Enzymatic Acylation of Flavonoids
J. Agric. Food Chem., Vol. 55, No. 23, 2007 9501
LITERATURE CITED
Effect on RegioselectiVity. The effect of the molar ratio of
substrates on the regioselectivity of the reaction is summarized
in Table 2. For a molar ratio of 5, only mono- (63%) and diester
(37%) were synthesized. For a higher molar ratio (10, 20, 40),
mono-, di-, and triacetates were formed. Moreover, higher molar
ratios favored the synthesis of triester. For PSL-C, to our
knowledge, no data are available in the literature about the effect
of the acyl donor/flavonoid molar ratio on the selectivity. For
CAL-B, several studies (31, 32) reported that the formation of
triester did not lead to a decrease in monoester.
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Effect of the Nature of the Flavonoid. The effect of flavonoid
structure on the regioselectivity of the acetylation catalyzed by
PSL-C was studied by using hesperetin (3), naringenin (4), and
chrysin (5) in acetonitrile, at 50 °C. Similarly to 1, the acetylation
yields at 96 h, were almost 100%, for naringenin, but only 30 and
15% for hesperetin and chrysin, respectively. The number of
synthesized products depended on the nature of the flavonoid. For
hesperetin and naringenin, two products were synthesized, while
with chrysin only one product was observed.
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Structural determination of the products (HPLC–ESI–MS, and
NMR analyses) allowed the acetylated positions to be located
(Figure 4). In the case of naringenin, the two products were
naringenin 4′-acetate (4a) [M – H-, m/z 313] and naringenin
7,4′-diacetate (4b) [M – H-, m/z 355]. With hesperetin, the two
isomeric monoesters detected were hesperetin 7-acetate (3a) and
hesperetin 3′-acetate (3b) [M – H-, m/z 343], while with chrysin
only chrysin 7-acetate (5a) [M – H-, m/z 295] was synthesized.
For quercetin, as previously shown, the acetylation took place
on the 3′,4′- and 7-OH groups. These results confirm that only
PSL-C is able to acylate phenolic groups, as was described by
Lambusta et al. (12) when using catechin as substrate.
The reactivity of the 4′-OH group of quercetin was also
reported by Van Acker et al. (33), Russo et al. (34), and Erkoc
et al. (35) using ab initio (STO-3G) and semiempirical (AM1
and PM3) methods. These authors also indicated the presence
of an intramolecular H-bond: H5-O4-H3. Thus, the engagement
of the 3-OH and 5-OH groups in the intramolecular H-bond
probably explains the absence of acylation in these positions in
our study.
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deprotection of bioactive compounds. J. Mol. Catal. B: Enzym.
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In the absence of a 4′-OH group (hesperetin), PSL-C shows
a preference for the 7-OH group, followed by the 3′-OH. The
low reactivity of the 3′-OH group of hesperetin compared to
quercetin may be explained by steric hinderance from the
methoxy group at C-4′.
Finally, with chrysin, that has only two hydroxy groups at
the 5-OH and 7-OH positions, the acylation took place on the
7-OH. This result confirms that the 5-OH group is not reactive
when a 4-oxo group is present in the structure of the flavonoid.
The reactivity of the 7-OH group was observed by Shin et al.
(36) during chemical acylation of chrysin. However, in the
absence of the 4-oxo group (catechin), Lambusta et al. (12)
reported that the acetylation of catechin by PSL occurred on
both the 5 and the 7-OH groups.
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M. Enzymatic acylation of flavonoids. Process Biochem. 2006,
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660.
In conclusion, our data showed that for aglycone flavonoids
the acylation takes place on 3′,4′-dihyroxy of B ring. This fact
could lead to a decrease of the biological activities. So, for their
use in food application it is necessary to evaluate the biological
activities of these derivatives.
Supporting Information Available: Molecular weights and
NMR characterizations of the used substrates (flavonoids) and
the obtained derivatives (products). This material is available