, the
biochemical processes mechanisms are often proved
by studies of interaction between model enzyme and
substrate fragments under the controlled conditions.
In order to fill in the gap in our knowledge on
transaldimination mechanism, in this work we studied
kinetics and mechanism of the Schiff’s bases interac-
tion with amino acids and aromatic amines under
varied conditions. To do so, we chose the following
objects: p-aminobenzoic acid, β-alanine, and the cor-
responding Schiff’s bases, products of p-aminobenzoic
acid and β-alanine condensation with pyridoxal. There
are no chiral centers in these compounds, and their
optical absorbance bands do not overlap [λmax (pyrid-
oxalidene–p-aminobenzoic acid) = 370 nm, and
λmax (pyridoxalidene-β-alanine) = 340 and 430 nm].
It is known that new Schiff’s bases formation is
likely to proceed via amino group addition to the C=N+
group (via transaldimination reaction), but not via addition
to the C=O group of PLP-dependent enzymes [5, 6].
The stage of addition to the L-lysine condensation
product is followed by lysine ε-amino group elimina-
tion. The indirect proof of this mechanism is the
assumption suggested in [5] that the addition to the
HC=NH+ bond proceeds faster than the addition to the
HC=O group. According to [5], this agrees with the
disappearance of the positive circular dichroism upon
addition of the substrate to the coenzyme. After having
the substrate reacted, the circular dichroism reappeared.
Kinetics and mechanism of pyridoxylidene-β-alanine
interaction with p-aminobenzoic acid were studied
following the absorbance at λmax = 430 nm. In the
initial stage the absorbance decreased sharply, this was
followed by the second stage accompanied by slow
continuous decrease in the absorbance. On the contrary,
during the reaction of pyridoxalidene-p-aminobenzoic
acid with β-alanine, the absorbance at λmax = 430 nm
initially sharply increased and then continued to grow
slowly (Figs. 1, 2).
However, the discussed mechanism has been still
an open question. Seemingly, it can be hardly solved
by studies of the enzyme reactions, as they proceed
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