Crude peroxidase from onion solid waste as a tool for organic synthesis. Part II: Oxidative dimerization-cyclization of methyl p-coumarate, methyl caffeate and methyl ferulate

Article abstract of DOI:10.1016/j.tetlet.2011.01.004

The ability of a crude onion peroxidase preparation to act as a biocatalyst for the oxidative dimerization-cyclization of methyl p-coumarate, methyl caffeate and methyl ferulate is presented. The products of the reaction have been fully characterized and were found to possess potent antioxidant activity in a ferric reducing antioxidant power (FRAP) assay.

Full text of DOI:10.1016/j.tetlet.2011.01.004

Tetrahedron Letters 52 (2011) 1165–1168
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Crude peroxidase from onion solid waste as a tool for organic synthesis.
Part II: oxidative dimerization–cyclization of methyl p-coumarate, methyl
caffeate and methyl ferulate
Sonia Moussouni a, , Maria-Liliana Saru a, , Efstathia Ioannou b, Mohamed Mansour a, Anastasia Detsi c,
Vassilios Roussis b, Panagiotis Kefalas a,
⇑
a Department of Food Quality and Chemistry of Natural Products, Mediterranean Agronomic Institute of Chania/Centre International de Hautes Etudes Agronomiques
Méditerranéennes, 73100 Chania, Crete, Greece
b Department of Pharmacognosy & Chemistry of Natural Products, School of Pharmacy, University of Athens, Panepistimiopolis Zografou, Athens 15771, Greece
c Laboratory of Organic Chemistry, Department of Chemical Sciences, School of Chemical Engineering, National Technical University of Athens, Heroon Polytechniou 9,
Zografou Campus, 15780 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
The ability of a crude onion peroxidase preparation to act as a biocatalyst for the oxidative dimerization–
cyclization of methyl p-coumarate, methyl caffeate and methyl ferulate is presented. The products of the
reaction have been fully characterized and were found to possess potent antioxidant activity in a ferric
reducing antioxidant power (FRAP) assay.
Received 23 September 2010
Revised 15 December 2010
Accepted 4 January 2011
Available online 12 January 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Onion peroxidase
p-Coumaric acid
Caffeic acid
Ferulic acid
Dihydrobenzofuran lignans
Antioxidant activity
Oxidative dimerization of phenolic compounds occurs in Nature
and leads to the formation of dihydrodimeric products that play a
significant role in the cross-linking of plant cell wall components,
which in turn influences various important cell wall properties.
This biotransformation is catalyzed by enzymes, such as peroxi-
dases and polyphenol oxidases including laccases, or, in some
cases, occurs via photochemical oxidation.1–5 Moreover, oxidative
cross-linking is very important for the properties and technological
applications of biopolymers in which phenolics (feruroyl and cou-
maroyl residues) are covalently attached.6,7
p-Coumaric acid (1), caffeic acid (2), and ferulic acid (3)
(Scheme 1) are components of plant cell walls, and are subjected
to oxidative dimerization to produce structurally diverse lignan-
type natural products. Characteristic examples of common pheno-
lic acid dihydrodimers isolated from plants are the ferulic acid di-
mers 8-O-4-diFA (compound I), 8,8-diFA (compound II), 5,5-diFA
(compound III), and the dihydrobenzofuran lignans (compounds
of type IV) (Fig. 1).8–10 The latter display a wide range of biological
activity including antitumor,11 antiparasitic,12 and antioxidant,5
and as a result, have attracted considerable attention as potential
synthetic targets with an important bioactivity.
The dimerization of phenolic acids to give lignans has been re-
ported to occur efficiently mainly using the biomimetic peroxi-
dase-H2O2 system, exploiting in most cases horseradish
peroxidase,4,13–16 but also potato peroxidase,9 Momordica charantia
peroxidase,5 and a peroxidase from the leaves of Bupleurum salicifo-
lium.1 The dihydrobenzofuran dimer of p-coumaric acid has also
been produced as a metabolic product of Curvularia lunata, as a re-
sult of the presence of peroxidase and laccase in the culture filtrates
of this microorganism.17 In addition, chemical single-electron oxi-
dizing systems, such as Ag2O or K3Fe(CN)6, are effective coupling
agents for the dimerization of phenolic acids or their esters.11,12,18
The exploitation of food residuals as sources of crude enzymes
can contribute not only to the reduction of the polluting load of
food industry wastes, but also to the development of high added
value products. As a continuation of our research on the potential
biocatalytic activity of crude peroxidase (POD) from onion solid
waste,19 we herein describe the reactions of methyl p-coumarate
(4), methyl caffeate (5), and methyl ferulate (6) using this enzymic
preparation as a catalyst. The antioxidant activity of the reaction
⇑
Corresponding author. Tel./fax: +30 2821035056.
E-mail address: panos@maich.gr (P. Kefalas).
These two authors contributed equally to this research.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.004

1166
S. Moussouni et al. / Tetrahedron Letters 52 (2011) 1165–1168
10'
HO
O
H3CO
O
CH3
9
O
O
9
10
CH3
9' O
8
2
6
7
6' 5'
crude onion POD
extract
CH3OH, H2SO4
reflux, 2 h
1
8
5
8'
7'
1
7
4' OH
O
6
5
1'
2
H2O2, pH 4 or 5
3'
4
O
2'
3
R
3
R
R
R
4
OH
OH
4 R = H
1 R = H
7 R = H
2
5
8
R = OH
R = OH
6 R = OCH3
R = OH
9 R = OCH3
3 R = OCH3
Scheme 1. Oxidative coupling of esters 4–6 catalyzed by crude onion POD extract.
O
HO
O
OH
OH
O
OH
HO
O
HO
O
O
OCH3
OH
OCH3
CH3O
CH3O
OCH3
OH
OCH3
OH
OH
O
OH
III
I
II
R
O
O
O
R
O
OH
O
R'
R'
IV R = H, CH3; R' = H, OH, OCH3
Figure 1. Chemical structures of common natural phenolic acid dihydrodimers.
products, as well as of their corresponding free acids and esters,
was evaluated using the ferric reducing antioxidant power (FRAP)
assay.
anol and the appropriate co-solvent, and the mixture was stirred at
room temperature for 30 minutes. The reaction was quenched by
adding trichloroacetic acid and, after centrifugation, the aqueous
supernatant was extracted with ethyl acetate.23
The starting esters 4–6 were synthesized via Fischer esterifica-
tion of p-coumaric, caffeic, and ferulic acids 1–3, respectively.20
The onion solid waste used as the enzyme source in this study
was obtained from a local catering facility (Chania, Crete, Greece)
after processing of brown-skin onion bulbs (Allium cepa). This is
the most widely cultivated horticultural crop in Europe, and the
common onion variety that can be found in typical supermarkets
in almost every region of Greece. The waste consisted of the apical
trimmings of the bulbs, as well as the outer dry and semi-dry
layers. The cell-free, POD-active extract was prepared as previously
described.19,21 In an effort to improve the enzyme activity we
concentrated the POD-active extract by treatment with ammo-
nium sulfate, stirring for one hour in an ice bath, and then centri-
fuging at 10,000 rpm at 4 °C for 15 min. The precipitate was
dispersed in phosphate buffer (pH 4) and used as the crude enzyme
source.22
The reaction conditions using methyl p-coumarate (4), methyl
caffeate (5) and methyl ferulate (6) as substrates for the crude en-
zyme preparation were optimized in terms of pH and solvents. We
performed the reactions at pH 4, 5 and 6, and optimum yields were
observed at pH 5 for methyl p-coumarate (4), at pH 4 for methyl
caffeate (5) and at pH 4 for methyl ferulate (6). In a typical exper-
iment (Scheme 1), to the crude POD extract were added H2O2, as a
solution in phosphate buffer, a solution of ester 4, 5 or 6 in meth-
The reaction progress was monitored by RP-18 LC-DAD-MS at
278 and 340 nm.24 In the case of methyl p-coumarate (4), a new
compound appeared at kmax 314 nm ([M+H]+ at m/z 355), whereas
in the case of methyl caffeate (5), a new compound appeared at
kmax 318 nm ([M+H]+ at m/z 387). For methyl ferulate (6), a new
compound appeared displaying a kmax at 328 nm ([M+H]+ at m/z
415). An LC-MS study indicated highly regiospecific dimerisation
resulting from oxidative coupling of the starting esters. The crude
products were purified using preparative TLC and HPLC to afford
compounds 7–9.23 Dihydrodimer 8 was unstable under these con-
ditions, and therefore was not isolated in pure form.
Based on 1D and 2D NMR data, the structures of the reaction
products were assigned unambiguously as the dihydrobenzofuran
lignans 7–9.25–27 Even though two different stereoisomers were
possible for each reaction product, analysis of their spectroscopic
characteristics suggested a single isomer, which was determined
as having a 70,80-trans configuration, in accordance with data re-
ported in the literature.11 Pure compounds 7 and 9 did not show
optical rotation. The ESI-MS spectra of pure compounds 7 and 9 in-
cluded, apart from the molecular ion, several diagnostic fragments.
To the best of our knowledge, this is the first report describing
the radical dihydrodimerisation of hydroxycinnamates involving
crude onion POD as the biocatalyst. The procedure is straightfor-

S. Moussouni et al. / Tetrahedron Letters 52 (2011) 1165–1168
1167
OCH3
O
H3CO
O
R
H3CO
O
R
OCH3
9
5
O
8
2
8
7
R
1
O
6
5
O
3
4OH
R
O
OCH3
O
CH3
O
O
O
CH3
H
O
H
OH
H3CO
4
R
O
O
R
R
7
O
R
O
Scheme 2. Proposed mechanism for the oxidative radical dihydrodimerisation of esters 4–6 catalyzed by crude onion POD extract.
ward and the compounds are obtained in good yields (70% and 90%
for compounds 7 and 9, respectively). The mechanism of the reac-
tion, as shown in Scheme 2, involves initial formation of a new C–C
bond arising from reaction between the C-8-centered semiquinone
radical and the C-5-centered quinone methide radical. The product
of this reaction is then cyclized regioselectively via nucleophilic at-
tack of the carbonyl oxygen C-4 at C-7 to produce the 2,3-dihydro-
benzofuran derivative.
coumaric and ferulic acids results in a decrease in the ferric reduc-
ing power activity. The most active compound of the series tested
is dihydrobenzofuran 9, derived from the oxidative dimerization–
cyclization of methyl ferulate. This compound has also been re-
ported as a potent inhibitor of cell-mediated LDL oxidation.12
In conclusion, we have demonstrated the potential of using crude
onion POD extract as a biocatalyst for the efficient oxidative dimer-
ization–cyclization of methyl p-coumarate, methyl caffeate, and
methyl ferulate leading to dihydrobenzofuran lignan-type com-
pounds. The antioxidant activity of compounds 7 and 9 indicates
that they are promising molecular scaffolds for the design and syn-
thesis of more potent antioxidants derived from phenolic acids.
The antioxidant activity of compounds 7 and 9, and the corre-
sponding starting phenolic acids and esters, was evaluated by the
ferric reducing antioxidant power (FRAP) assay, which uses antiox-
idants as reductants in
a redox-linked colorimetric method,
employing an easily reduced oxidant system present in stoichiom-
etric excess. At low pH, reduction of ferric tripyridyl triazine (Fe III
TPTZ) complex to the ferrous form (which has an intense blue col-
or) can be monitored by measuring the change in absorption at
593 nm.28,29 The reaction is non-specific, in that any half reaction
that has a lower redox potential than that of the ferric-ferrous half
reaction, will lead to ferrous ion formation. The change in absor-
bance is therefore, directly related to the combined or ‘total’ reduc-
ing power of the electron-donating antioxidants present in the
reaction mixture. The results of the antioxidant activity evaluation
(Table 1) show that the dihydrobenzofuran lignans 7 and 9 exhibit
significantly higher activity in this assay than the corresponding
phenolic acids, 1 and 3, or the esters 4 and 6. Esterification of p-
Acknowledgment
E.I. is grateful to PNG Gerolymatos for a Postdoctoral fellowship.
References and notes
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Med. Chem. 1999, 42, 5475–5481.
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Belval, F.; Rasoanaivo, P.; Negre-Salvayre, A.; Gornitzka, H. Bioorg. Med. Chem.
2007, 15, 6018–6026.
Table 1
Antioxidant activity of compounds 1, 3, 4, 6, 7 and 9 as evaluated using the FRAP
assay.
Compound
FRAP value (lM)
1
3
4
6
7
9
1.136
0.986
0.271
0.315
2.186
4.357

1168
S. Moussouni et al. / Tetrahedron Letters 52 (2011) 1165–1168
13. Chioccara, F.; Poli, S.; Rindone, B.; Pilati, T.; Brunow, G.; Pietikainen, P.; Setala,
H. Acta Chem. Scand. 1993, 47, 610–616.
14. Ralph, J.; Garcia Conesa, M. T.; Williamson, G. J. Agric. Food Chem. 1998, 46,
2531–2532.
24. ESI+ conditions: probe voltage 4 kV; probe temperature 350 °C; collision
induced fragmentation at 12 and 60 eV. The LC–MS carrier solvent was a
gradient of 2.5% AcOH in H2O and MeOH.
25. Methyl
(E)-2-(4-hydroxyphenyl)-5-(3-methoxy-3-oxoprop-1-enyl)-2,3-
15. Larsen, E.; Andreasen, M. F.; Christensen, L. P. J. Agric. Food Chem. 2001, 49,
3471–3475.
16. Wakimoto, T.; Nitta, M.; Kasahara, K.; Chiba, T.; Yiping, Y.; Tsuji, K.; Kan, T.;
Nukaya, H.; Ishiguro, M.; Koike, M.; Yokoo, Y.; Suwa, Y. Bioorg. Med. Chem. Lett.
2009, 19, 5905–5908.
17. Torres y Torres, J. L.; Rosazza, J. P. N. J. Nat. Prod. 2001, 64, 1408–1414.
18. Wasserman, H. H.; Brunner, R. K.; Buynak, J. D.; Carter, C. G.; Oku, T.; Robinson,
R. P. J. Am. Chem. Soc. 1985, 107, 519–521.
19. Moussouni, S.; Detsi, A.; Majdalani, M.; Makris, D. P.; Kefalas, P. Tetrahedron
Lett. 2010, 51, 4076–4078.
dihydrobenzofuran-3-carboxylate (7): Yield: 70%; UV (CH3OH) kmax 314 nm;
1H NMR (CDCl3, 400 MHz) d 7.65 (1H, d, J = 16.0 Hz, H-7), 7.53 (1H, br s, H-6),
7.41 (1H, dd, J = 8.4, 1.7 Hz, H-2), 7.25 (2H, dd, J = 8.9, 2.2 Hz, H-20 and H-60),
6.87 (1H, d, J = 8.4 Hz, H-3), 6.82 (2H, dd, J = 8.9, 2.2 Hz, H-30 and H-50), 6.30
(1H, d, J = 16.0 Hz, H-8), 6.08 (1H, d, J = 7.5 Hz, H-70), 4.25 (1H, d, J = 7.5 Hz, H-
80), 3.82 (3H, s, H-100), 3.78 (3H, s, H-10); 13C NMR (CDCl3, 100 MHz) d 170.8
(C-90), 167.8 (C-9), 161.2 (C-4), 156.0 (C-40), 144.6 (C-7), 132.2 (C-10), 130.8 (C-
2), 127.9 (C-1), 127.5 (C-20 and C-60), 125.1 (C-5), 125.0 (C-6), 115.7 (C-30 and
C-50), 115.3 (C-8), 110.3 (C-3), 86.4 (C-70), 55.1 (C-80), 52.9 (C-100), 51.7 (C-10);
ESI-MS m/z 355 [M+H]+, 323, 291, 263, 235, 207.
20. Synthesis of methyl p-coumarate (4), methyl caffeate (5), and methyl ferulate (6):
The appropriate phenolic acid (100 mg) was dissolved in MeOH (50 ml) to
which 3 drops of H2SO4 had been added. The solution was refluxed for 2 h,
NaHCO3 (100 mg) was added, and the solvent was evaporated in vacuo at
40 °C. The residue was partitioned between Et2O (15 mL) and H2O (15 mL), the
organic layer separated, dried (Na2SO4), and the solvent was evaporated in
vacuo to afford the desired ester, as a white solid. The esters were used in the
oxidation reaction without further purification.
26. Methyl (E)-2-(3,4-dihydroxyphenyl)-7-hydroxy-5-(3-methoxy-3-oxoprop-1-
enyl)-2,3-dihydrobenzofuran-3-carboxylate (8): UV (CH3OH) kmax 318 nm; 1H
NMR (CDCl3, 400 MHz) d 7.57 (1H, d, J = 15.9 Hz, H-7), 7.08 (1H, br s, H-6), 7.03
(1H, br s, H-2), 6.87 (1H, d, J = 1.4 Hz, H-20), 6.84 (1H, d, J = 8.0 Hz, H-50), 6.80
(1H, dd, J = 8.0, 1.4 Hz, H-60), 6.26 (1H, d, J = 15.9 Hz, H-8), 6.05 (1H, d,
J = 7.5 Hz, H-70), 4.28 (1H, d, J = 7.5 Hz, H-80), 3.81 (3H, s, H-100), 3.78 (3H, s, H-
10); ESI-MS m/z 387 [M+H]+.
27. Methyl
(E)-2-(4-hydroxy-3-methoxyphenyl)-7-methoxy-5-(3-methoxy-3-
21. Khiari, Z.; Makris, D. P.; Kefalas, P. Food Bioprocess Technol. 2009, 2, 337–343.
22. Preparation of the crude onion peroxidase (POD) extract: Parts of the onion bulb
considered as waste (non edible) material, consisting of the apical trimmings,
were used for preparing the crude POD extract. The material was transferred to
the laboratory immediately after processing and ground in a domestic blender.
An aliquot of the ground tissue (10 g) with solid polyvinylpyrrolidone (PVPP,
5 g) were suspended in phosphate buffer (100 ml, pH 4, containing 2 mM
CaCl2). The suspension was centrifuged at 3000 rpm for 10 min and filtered
through filter paper to remove cell debris. The clear supernatant obtained was
subjected to precipitation by the addition of solid NH4SO4, stirred for 1 h in an
ice bath, and then centrifuged at 10,000 rpm at 4 °C for 15 min. The precipitate
was dispersed in phosphate buffer (pH 4) and used as the crude enzyme
source.
oxoprop-1-enyl)-2,3-dihydrobenzofuran-3-carboxylate (9): Yield: 90%; UV
(CH3OH) kmax 328 nm; 1H NMR (CDCl3, 400 MHz) d 7.63 (1H, d, J = 15.9 Hz,
H-7), 7.16 (1H, d, J = 1.1 Hz, H-6), 7.00 (1H, d, J = 1.1 Hz, H-2), 6.89 (2H, m, H-50
and H-60), 6.88 (1H, m, H-20), 6.30 (1H, d, J = 15.9 Hz, H-8), 6.09 (1H, d,
J = 8.2 Hz, H-70), 4.33 (1H, d, J = 8.2 Hz, H-80), 3.89 (3H, s, H-11), 3.86 (3H, s, H-
110), 3.81 (3H, s, H-100), 3.79 (3H, s, H-10); 13C NMR (CDCl3, 100 MHz) d 170.7
(C-90), 167.6 (C-9), 149.9 (C-4), 146.7 (C-30), 146.1 (C-40), 144.8 (C-3), 144.7 (C-
7), 131.4 (C-10), 128.6 (C-1), 125.7 (C-5), 119.4 (C-60), 117.9 (C-6), 115.6 (C-8),
114.5 (C-50), 112.2 (C-2), 108.7 (C-20), 87.5 (C-70), 56.1 (C-11), 56.0 (C-110), 55.5
(C-80), 52.8 (C-100), 51.6 (C-10); ESI-MS m/z 415 [M+H]+, 383, 351, 323, 295,
267.
28. FRAP assay: the acetate buffer solution (a) 300 mM, pH 3.6 was prepared by
weighing 3.1 g NaOAcꢁ3H2O and adding 16 ml of glacial AcOH and the volume
was made up to 1 L with distilled H2O. The second solution (b) contains 10 mM
TPTZ (2,4,6-tripyridyl-s-triazine) in 40 mM HCl, and solution (c) contains
20 mM FeCl3ꢁ6H2O dissolved in distilled H2O. The working FRAP reagent was
prepared by mixing (a), (b) and (c) in the ratio of 10:1:1 at the time of use. The
23. Dimerization reaction of phenolic esters using crude onion POD:
A solution
containing 2 mM of phenolic ester 4, 5 or 6 dissolved in MeOH, DMF or glycerol
(1 mL) was added to a mixture containing crude onion POD extract (10% of the
total volume) and 8ml H2O2 (3 mM in phosphate/citrate buffer, pH
5
containing 2 mM CaCl2) for 30 min at room temperature. The reaction was
stopped by the addition of TCA (0.1 ml, 10% in EtOH) and then subjected to
centrifugation at 5000g for 10 min. The mixture was extracted with EtOAc
(3 ꢀ 20 mL) and the organic layer was dried over Na2SO4. After vacuum
distillation of the solvent, the residue was redissolved in MeOH (1 mL), filtered
sample (100 lL, 1 mM) was mixed with 3 mL of working FRAP reagent and the
absorbance (593 nm) was measured at 0 min after vortexing. Thereafter,
samples were placed in a water bath (37 °C) and the absorption was measured
after 4 min. The FRAP reagent was used as the blank solution, and ascorbic acid
(1 mM) was used as the standard. Blank and ascorbic acid standards were
through 0.45
l
m syringe filters, and the filtrate was used for chromatographic
processed in the same way. FRAP value of sample
absorbance of sample from 0 to 4 min/change in absorbance of standard
from 0 to 4 min) ꢀ FRAP value of the standard (1000 M).
29. Benzie, F. F.; Strain, J. J. Methods Enzymol. 1999, 299, 15–23.
(lM) = (change in
analyses. The crude reaction products were purified using preparative silica gel
TLC (developed with petroleum ether/EtOAc 7:3), followed by preparative
normal phase HPLC using a CECIL 1100 Series liquid chromatography pump
l
equipped with
a GBC LC-1240 refractive index detector and a Supelcosil
SPLC-Si (25 cm ꢀ 10 mm) column with cyclohexane/EtOAc (6:4) as eluent.