Chemistry Reference
In-Depth Information
Murakami et al. also found that the transamination reaction between hydrophobic
pyridoxals (
36
and
37
) and
-keto acids, was extremely slow
for neutral pyridoxals even in the presence of Cu(
II
) ions [24]. Detailed kinetic analysis
of the reactions carried out in the vesicular system indicated that the transformation of
the Cu(
II
)-quinonoid chelate into the Cu(
II
)-ketimine chelate was kinetically unfavorable
compared with the competing formation of the Cu(
II
)-aldimine chelate from the same
quinonoid species. This problem was solved to a certain extent by quaternization of the
pyridyl nitrogen in pyridoxal, as Murakami et al. successfully accomplished transami-
nation between catalyst
36
and
L-
phenylalanine to produce phenylpyruvic acid.
Having successfully accelerated the reversible isomerization between the aldimine
and ketimine Schiff bases, Murakami et al. then studied how to obtain turnovers in the
full transamination reaction between one amino acid and one keto acid [25]. They
found that the bilayer vesicle system constituted with
33
,
36
, and Cu(
II
) ions showed
some turnovers for the transamination between
L-
phenylalanine and pyruvic acid.
However, such turnover behavior was not observed in a vesicular system composed
of
32
,
36
, and Cu(
II
) ions, and an aqueous system involving N-methylpyridoxal and
Cu(
II
) ions without amphiphiles. Therefore, both the hydrophobic effect and the imi-
dazole catalysis effect were proposed as important for the turnover behavior.
Murakami et al. also examined the enantioselectivity of the catalyzed transamination
reaction in a bilayer membrane [26]. They contrasted a system composed of a peptide
lipid bearing an
L-
lysine residue (
34
), a hydrophobic pyridoxal derivative quaternized at
the pyridyl nitrogen (
37
), and Cu(
II
) ions. This system exhibited turnover behavior for
a
-amino acids, to produce
a
Figure 2.2 Proposed mechanism for enantioselective transamination
reaction catalyzed by Murakami's bilayer membrane systems.
(Reprinted with permission from Ref. 27. Copyright 1995 American
Chemical Society.)
