Microbial synthesis of dihydrochalcones using Rhodococcus and Gordonia species

Article abstract of DOI:10.1016/j.molcatb.2013.09.009

Chalcones are important compounds in food and cosmetics industry, and in food chemistry research. We have developed a method of synthesis of dihydrochalcones from flavanone and α,β-unsaturated chalcones by microbial hydrogenation. It has been found that bacterial strains of Rhodococcus sp. and Gordonia sp. can be successfully used in the key step of dihydrochalcones synthesis. This kind of activity has not been previously examined. Twelve microorganisms were initially screened for their ability to catalyze biotransformation reactions of selected flavonoid compounds. Of these, Rhodococcus sp. and Gordonia sp. transformed flavanone and chalcones to hydrogenation products in good isolated yield of 13-94%.

Full text of DOI:10.1016/j.molcatb.2013.09.009

Journal of Molecular Catalysis B: Enzymatic 97 (2013) 283–288
Contents lists available at ScienceDirect
Journal of Molecular Catalysis B: Enzymatic
journal homepage: www.elsevier.com/locate/molcatb
Microbial synthesis of dihydrochalcones using Rhodococcus and
Gordonia species
Monika Stompor, Bartłomiej Potaniec, Anton∗i Szumny, Paweł Zielin´ ski,
˙
Anna Katarzyna Zołnierczyk, Mirosław Anioł
Department of Chemistry, Wrocław University of Environmental and Life Sciences, Norwida 25, 50-375 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 May 2013
Received in revised form 31 August 2013
Accepted 4 September 2013
Available online 13 September 2013
Chalcones are important compounds in food and cosmetics industry, and in food chemistry research.
We have developed a method of synthesis of dihydrochalcones from flavanone and ␣,␤-unsaturated
chalcones by microbial hydrogenation. It has been found that bacterial strains of Rhodococcus sp. and
Gordonia sp. can be successfully used in the key step of dihydrochalcones synthesis. This kind of activity
has not been previously examined.
Twelve microorganisms were initially screened for their ability to catalyze biotransformation reactions
of selected flavonoid compounds. Of these, Rhodococcus sp. and Gordonia sp. transformed flavanone and
chalcones to hydrogenation products in good isolated yield of 13–94%.
Keywords:
Biotransformation
Flavonoids
© 2013 Elsevier B.V. All rights reserved.
Gordonia sp.
Rhodococcus sp.
Hydrogenation
1. Introduction
The whole cells of various microorganisms have also been
employed to catalyze asymmetric reduction of ketones, aldehydes,
Flavonoids are the group of natural polyphenolic compounds
that are widespread in the plant kingdom [1–3]. Due to their health-
promoting properties, in recent years there has been a growing
interest in methods of synthesis of flavonoids and their application
as food additives [4]. Among others, flavonoids have found applica-
tion in food industry as a rich source of food colourings. Due to their
antioxidant activity they play an important role as natural antioxi-
dants and also they are used as protecting agents against microbial
infections [5–7].
The use of microorganisms may be a highly efficient method of
production of these compounds. This process is considered more
effective than extraction from plants and than classic chemical syn-
thesis. The major advantage of microbial biosynthesis of flavonoids
is the possibility of cost reduction by using modified microbial
strains or low-cost cultivation medium components.
␤-ketoesters and other ␣,␤-unsaturated systems [12]. The applica-
tion of whole growing or resting cells in the reduction reactions has
clear advantages. Among them, the most important is production
of chiral synthons and the ability to catalyze the reactions of high
regio-, chemo- and enantioselectivity.
Many derivatives of flavone are known to have important bio-
logical functions. These natural products have been examined
because of their diverse physiological and pharmaceutical proper-
ties such as estrogenic, antitumor, antimicrobial, antiallergic, and
anti-inflammatory action [13–16]. In the literature there were also
described studies on natural chalcone precursors and some other
flavonoid derivatives, which aimed at determining the impact of
structural modifications on pharmacological properties of the com-
pounds (cytotoxic, antioxidant and anticancer) [17,18].
Satisfactory results of the original research concerning eval-
uation of pharmacological relevance of flavonoids isolated from
natural sources, mainly from plants, inspired us to undertake
the study on biotransformation of health-promoting polyphenols
and their analogues. There are many reports in the literature on
transformations 2-phenylchromane and its biosynthetic deriva-
tives using plants, fungi and yeasts, leading to hydroxy-, methoxy-
and glycosidic compounds [19,20].
The reactions involved in biotransformation of organic com-
pounds by whole cells of various microorganisms include
oxidation, reduction, hydroxylation, esterification, methylation,
demethylation, isomerization, hydrolysis, glucosylation and hydro-
genation [8–10]. In studies on intensification methods of obtaining
natural compounds the mutagenesis techniques are also used [11].
Escherichia coli containing biphenyl dioxygenase from Pseu-
domonas pseudoalcaligenes transformed isoflavone into its
2ꢀ,3ꢀ-dihydro-2ꢀ,3ꢀ-cis-dihydroxy derivative [21]. When genes
of naphthalene dioxygenase ware incorporated into E. coli cells,
∗
Corresponding author. Tel.: +48 71 320 5468; fax: +48 71 320 7744.
E-mail address: miroslaw.aniol@up.wroc.pl (M. Anioł).
1381-1177/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcatb.2013.09.009

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M. Stompor et al. / Journal of Molecular Catalysis B: Enzymatic 97 (2013) 283–288
additionally analogous 5,6- and 7,8-dihydrodiol products were
formed. In this case, flavone gave only 7,8-flavonedihydrodiol
[22]. Above biphenyl dioxygenase produced 2ꢀ,3ꢀ-dihydroepoxy
compounds from flavanone and cis-/trans-isoflavan-7-ols [23,24].
Another interesting biotransformation, occurred in the case of
flavone and its derivatives, was C2-C3 double bond reduction
leading to flavanone moiety [25]. Seo et al. reported also regio-
and stereo-specific hydroxylation of flavanone and isoflavanone
using sitedirected mutants of naphtalene dioxygenase which was
impossible to carry out with the use of the native enzyme [26].
The big advantages of dihydrochalcones as potential food addi-
tives are their safety and stability. Nakamura et al. [5] reported that
dihydrochalcones exhibited higher antioxidant activities than the
corresponding flavanones. Lin et al. [27] characterized phloretin,
dihydrochalcone of naringenin, and 12 other compounds isolated
from apples for their hydroxyl radical scavenging activity and cel-
lular tyrosinase inhibitory activity, suggesting their possible use as
cosmetic agents. Phloretin was suggested to induce apoptosis in
HL60 cells through the inhibition of protein kinase C activity [28].
Dihydrochalcones have also received considerable attention as food
sweeteners, for instance neohesperidin dihydrochalcone, which is
more than 300 times sweeter than sugar.
Chemical methods of hydrogenation with molecular hydrogen
involve the use of catalysts, such as metal complexes and other
compounds harmful to the environment. Selective hydrogenation
of ␣,␤-unsaturated carbonyl compounds has attracted great atten-
tion due to its wide application in synthesis of many fine chemicals
and pharmaceutical compounds. This type reaction is a challeng-
ing task and many transition metals such as rhodium, ruthenium,
palladium, iridium, and other metal complexes were used for
these transformations [29,30]. Chemoselective hydrogenation of
an olefinic fragment linked to an aromatic ring in ␣,␤-unsaturated
ketones has also been noted [31–33]. Kim et al. [34] reported about
the method of reduction of flavone to dihydrochalcone using an
excess of ammonium formate in the presence of Pd-C.
2.1.2. Xanthohumol (4)
Xanthohumol (4) was isolated from supercritical carbon diox-
ide extracted hops obtained from Production of Hop Extracts of
Fertilizer Institute, Puławy, Poland. The method of isolation and
purification was reported by Anioł et al. [36]. The product was
obtained as yellow-orange crystals, mp. 170–172 ◦C. The spectro-
scopic data were in accordance with those previously reported [37].
2.2. General methods
The course of biotransformations was controlled by means of
TLC and HPLC.
Thin layer chromatography was carried out using Kiesgel 60 F254
silica gel (0.2 mm, Merck) with various solvent systems as eluents.
Compounds were visualized by spraying the plates with a solution
of 10 g of Ce(SO4)2 and 20 g of phosphomolybdic acid in 1 L of 10%
H2SO4 and heating.
Composition of reaction mixtures was established by high-
performance liquid chromatography (HPLC), performed on
a
Waters 2690 instrument fitted with 2690 separations module and
996 photodiode array detector and Millennium 32 software. A
reverse phase C-18 column (Marcherey Nagel, NUCLEODUR 100-
5 C18 ec, 4.6 mm × 250 mm) was used. The mobile phase consisted
of two solvents: A – 1% HCOOH in MeCN, B – 1% HCOOH in H2O. A
gradient elution from 40% solvent A to 60% solvent B over 28 min
was used at the flow rate of 1 mL/min.
Products were separated by column chromatography using sil-
ica gel Kieselgel 60 (230–240 mesh, Merck) and hexane/acetone
or hexane/dichloromethane/diethyl ether mixtures. Melting points
were recorded on a Boetius apparatus.
Structures of isolated products were confirmed by spectroscopic
methods. 1H NMR and 13C NMR spectra were recorded on NMR
Bruker Avance II DRX 300 and Bruker Avance II DRX 600 MHz
spectrometers with the tetramethylsilane (TMS) as an internal ref-
erence. The NMR spectra were measured in CDCl3 or acetone-d6. To
confirm the presence of characteristic absorption bands, IR spectra
were measured using a FT-IR Thermo-Nicolet IR 300 spectrometer.
HRESI-MS spectra were taken on a Bruker micrOTOF-Q spectrom-
eter. The structures of the known compounds were confirmed by
comparison of their spectroscopic properties with data published
in the literature.
Due to the potential ability of bacterial cells to promote bio-
transformation reactions, we decided to study biotransformation
of chalcone (1), 4-methoxychalcone (2), chalconaringenin (3), xan-
thohumol (4), flavanone (5) and naringenin (6). In this report, the
whole cells of Rhodococcus sp. DSM 364 and Gordonia sp. DSM
44456 were used to obtain hydrogenation products of flavonoid
compounds.
2.3. Microorganisms
2. Experimental
Microorganisms (Rhodococcus, Gordonia, Micrococcus, Strepto-
myces, Bacillus, Pseudomonas, Dietzia) were purchased from the
Polish Collection of Microorganisms (PCM) of the Institute of
Immunology and Experimental Therapy Polish Academy of Sci-
ences in Wroclaw and from German Microorganisms Culture
Collection (DSMZ).
Biomass was determined by the dry weight method. Samples
(10 mL) were filtered on Milipore HA filters, the residue was washed
with distilled water (2 × 5 mL) and dried at 105 ◦C to a constant
weight using the MAX 50/1/NH apparatus, RADWAG.
2.1. Chemicals
All used chemicals were commercially available. Substrates
for biotransformation were purchased from Sigma–Aldrich or
obtained according to procedures described below.
2.1.1. Chalconaringenin (3)
Chalconaringenin (3) was prepared according to the general
procedure described previously [35], which was modified. Thus,
naringenin (1 g, 3.67 mmol) was dissolved in methanol (10 mL) and
added to 200 mL of 20% KOH. The reaction mixture was stirred
at 40 ◦C for 4 h, acidified with 2 M HCl to pH = 6 and extracted
with diethyl ether (3 × 100 mL). The organic layer was separated,
washed with water (3 × 100 mL). The combined organic extracts
were dried over Na2SO4, filtered and concentrated in vacuo. The
crude residue was purified by column chromatography on silica
gel (chloroform/methanol 5/0.5, v/v) to afford 0.80 g (80%) of yel-
low crystals, mp. 256–258 ◦C. IR, 1H NMR, 13C NMR spectra and HR
ESI-MS are given in supplementary data.
2.4. Screening procedure
25 mL of the medium in a 100 mL Erlenmeyer flask containing
1 g of glucose, 2.5 g of malt extract, and 1 g of yeast extract (pH 7.2)
were inoculated with a suspension of a microorganism and then
incubated for 48 h at 28 ◦C on a rotary shaker at 200 rpm. After that,
1 mg of a substrate dissolved in 1 mL of acetone was added. Every
24 h, portions of 5 mL of the transformation mixture were taken
out and extracted with 10 mL of ethyl acetate and centrifuged at
4 ◦C for 10 min at 15,000 rpm. The extracts were dried over Na2SO4,
concentrated in vacuum and analyzed by HPLC and TLC.

M. Stompor et al. / Journal of Molecular Catalysis B: Enzymatic 97 (2013) 283–288
285
OH
3'
3
6
HO
O
HO
OH
OH
2'
1'
4'
5'
4
2
1
KOH, MeOH
5
6'
OH
O
OH
O
6
3
Fig. 1. Synthesis of chalconaringenin (3).
All experiments were done in duplicates and the results were
reproducible. Verification of the stability of the substrates were per-
formed by preparing and incubating control samples consisting of
a tested flavonoid and the sterile cultivation medium. The culture
controls consisted of the microbial cultures in fermentation media
in which the microorganisms were cultivated, without substrate
addition.
2.6.3. Dihydrochalconaringenin–phloretin (3a)
Chalconaringenin (3, 25 mg, 0.092 mmol) was used as
a
substrate. The crude product was purificated by column chro-
matography (hexane:acetone 5/1, v/v) to obtain pale yellow solid
of 3a (DSM 364: 21.2 mg, yield 84%), mp. 259–260 ◦C. Literature
melting point (258–260 ◦C), chemical synthesis and IR, 1H–13C NMR
spectra were reported previously [40].
2.6.4. 2ꢀ-Hydroxychalcone (5a) and 2ꢀ-hydroxydihydrochalcone
(5b)
2.5. Preparative biotransformation
Flavanone (5, 25 mg, 0.111 mmol) was used as a substrate. The
crude product mixture was purificated by column chromatography
(hexane/diethyl ether/chloromethane 8/1.5/0.5, v/v/v) to obtain
yellowish solid of 5a (DSM 364: 7.5 mg, yield 30%), mp. 86–88 ◦C
(hexane). Literature melting point (88–90 ◦C) and chemical synthe-
sis were described by Rajput et al. [41]. Product (5b) was isolated as
a white solid (DSM 364: 3.3 mg, yield 13%), mp. 127–129 ◦C (hex-
ane). The chemical preparation and spectra data were described
previously [20,42].
All transformations were performed on the preparative scale
in 2000 mL flasks, each containing 250 mL of a cultivation medium.
Precultures of Gordonia sp. DSM 44456 and Rhodococcus sp. DSM 364
were prepared in two steps. At first, cells from the agar slants were
transferred into 100 mL Erlenmeyer flask, each containing 25 mL
of the medium (2.5% malt extract, 1% yeast extract, 4% glucose,
pH 7.2), and incubated in a shaker at 28 ◦C and 200 rpm for 48 h.
Then, 1 mL of this medium was added to a 250 mL Erlenmeyer flask
containing 100 mL of the cultivation medium, and grown at 28 ◦C
for 24 h. Portions of 1 mL of the pre-incubation culture suspension
were used to inoculate four 2000 mL flasks, then 25 mg of substrate
dissolved in 1 mL of acetone was added to the grown cultures under
the same conditions as described above (24 h at 28 ◦C). After 72 h
(biotransformations of chalcones 1–3) and 7 days (biotransforma-
tions of flavanone 5) of incubation the mixtures were extracted
with ethyl acetate 3 × 200 mL and dried over Na2SO4. After the
solvent evaporation, transformation products were separated by
column chromatography and analyzed by chromatographic meth-
ods: TLC, HPLC, NMR and IR. The products were crystallized in
hexane to obtain dihydrochalcones. IR, 1H NMR, 13C NMR spectra
and HR ESI-MS are given in supplementary data.
3. Results and discussion
The substrates containing 1,3-diphenyl-2-propen-1-one moi-
ety: commercially available chalcone (1) and 4-methoxychalcone
(2), obtained by chemical synthesis chalconaringenin (3, Fig. 1),
isolated from spent hop xanthohumol (4), flavanone (5) and narin-
genin (6) were subjected to screening tests in order to select
bacterial strains which were capable of enzymatic transformation.
Among 12 microorganisms tested, Rhodococcus sp. DSM 364 and
Gordonia sp. DSM 4456 were found to be the most effective in
hydrogenation of some of these compounds.
The available literature evidence and our preliminary study indi-
cate that it is difficult to select a biocatalyst capable of transforming
flavonoids. For biotransformation of the compounds prepared by
chemical synthesis we used the microorganisms selected on the
basis of literature data. We expected to find the products of micro-
bial aromatic ring hydroxylation, hydroxyl group acylation and
glycosidation, carbonyl groups reduction and reduction of unsat-
urated bonds.
Rhodococcus and Gordonia strains are ubiquitous in the envi-
ronment and have diverse biodegradative capabilities, breaking
down a variety of alkanes, halogenated aliphatics and aromat-
ics, and other xenobiotic pollutants [43–48]. To the best of our
knowledge there are not many reports regarding biotransforma-
tion of chalcones with the help of bacteria belonging to Gordonia
or Rhodococcus. In fact, known reports describing hydrogenation
of chalcones by microorganisms to the corresponding saturated
ketones concerned the reaction catalyzed by Corynebacterium equi
IFO 3730, Aspergillis flavus isolated from Paspalum maritimum – in
aqueous medium and Saccharomyces cerevisiae yeasts in biphasic
system [31–33].
2.6. Biotransformation products
2.6.1. Dihydrochalcone (1a)
Chalcone (1, 25.0 mg, 0.120 mmol) was used as a substrate. The
crude product was purificated by column chromatography (hex-
ane:acetone 5/1, v/v) to obtain white crystals of 1a (DSM 364:
23.7 mg, yield 94%; DSM 44456: 16.4 mg, yield 65%), mp. 71–72 ◦C
(hexane), lit. 71–73 ◦C [38]. The chemical synthesis of the com-
pound and IR, 1H–13C NMR and MS spectra have been described
previously [38,39].
2.6.2. 4-Methoxydihydrochalcone (2a)
4-Methoxychalcone (2, 25 mg, 0.105 mmol) was used as a
substrate. The crude product was purificated by column chro-
matography (hexane:acetone 5/1, v/v) to obtain white crystals of
2a (DSM 364: 20.2 mg, yield 80%; DSM 44456: 11.4 mg, yield 45%),
mp. 58–59 ◦C (hexane), lit. 57.5–58.5 ◦C [38]. The chemical synthe-
sis of the compound and IR, 1H–13C NMR and MS spectra have been
described previously [38,39].
After the screening, we determined the biomass produc-
tion at the chosen temperature of 28 ◦C by both the bacterial

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M. Stompor et al. / Journal of Molecular Catalysis B: Enzymatic 97 (2013) 283–288
Fig. 2. Biomass production by (A) Rhodococcus sp. DSM 364, and (B) Gordonia sp. DSM.
strains – Rhodococcus sp. DSM 364 and Gordonia sp. DSM 4456
(Fig. 2). On the basis of the results obtained, in the preparative bio-
transformations the substrates were added after 72 h of incubation,
when microorganisms reached the stationary phase of growth.
The biohydrogenation of chalcone (1), 4-methoxychalcone
(2) and chalconaringenin (3) was completely chemoselective,
producing only the corresponding saturated derivatives of 1,3-
diphenyl-1-propanone (1a, 2a, 3a and 5a) with a yield ranging from
13 to 94% after 72 h of the reaction. No other products were detected
by HPLC and TLC. In the case of xanthohumol (4) we did not observe
any product formation (Fig. 3 and Table 1), although in our previ-
ous study using the strain Rhodotorula marina this substrate was
transformed into its dihydroderivative with 18% yield. This strain
produced also compound (2a) from 2 [37].
showed maxima (ꢀmax = 284, 233 and 213 nm), that were compara-
ble with a chalcone chromophore. The infrared spectrum exhibited
absorption bands typical for hydroxyl (3285 cm−1), methylene
(2924 cm−1), conjugated carbonyl (1605 cm−1) and aromatic (1573
and 828 cm−1) groups. In the 13C NMR spectrum of 3a 11 carbon
signals, representing 15 carbon atoms, were observed. Compari-
son of the 1H and 13C NMR spectra of 3 and 3a also revealed a
structural similarity of these two compounds. The most significant
difference observed in the 1H NMR was the presence of protons of
two methylene groups at ␣- and ␤ positions in the spectrum of 3a,
and their absence in the spectrum of 3. In addition, compound 3
exhibited a molecular ion at m/z 271.0608 in HR-EIMS, lower by
2.0141 compared to the product. Thus, compound 3a was assigned
as dihydrochalconarinenin (phloretin).
Initially, we used two strains of bacteria. The experiments car-
ried out on substrates 1 and 2 showed that better yields were
obtained for Rhodococcus sp. DSM 364 – 94% and 80%, respectively.
Thus, this strain was applied for the next biotransformations, using
compounds 3–5.
The structures of products were confirmed by means of IR,
1H NMR, 13C NMR, correlation spectroscopy and comparison with
reported physical and spectroscopic data.
Biotransformation of chalcones 1 and 2 in the cultures of
Gordonia sp. and Rhodococcus sp. led to the corresponding dihy-
drochalcones 1a and 2a. Comparison of the 1H NMR, 13C NMR and
IR spectra of the products with the substrates clearly showed the
lack of the double C C bond and the presence of the unchanged
carbonyl group C O. Transformation of chalconaringenin (3) in
the culture of Rhodococcus sp. led to phloretin (3a) in 84% yield.
Compound 3a, isolated as a white powder, exhibited a molecu-
lar ion at m/z 273.0749 in the high resolution electron ionization
mass spectrum (HR-EIMS), corresponding to the molecular for-
mula of C15H14O5 (calcd. = 274.2686). The UV spectrum of 3a
Phloretin (3a) was isolated from Formosan apple (Malus doumeri
var. formosana) [27] and from tropic medicinal plant Syzygium
aqueum and have potential as a cosmetic and antihyperglycaemic
agent [49]. Furthermore, other research suggest that phloretin
inhibits tumour cell growth by induction of apoptosis in the B16
mouse melanoma cells [28]. A literature search disclosed that
phloretin (3a) has been synthesized by Siddaiah et al. [40] but to
our knowledge, this paper is the first report of its production using
bacterial biocatalysis.
Biotransformation of flavanone (5) catalyzed by the strain of
Rhodococcus sp. DSM364 led to formation of two products. Thus,
compound (5) was transformed into 2ꢀ-hydroxychalcone (5a) and
next, like compounds (1–3), to 2ꢀ-hydroxydihydrochalcone (5b) by
a bacterial enzyme system with 30 and 13% yield, respectively.
Trihydroxy derivative of this compound: naringenin (6), was bio-
transformed neither by this strain nor by Gordonia sp. DSM 44456
(Fig. 4).
Compound 5a, isolated as a yellowish powder, exhibited a
molecular ion at m/z: [M−H+] = 223.0764 corresponding to the
molecular formula of C15H12O2. The IR spectrum exhibited absorp-
tion bands typical for hydroxyl (3467 cm−1), aromatic or alkenyl
Table 1
C
H (3058 cm−1), conjugated carbonyl (1640 cm−1) and aromatic
Synthesis of dihydrochalcones by Rhodococcus sp. DSM 364 (I) and Gordonia sp. DSM
(1573 cm−1) groups. The 13C NMR spectrum of 5a showed 13 carbon
signals representing 15 carbon atoms, all of them having chemical
shifts of sp2 carbon, in which two of the signals were assignable
to a conjugated carbonyl (193.7 ppm) and an oxyaryl (163.5 ppm)
carbon atom. The 1H NMR spectrum showed a characteristic sig-
nal of a chalcone structure by the presence of a pair of doublets at
7.94 and 7.67 ppm with a trans coupling constant J = 15.3 Hz (trans-
chalcone J = 15.6 Hz, cis-chalcone J = 13.0 Hz [50]). The spectroscopic
data suggested that 5a is a simple monohydroxylated chalcone
derivative.
44456 (II).a
Strain of
bacteria
Substrate
Product
Time of biotransformation
(days)
Yield (%)
I
II
I
II
1
1
2
2
3
4
5
1a
1a
2a
2a
3a
None
5a
5b
None
3
3
3
3
3
7
7
94
65
80
45
84
0
30
13
0
I
I or II
I
The 1H NMR spectrum of 5b showed presence of two methylene
groups in the place of the chalcone double bond by the presence of
a pair of triplets at 3.32 and 3.06 ppm with a coupling constant
I or II
6
7
a
Culture conditions: 200 rpm, 28 ◦C. After 48 h of incubation 25 mg of substrate
dissolved in 1 mL of acetone was added.

M. Stompor et al. / Journal of Molecular Catalysis B: Enzymatic 97 (2013) 283–288
287
R1
R2
R3
R1
R2
R3
R4
O
R4
O
1-3
1a-3a
HO
O
OH
OH
O
4
Fig. 3. Biotransformation of the chalcones (1–4) catalyzed by the selected bacteria strains.
O
OH
OH
Rhodococcus sp. DSM 364
+
O
O
O
5
5a
5b
Fig. 4. Biotransformation of flavanones 5 catalyzed by Rhodococcus sp. DSM364.
J = 7.7 Hz. The phenolic group was determined to be at 12.28 ppm,
whereas the multiplicities of four aromatic signals 7.73, 7.45, 6.96
and 6.86 ppm were typical for a 1,2-disubstituted benzene. Con-
sequently, the ring B in 5b was an unsubstituted phenyl group
(7.18–7.33 ppm, 5H).
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2ꢀ-Hydroxydihydrochalcone (5b) has been isolated from the
Pepermia obtusifolia [51] and has been obtained in biotransforma-
tion of flavanone (5) by the culture of a UV mutant of Aspergillus
niger MB [20]. It has been popular substrate in the solvent-free syn-
theses for generation of a variety of flavonoids [52]. Some of these
compounds possess a wide range of antimicrobial activities [53].
4. Conclusion
In conclusion, the present study reports a development of
highly efficient protocol for chemoselective conjugate reduction
of ␣,␤-unsaturated carbonyl compounds in aqueous media using
bacterial cultures. The products obtained in the biotransformations
of chalcones (1-2) using the strains of Gordonia sp. DSM 44456
and Rhodococcus sp. DSM 364 contained 1,3-diaryl-2-propan-1-one
structure. The strain of Rhodococcus sp. DSM 364 was capable of
hydrogenation of chalconaringenin (3) to phloretin (3a) and cleav-
age of the C-ring in flavanone (5), leading to 2ꢀ-hydroxychalcone
(5a) and next to 2ꢀ-hydroxydihydrochalcone (5b). Xanthohumol
(4) and naringenin (6) were resistant to biotransformation by these
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