1. Azachalcones: A new class of potent polyphenol oxidase inhibitors
Sini Karanayil Radhakrishnan ⇑
, Ronald Gibrial Shimmon, Costa Conn, Anthony T. Baker
School of Chemistry and Forensic Science, University of Technology Sydney, 15 Broadway, Ultimo, NSW 2007, Australia
a r t i c l e i n f o
Article history:
Received 31 January 2015
Revised 19 February 2015
Accepted 23 February 2015
Available online 2 March 2015
Keywords:
PPO
Tyrosinase
Azachalcone
Competitive inhibitor
a b s t r a c t
A library of potent inhibitors of polyphenol oxidase and their structure activity relationships are
described. Azachalcone derivatives were synthesized and tested for their tyrosinase inhibitory activity.
Their inhibitory activities on mushroom tyrosinase using L-DOPA as a substrate were investigated. Two
compounds that are the reduction congeners of the pyridinyl azachalcones strongly inhibited the enzyme
activity and were more potent than the positive control kojic acid.
Ó 2015 Elsevier Ltd. All rights reserved.
Skin color depends on the type and quantity of melanin, a natu-
ral pigment produced by melanocytes. Melanin biosynthesis
involves the enzyme tyrosinase [EC 1.14.18.1], also called polyphe-
nol oxidase (PPO), that catalyzes the first two steps in the melanin-
biosynthesis pathway: the hydroxylation of L-tyrosine to L-DOPA
(L-3,4-dihydroxyphenylalanine) and oxidation of L-DOPA to DOPA
quinone (Fig. 1).1
These quinone compounds are highly reactive
and spontaneously polymerize to form compounds of high molecu-
lar weight, observed as brown pigments. The quinone–protein reac-
tion decreases the digestibility and causes inhibition of proteolytic
and glycolytic enzymes, also reducing the bioavailability of essen-
tial amino acids like cysteine and lysine. Therefore, tyrosinase inhi-
bitors are commonly applied during food processing to slow the
degradation of the foodstuffs and retain full value in the products.2
Melanin plays a crucial role against skin photo carcinogenesis.
However, an excessive accumulation of the pigment could lead to
serious aesthetic issues. Alterations in tyrosinase dysfunction
could culminate with serious pigmentation disorders like mel-
asma, chloasma, lentigo, age spots, inflammatory hypermelanosis
and trauma-induced hyperpigmentation. These maladies can be
initiated by ultraviolet light, chronic inflammation as well as
abnormal levels of a-melanocyte stimulating hormone (a-MSH)
which results in overproduction of melanin.3,4
PPO inhibitors have
also been used as herbicides to control weeds.5
It has also been
suggested that tyrosinase may contribute to the neurodegenera-
tion associated with Parkinson’s disease.6
A number of studies have
been devoted to producing safe and efficient depigmenting agents.
However, many popular depigmenting compounds either lack
potency or produce undesirable side effects. This necessitates the
need to develop new polyphenol oxidase inhibitors that could be
used in the food industry as well as in medicinal and cosmetic
products.
Chalcones (1,3-diaryl-2-propen-1-ones) are flavonoids lacking a
heterocyclic C ring. Chalcones consist of two aromatic rings in trans
configuration, separated by three carbons, an a,b-unsaturated car-
bonyl (Scheme 1).7
The presence in chalcones of a conjugated dou-
ble bond and a completely delocalized p electron system reduces
their redox potentials and makes them prone to electron transfer
reactions. The free radical scavenging potential of chalcones is
attributed to the presence of an enone functionality.8
Chalcones
are readily synthesized by the base catalyzed Claisen–Schmidt
condensation of an aldehyde and an appropriate ketone in a polar
solvent like methanol. The method is versatile and convenient,
though yields may be variable. Azachalcones and their derivatives
have been reported to have a wide variety of biological activities
such as antibacterial, tuberculostatic, and anti-inflammatory
potential.9–12
These activities are largely attributed to the unsatu-
rated ketone moiety.13
The most important factor determining tyrosinase inhibition
efficiency is the position of hydroxyl groups on the aromatic
rings.14
Our hypothesis was that the presence of the 20
-hydroxyl
group attached to ring A of the azachalcone must be of major
importance. Therefore, we synthesized a series of azachalcones
(Scheme 1) and their derivatives and investigated their inhibitory
effects on tyrosinase activity. Assays were performed with
L-DOPA as the substrate, using kojic acid, a well-known strong
tyrosinase inhibitor as the positive control. Synthetic reactions
for the target compounds proceeded smoothly under reflux
http://dx.doi.org/10.1016/j.bmcl.2015.02.060
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +61 426687300.
E-mail address: Sini.KaranayilRadhakrishnan@student.uts.edu.au
(S.K. Radhakrishnan).
Bioorganic & Medicinal Chemistry Letters 25 (2015) 1753–1756
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl

2. conditions in methanol giving compounds 1–11 (Table 1).
Reduction of the carbonyl group in compounds 2 and 3 yielded
azachalcone derivatives, 12 and 13 (Scheme 2). The structures of
the compounds synthesized were confirmed by 1
H NMR, 13
C
NMR, FTIR and HRMS.
Among the azachalcone derivatives synthesized, compounds 12
and 13 exhibited the greatest inhibition of the L-DOPA oxidase
activity of mushroom tyrosinase. These compounds were found
to be more potent than the positive control, kojic acid. Thus the
reduction of the carbonyl group brought about a substantial
increase in potency. Some possible structure–activity relationships
could be inferred from tyrosinase inhibitory assay results: the
nitrogen atom of substituted pyridinyl chalcone derivatives can
possibly get protonated at physiological pH and might act as a
positive center capable of interacting with anionic or partially
anionic groups of amino acid residues existing in the tyrosinase
active site. It is likely for the nitrogen atom in pyridine skeleton
to coordinate with the copper atoms present in the tyrosinase
active site. Compounds 8 and 9, the novel azachalcone derivatives
with a 2-quinolin-2-yl and 3-quinolin-2-yl groups showed poor
tyrosinase inhibition. This could be possibly due to steric hin-
drance caused by the presence of the bulky quinoline group that
hinders the binding of the ligand with the active site of the enzyme
(Tables 2 and 3).
To explore the mechanism of active inhibitors, we conducted a
study of the kinetic behavior of tyrosinase activity in the presence
of inhibitors. In the present study, tyrosinase extracted from the
edible mushroom Agaricus bisporus is used due to its easy availabil-
ity and high homology with the mammalian enzyme that renders
it well suited as a model for studies on melanogenesis.15
We mea-
sured the reaction rates in the presence of active inhibitors 12 and
13 with various concentrations of L-DOPA as a substrate. As the
concentrations of active inhibitors 12 and 13 increased, Km values
gradually increased, but Vmax values did not change, thereby indi-
cating the inhibitors act as competitive inhibitors of mushroom
tyrosinase (Fig. 2). Tyrosinase inhibitors like tropolone and kojic
acid have been found to be competitive inhibitors by chelating
the copper in the active site of the enzyme.16,17
The inhibition
kinetics were illustrated by Dixon plots, which were obtained by
plotting 1/V versus [I] with varying concentrations of substrate.
Dixon plots gave a family of straight lines passing through the
same point at the second quadrant, giving the inhibition constant
(Ki) (Fig. 3). The Ki value estimated from this Dixon plot was 2.62
and 8.10 lM for the compounds 12 and 13, respectively.
Plots of the initial velocity versus enzyme concentration in the
presence of different concentrations of compounds 12 and 13 gave
a family of straight lines, all of which passed through the origin
(Fig. 4).
In summary, we have rigorously investigated a series of
azachalcone derivatives employed for use as PPO inhibitors. Two
of these compounds emerged to be potent inhibitors of mushroom
tyrosinase of which compounds 12 and 13 were found to be more
potent than the positive control, kojic acid. This study thus
explored novel azachalcones that could be targeted as effective
depigmentation agents and also be employed to curb tyrosinase
affected food depreciation. The authors are looking forward to
Table 1
Substitution pattern and tyrosinase inhibition effects of azachalcone derivatives
(1–11)
W
-
O
Z
Y
X
R1
R2
Compound R1 R2 W X Y Z Yield
(%)
Tyrosinase
inhibitiona
(%)
1 H — CH N CH CH 32.20 34.0 ± 1.72
2 OH — CH N CH CH 42.25 56.8 ± 0.32
3 OH — CH CH CH N 83.05 39.7 ± 0.20
4 OH — CH CH N CH 98.25 49.3 ± 1.60
5 H 3-NO2 CH CH CH CH 45.35 18.0 ± 4.02
6 OH 3-NO2 CH CH CH CH 80.20 42.2 ± 1.50
7 OH 4-N,N-Dimethyl
amino
CH CH CH CH 80.10 27.2 ± 1.00
8 OH 2-Quinolin-2-yl CH CH CH CH 86.15 8.9 ± 1.10
9 OH 3-Quinolin-2-yl CH CH CH CH 40.15 12.3 ± 0.60
10 H 2,4-Dimethoxy N CH CH CH 90.25 42.7 ± 0.10
11 H 4-N,N-Dimethyl
amino
N CH CH CH 66.25 39.7 ± 2.50
a
Values indicate means ± SE for three determinations.
O
Y
X
OH
b
OH
Y
X
OH
X = N; Y = CH: Compound 2
X = CH; Y = N: Compound 3
X = N; Y = CH: Compound 12
X = CH; Y = N: Compound 13
Scheme 2. Reduction of azachalcones. Reagents and conditions: (b) CeCl3Á7H2O, NaBH4.
HO
NH2
OH
O
HO
NH2
OH
O
HO
O
NH2
OH
O
O
Tyrosinase Tyrosinase
L- Tyrosine L-DOPA Dopaquinone
Melanin
Figure 1. Pathway for melanin biosynthesis.
R
O
CH3
O
R1H R
O
R1
+
a
Scheme 1. (General method for synthesis of azachalcones 1–11). Reagents and
conditions: (a) MeOH, NaOH, 0 °C, 24 h.
1754 S. K. Radhakrishnan et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1753–1756

3. extend this research with further toxicological analysis of the syn-
thesized azachalcones.
Notes
1. General method for the synthesis of azachalcone derivatives
(1–11)
To a stirred solution of the respective ketone (2.0 mmol) and the
corresponding aldehyde (2.0 mmol) in methanol (6 mL) at room
temperature was added freshly pulverized sodium hydroxide
(160 mg, 4.0 mmol). The solution was stirred overnight (monitored
by TLC) and then poured onto crushed ice. Once all the ice had
melted, pH was adjusted to 5 using hydrochloric acid (5 M). The
resulting solution was then extracted with ethyl acetate, washed
with brine and filtered after drying with anhydrous sodium sulfate.
The solution was then concentrated with a rotary evaporator under
vacuum to give the respective azachalcone.
2. Method for synthesis of compounds 12 and 13
A solution of the respective azachalcone; 2 and 3 (1 mmol) in
methanol was treated with 2.5 equiv of cerium(III) chloride hep-
tahydrate at room temperature, followed by the addition of six
equivalents of sodium borohydride in portions at 0 °C with vigor-
ous stirring. The reaction mixture was allowed to warm to room
temperature within 5 h. After monitoring the completion of the
1/V(µM/min)-1
1/S (mM/L)
12
1/V(µM/min)-1
1/S (mM/L)
13
Figure 2. Lineweaver–Burk plots for inhibition of compounds 12 and 13 against
mushroom tyrosinase for the catalysis of L-DOPA. The inhibitor concentrations were
0, 0.15, 0.30 and 0.60 mM. The final enzyme concentration was 2.9 lg/ml.
Figure 3. Dixon plot for the inhibitory effect of compounds 12 and 13 on L-DOPA
oxidation catalyzed by mushroom tyrosinase. The inhibitor concentrations were 0,
10 and 20 lM. The L-DOPA concentrations were 100, 200 and 300 lM.
Table 2
Substitution pattern and tyrosinase inhibition effects of azachalcone derivatives (12
and 13)
OH
Y
X
OH
X = N; Y = CH: Compound 12
X = CH; Y = N: Compound 13
Compound X Y Yield (%) Tyrosinase inhibitiona
(%)
12 N CH 34.78 80.2 ± 2.12
13 CH N 72.33 57.22 ± 1.05
a
Azachalcone derivatives were synthesized according to the details in Schemes 1
and 2. Values indicate means ± SE for three determinations.
Table 3
Effects on mushroom polyphenol oxidase activity and kinetic analysis of compounds
Compound Type of inhibitionb
IC50
c
(lM) Ki
d
(lM)
12 Competitive 1.70 ± 1.02 2.62
13 Competitive 2.30 ± 0.50 8.10
Kojic acid Competitive 27.30 ± 0.40 8.89
b
Lineweaver–Burk plot of mushroom tyrosinase: data are presented as mean
values of 1/V, which is the inverse of the increase in absorbance at wavelength
492 nm/min (A492/min), for three independent tests with different concentrations
of L-DOPA as the substrate.
c
(IC50): refers to the concentration of compound that caused 50% inhibition.
d
Values were measured at 5 lM of active compounds and Ki is the inhibitor
constant.
S. K. Radhakrishnan et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1753–1756 1755

4. reaction with TLC, the reaction was quenched with ammonium
chloride to obtain the respective compound.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.02.
060.
References and notes
1. Seo, S. Y.; Sharma, V. K.; Sharma, N. J. Agric. Food Chem. 2003, 51, 2837.
2. Jimenez-Atienzar, M.; Escribano, J.; Cabanes, J.; Gandıa-Herrero, F.; Garcıa-
Carmona, F. Plant Physiol. Biochem. 2005, 43, 866.
3. Fu, B.; Li, H.; Wang, X.; Lee, F. S. C.; Cui, S. J. Agric. Food Chem. 2005, 53, 7408.
4. Mcevily, J. A.; Iyengar, R.; Otwell, Q. S. Crit. Rev. Food Sci. Nutr. 1992, 32, 253.
5. Hao, G. F.; Zuo, Y.; Yang, S. G.; Yang, G. F. Chimia (Aarau) 2011, 65, 961.
6. Xu, Y.; Stokes, A. H.; Freeman, W. M.; Kumer, S. C.; Vogt, B. A.; Vrana, K. E. Mol.
Brain Res. 1997, 45, 159.
7. Nowakowska, Z. Eur. J. Med. Chem. 2007, 42, 125.
8. Mukherjee, V. K.; Prasad, A. K.; Raj, A. G.; Brakhe, M. E.; Olsen, C. E.; Jain, S. C.;
Parmer, V. P. Bioorg. Med. Chem. 2001, 9, 337.
9. Nowakowska, Z.; Wyrzykiewicz, E.; Kedzia, B. Farmaco 2001, 56, 325.
10. Nowakowska, Z.; Wyrzykiewicz, E.; Kedzia, B. Farmaco 2002, 57, 657.
11. Yaylı, N.; Kuecuek, M.; Uecuencue, O.; Yasar, A.; Yaylı, N.; Karaoglu, A. J.
Photochem. Photobiol. 2007, 188, 161.
12. Yaylı, N.; Ucuncu, O.; Yasar, A.; Kucuk, M.; Yaylı, N.; Akyuz, E. S.; Karaoglu, A.
Turk. J. Chem. 2006, 30, 505.
13. Rao, Y. K.; Fang, S.-H.; Tzeng, Y.-M. Bioorg. Med. Chem. 2009, 17, 7909.
14. Khatib, S.; Nerya, O.; Musa, R.; Shmuel, M.; Tamir, S.; Vaya, J. Bioorg. Med. Chem.
2005, 13, 433.
15. Khan, M. T. H. Heterocycl. Chem. 2007, 9, 119.
16. Son, S. M.; Moon, K. D.; Lee, C. Y. J. Agric. Food Chem. 2000, 48, 2071.
17. Battaini, G.; Monzani, E.; Casella, L.; Santagostini, L.; Pagliarin, R. J. Biol. Inorg.
Chem. 2000, 5, 262.
Figure 4. Relationship between the catalytic activity of tyrosinase and concentra-
tions of compounds 12 and 13. Concentrations of compounds for lines from top to
bottom were 0, 10, 15, and 20 lM, respectively.
1756 S. K. Radhakrishnan et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1753–1756