Antioxidant and tyrosinase docking studies of heterocyclic sulfide derivatives containing a thymol moiety

Article abstract of DOI:10.1016/j.ica.2020.119495

Fourteen heterocyclic sulfide derivatives (4–17) containing a thymol moiety and oxadiazole, thiadiazole, triazole, oxazole, thiazole, imidazole, pyridine or purine heterocycles were synthesized in three steps. The cupric, Cu(II), ion reducing antioxidant capacity of the compounds was examined, and molecular docking studies were performed to determine whether the sulfur, thymol or heterocyclic moieties interact with the Cu ions in tyrosinase, a type-3 copper enzyme. Using the CUPRAC assay, eight compounds (5–8, 10, 15–17) showed equal or better Cu (II) reducing capacity than trolox at neutral pH, with trolox equivalent antioxidant capacity (TEAC) coefficients ranging between 1.00 and 1.48. The compounds containing a thiadiazole moiety were most effective, with the methyl thiadiazole derivative (8) having the highest Cu(II) reducing capacity. Molecular docking studies of the sulfide derivatives with tyrosinase revealed that there were no direct interactions between the sulfur atom and the active site copper ions. However, the compounds displayed two different binding interactions with the histidine-Cu catalytic center. For compounds 4–13, the thymol portion was embedded in the active site cavity, while for compounds 14–17 the heterocyclic portion of the molecule approached the cavity.

Full text of DOI:10.1016/j.ica.2020.119495

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Research paper
Antioxidant and tyrosinase docking studies of heterocyclic sulfide derivatives
containing a thymol moiety
Mia H. Havasi, Andrew J. Ressler, Eden L. Parks, Alexander H. Cocolas, Ashton
Weaver, Navindra P. Seeram, Geneive E. Henry
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S0020-1693(19)31817-1
https://doi.org/10.1016/j.ica.2020.119495
ICA 119495
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Inorganica Chimica Acta
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Please cite this article as: M.H. Havasi, A.J. Ressler, E.L. Parks, A.H. Cocolas, A. Weaver, N.P. Seeram, G.E. Henry,
Antioxidant and tyrosinase docking studies of heterocyclic sulfide derivatives containing a thymol moiety,
Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119495
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Antioxidant and tyrosinase docking studies of heterocyclic sulfide derivatives
containing a thymol moiety
Mia H. Havasia1, Andrew J. Resslera1, Eden L. Parksa, Alexander H. Cocolasa, Ashton
Weavera, Navindra P. Seeramb, Geneive E. Henrya*
aDepartment of Chemistry, Susquehanna University, 514 University Avenue, Selinsgrove, PA
17870, USA
bBioactive Botanical Research Laboratory, Department of Biomedical and Pharmaceutical
Sciences, College of Pharmacy, University of Rhode Island, Kingston, RI 02881, USA
1These authors contributed equally to the work.
Correspondence to:
Geneive E. Henry
henry@susqu.edu

Abstract
Fourteen heterocyclic sulfide derivatives (4-17) containing a thymol moiety and oxadiazole,
thiadiazole, triazole, oxazole, thiazole, imidazole, pyridine or pyridine heterocycles were synthesized
in three steps. The cupric, Cu(II), ion reducing antioxidant capacity of the compounds was examined,
and molecular docking studies were performed to determine whether the sulfur, thymol or
heterocyclic moieties interact with the Cu ions in tyrosinase, a type 3 copper enzyme. Using the
CUPRAC assay, eight compounds (5-8, 10, 15-17) showed equal or better Cu (II) reducing capacity
than trolox at neutral pH, with trolox equivalent antioxidant capacity (TEAC) coefficients ranging
between 1.00 and 1.48. The compounds containing a thiadiazole moiety were most effective with the
methyl thiadiazole derivative (8), having the highest Cu(II) reducing capacity. Molecular docking
studies of the sulfide derivatives with tyrosinase revealed that there were no direct interactions
between the sulfur atom and the active site copper ions. However, the compounds displayed two
different binding interactions with the histidine-Cu catalytic center. For compounds 4-13, the thymol
portion was embedded in the active site cavity, while for compounds 14-17 the heterocyclic portion
of the molecule approached the cavity.
Keywords: Thymol; heterocyclic sulfides; antioxidant; copper (II) ion reduction; tyrosinase docking
studies

1. Introduction
Endogenous sulfur containing molecules such as glutathione, thioredoxin and glutaredoxin
play an important role in the body. They act as antioxidants and reduce the amount of excess reactive
oxygen species (ROS), which are causative agents in oxidative stress-related diseases such as
inflammation, cancer, cardiovascular and neurodegenerative diseases [1]. Synthetic sulfur containing
compounds, as well as those derived from natural sources, have also displayed a wide range of
biological properties [2-6]. In particular, sulfides have found applications as treatment for cancer,
bacterial infections, inflammation, Alzheimer’s, Parkinson, tuberculosis, and HIV diseases [7].
However, unlike thiols which have been widely studied as antioxidants [8-9], the role of sulfides as
antioxidant agents, particularly reduction of transition metal ions such as copper and iron, is largely
understudied [10].
Melanogenesis is regulated by tyrosinase, a metalloenzyme which is responsible for the
oxidation of tyrosine to dopaquinone, via L-Dopa as an intermediate. Tyrosinase is a type 3 copper
enzyme, in which the active site contains two copper ions associated with histidine residues, which
play a vital role in the oxidative mechanism of the enzyme [11-12]. While melanin serves a protective
role against skin damage by UV light, overproduction of melanin can lead to undesirable
hyperpigmentation disorders, such as solar lentigines, freckles, and melasma. Tyrosinase is also
responsible for the browning in fruit and vegetables. In order to reduce these undesirable effects,
several agents that target tyrosinase function directly and indirectly have been developed. Inhibition
of tyrosinase function occurs by a variety of mechanisms, including copper ion chelation, reversible
inhibition, irreversible inactivation, dopamine scavenging and reduction of dopaquinone [11-12].
Many compounds incorporating N, O and S heterocyclic moieties have been reported to
possess both strong antioxidant [13] and tyrosinase inhibitory [14] activities. These include

oxadiazole, thiadiazole, triazole, imidazole, thiazole, oxazole, pyridine and purine moieties. In recent
years, our lab has been focused on the development of derivatives of carvacrol and thymol, the
primary monoterpenoids in thyme and oregano, and evaluation of their antioxidant and tyrosinase
inhibitory activities. We have reported the tyrosinase inhibitory activities of alkyl oxobutanoate
derivatives of carvacrol and thymol [15] and the cupric ion reducing capacity of N, O, S heterocyclic
sulfide derivatives containing a carvacrol core [16]. As an extension of those studies, we herein report
the cupric ion reducing capacities of N, O, S heterocyclic thioether derivatives containing a thymol
core, and evaluation of their binding affinity for the mushroom tyrosinase enzyme (PDB: 2Y9X). We
examined two factors, 1) Given that sulfur has a high affinity for copper, does the exocyclic sulfur
atom show an interaction with the Cu ions in the tyrosinase active site, and 2) does the structure of
the heterocycle affect the binding mode.
2. Results and discussion
2.1 Chemistry
The mono and bicyclic heterocyclic sulfide derivatives of thymol (4-17) were prepared
using the method previously described for heterocyclic carvacrol derivatives [16], with slight
modifications (Scheme 1). Methyl thymol (2) was obtained in 88% yield by treating thymol (1) with
cesium carbonate and methyl iodide in DMF. The yield and spectroscopic data of 2 were in good
agreement with those reported by Silva, et al. [17]. Friedel-Crafts acylation of the methyl thymol in
the presence of chloroacetyl chloride and aluminum chloride in dichloromethane afforded
compound (3) in 48% yield. The heterocyclic sulfides were obtained by nucleophilic displacement
of the chloro group with a heterocyclic aromatic thiol in the presence of potassium iodide and
potassium iodide in acetonitrile. The products were obtained in 45-96 yield, after purification by

silica gel column chromatography using ethyl acetate-hexane solvent mixtures. The products were
characterized by IR and NMR spectroscopic data, together with HRMS data (See experimental
section).
O
HO
O
O
c
a
b
Ar
S
Cl
O
O
1
2
3
4-17
Oxadiazole/triazole/thiadiazole series: Ar =
Thiazole/oxazole/imidazole/purine series: Ar =
N
N
N
N
N
N
N
N
N
NH
N
G
G
G
S
S
O
N
X
H
N
17
13
9: G = Ph
14: X = S
15: X = O
16: X = NH
4: G = H
5: G = Ph
6: G = NH2
7: G = NH2
8: G = CH3
10: G = 2-pyridyl
11: G = 3-pyridyl
12: G = 4-pyridyl
Reagents and conditions: a) Cs2CO3, CH3I, dry DMF, heat, 88%; b) Chloroacetyl chloride, AlCl3,
dry CH2Cl2, 0°C to rt, 48%; c) Ar-SH, K2CO3, KI, dry CH3CN, rt, 45-96%.
Scheme 1. Synthesis of heterocyclic thioether derivatives of thymol.
2.2 CUPRAC assay of compounds 4-17
Exposure of Cu(II) ions to neocuproine (2,9-dimethyl-1,10-phenathroline) leads to the
formation of a green bis-neocuproine Cu(II) complex. In the presence of an antioxidant, Cu(II) is
reduced to Cu(I) by an outer sphere single electron transfer to form a yellow-orange charge-transfer
neocuproine-Cu(I) complex. The cupric, Cu(II), ion reducing antioxidant capacity (CUPRAC) assay
is used to measure the extent of reduction of Cu(II) to Cu(I) and gives a measure of the strength of
the antioxidant [18]. The charge-transfer complex displays an absorption maximum at 450 nm, thus,
increased absorbance at 450 nm indicates increased antioxidant activity. CUPRAC data are reported
as trolox equivalent antioxidant capacity (TEAC) coefficients, which represents the ratio of the molar
absorptivity (obtained from calibration plots using varying concentrations) of the antioxidant sample
to that of trolox (TEAC = sample/trolox).

As previously reported, when the antioxidant is a sulfide, the sulfur atom is thought to undergo
a one-electron oxidation to a radical cation, which is the subsequently oxidized by molecular oxygen
to the corresponding sulfone (Fig. 1) [16, 19]. We also demonstrated that the sulfur atom was the
primary group involved in Cu(II) reduction of heterocyclic sulfide derivatives based on carvacrol,
because conversion to the corresponding sulfone led to elimination of Cu(II) ion reducing capacity
[16].
2+
+
N
N
N
N
N
N
N
N
Reduction
Cu
Cu
+e
Bis-neocuproine-Cu(II)
+
Bis-neocuproine-Cu(I)
+
-e
O
O2
S
S
R
R
R
R
S
R
R
Oxidation
Fig. 1. Bis-neocuproine-Cu(II) reduction by sulfides
With the foregoing in mind, our goal was to determine the influence of the type of heterocyclic
moiety on the Cu(II) ion reducing capacity of the thymol containing heterocyclic sulfide derivatives
(4-17), using ascorbic acid and the parent thymol as additional standards. The compounds are
divided into two groups: 1) oxadiazole/triazole/thiadiazole series (4-12) and
thiazole/oxazole/imidazole/purine series (13-17). TEAC coefficients are shown in Table 1.

Table 1. Trolox equivalent antioxidant capacity (TEAC) and tyrosinase binding energy for
compounds 4-17.
Cpd Ar
TEAC Tyr BE
(kcal/mol)
Cpd Ar
TEAC TBE
(kcal/mol)
N
N
N
N
N
4
11
0.73
-6.1
0.50
-7.9
N
N
O
H
N
N
N
N
5
6
12
13
1.00
1.14
-6.8
-6.4
0.84
0.91
-7.4
-5.9
N
O
N
H
N
N
NH2
NH2
N
H
S
N
N
N
N
N
7
8
9
14
15
16
1.25
1.48
ND
-6.3
-6.5
-7.8
0.64
1.08
1.20
-6.5
-6.5
-6.7
S
S
N
N
S
O
N
N
O
N
H
N
N
10
N
N
N
1.25
-7.8
17
1.06
0.86
-6.7
-5.7
O
NH
N
KA
AA
NA
0.85
-5.6
ND
THY
Trolox:  = 16,100 M-1 cm-1; r2 = 0.9999
KA: Kojic acid ; AA: Ascorbic acid; THY: Thymol
NA: No activity; ND: Not determined
The triazole derivative (4) showed a lower antioxidant capacity than trolox, with a TEAC coefficient
of 0.73, and was also less effective than ascorbic acid and thymol with TEAC coefficients of 0.85 and
0.86 respectively. Incorporation of an electron donating phenyl group at position 5 on the triazole

ring led to the phenyl triazole derivative (5), which is equipotent to trolox, with a TEAC coefficient
of 1.00. Replacement of the phenyl group with an amino group led to a slight increase in reducing
capacity, with the amino triazole derivative (6) having a TEAC coefficient of 1.14. Isosteric
replacement of the NH in the triazole ring of compound 6 with sulfur (7), led to a further increase in
reducing capacity, with 7 showing slightly enhanced activity over 6. Conversion of the amino
thiadiazole derivative to the corresponding methyl derivative (8) gave a compound with
approximately 1.5 fold increase in activity over trolox.
The TEAC coefficient of phenyl oxadiazole (9) was not determined owing to insolubility in
the assay medium. The pyridyl oxadiazole derivatives, 10-12, exhibited a range of TEAC values
between 0.50 and 1.25, with the 2-pyridyl derivative (10) having the highest reducing capacity and
the 3-pyridyl derivative (11) having the lowest reducing capacity. These data are consistent with those
observed for the corresponding carvacrol derivatives [16]. Furthermore, the 3-pyridyl derivative has
the lowest reducing capacity of all the compounds tested.
The thiazole derivative (13) showed comparable antioxidant activity to trolox, with a TEAC
coefficient of 0.91. However, the incorporation of a fused ring in benzothiazole (14) resulted in
lowered activity (0.64). The benzoxazole (15) and benzimidazole (16) derivatives, showed 1.7 and
1.9 fold increase in activity, respectively, relative to the benzothiazole. The increase in antioxidant
activity among this series trends similarly to the basicity of the heteroatom. Finally, purine derivative
(17) showed slightly less activity than benzimidazole 16, likely due to the electron-withdrawing
pyrimidine ring of 17 being closer to the sulfur atom.

2.3 Molecular docking analysis of compounds 4-17 with mushroom tyrosinase
Given the copper reducing activities of the heterocyclic sulfide derivatives of thymol, and the
high affinity of sulfur for copper ions, it was envisaged that the sulfur atom may interact with the
copper ions in the active site of the tyrosinase enzyme. Thus, the compounds were subjected to in
silico docking studies with mushroom tyrosinase (PDB ID: 2Y9X), using Autodock Vina to determine
their binding affinity for the enzyme. Docking experiments were performed in triplicate, where all
residues on tyrosinase were rigid, and all rotatable bonds on the test compound were made flexible.
The charges on the copper atoms in tyrosinase were set to 2.00, in agreement with recent literature
[20]. The search volume was defined as a 15 Å × 15 Å × 15 Å cube containing the two-Cu atom
active site in the center. All other parameters were left at their default values.
Docking experiments using kojic acid and thymol, known tyrosinase inhibitors, were
performed for comparison with the synthesized derivatives. Kojic acid has been shown to inhibit
tyrosinase by multiple mechanisms, including copper-chelation and by embedding itself into the
tyrosinase active site [21]. As demonstrated in Fig. 2, the hydroxymethyl group of kojic acid is
oriented close to the active site copper atoms, while the phenol is associated with a histidine residue.
The observed docking store of -5.6 kcal/mol for kojic acid is in good agreement with the reported
docking affinity of -5.5 kcal/mol [22]. Molecular docking of thymol revealed that thymol was
embedded into the active site in a similar conformation as kojic acid, with their phenol groups oriented
in a similar fashion. However, thymol sits further outside the active site than kojic acid, likely due to
the bulkier isopropyl group (Fig. 2). The docking score of thymol (-5.7 kcal/ mol) is similar to that of
kojic acid. A study by da Silva, using the MolDock program, also showed that kojic acid and thymol
had similar binding affinity to the tyrosinase enzyme, although the docking scores were different than

those observed using the AutoDock Vina program [23]. Interestingly, unlike thymol, kojic acid
showed no activity in the CUPRAC assay.
Fig. 2. Docking conformations of kojic acid (green) and thymol (orange).
The heterocyclic sulfide derivatives adopted one of two general conformations within the
tyrosinase active site: 4-13 assumed conformations in which the thymol moiety was oriented toward
the copper-histidine catalytic center, whereas for derivatives containing fused rings, 14-17, the
heterocyclic moiety was oriented toward the catalytic center. Triazoles 4-6 and thiazole 13 interact
with tyrosinase similarly, as shown in Fig. 3. Each compound is stabilized by nonpolar interactions
between the hydrophobic thymol region and residues Ala286, Phe264, and Val283. Thiazole 13 had
the weakest docking score of all derivatives (-5.9 kcal/mol), due the absence of hydrogen bonding
interactions with the site residues. Triazole 4 showed increased binding affinity due to hydrogen
bonding between the triazole and the backbone of Val 283. Addition of the phenyl substituent in 5

lead to a significant increase in docking affinity (-6.8 kcal/mol), due to hydrogen bonding with
Asn260 and numerous hydrophobic interactions. Substitution of the phenyl group for an amino as
in 6 also led to a slight increase in docking affinity relative to triazole 4 (6.4 kcal/mol), due to
hydrogen bonding to His85 via the amino group.
Fig. 3. A) Docking conformations of triazole 4 (green) and thiazole 13 (orange). B) Conformations
of triazole derivatives 5 (green) and 6 (orange). Copper atoms are shown in purple.
Thiadiazole derivatives 7 and 8 displayed similar docking affinity to the amino triazole (6).
Both compounds are stabilized by hydrogen bonding between the thiadiazole N atoms and His85
(Fig. 4). Compound 7 is further stabilized by hydrogen bonding between the amino group and
His85. Both structures also interact with Val283 and Asn260, as well as show pi-sigma interactions
with Phe264 and His263. Surprisingly, methyl derivative 8 has a slightly more favorable binding

energy (-6.5 kcal/mol) than amino derivative 7 (-6.3 kcal/mole), even though the amino group of 7
is involved in hydrogen bonding.
Fig. 4. A) Docking conformations of thiadiazoles 7 (green) and 8 (orange). B) Ligand-protein
interaction map of thiadiazole 7.
The oxadiazole derivatives containing an additional aromatic ring (9-12) showed binding
affinities markedly stronger than any other class of compounds (Table 1, Fig. 5). Among these
compounds, 3-pyridyl oxadiazole 11 showed the highest affinity (-7.9 kcal/mol), while 4-pyridyl
oxadiazole 12 showed the lowest affinity (-7.4 kcal/mol). The increased stability of 11, and to a
lesser degree, the 2-pyridyl derivative 10 (-7.8 kcal/mol), is attributable to the capability of the
pyridyl group to hydrogen bond to Asn81 and His85. Phenyl oxadiazole 9 also showed significant
binding affinity, likely due to pi-type interactions with His 85.

Fig. 5. A) Docking conformations of oxadiazoles 9 (green), 10 (light green), 11 (light blue-green),
and 12 (light blue). B) Ligand-protein interaction map for nicotinic oxadiazole 11.
The fused heterocyclic derivatives 14-17 interacted with the tyrosinase active site by
embedding the heterocyclic portion into the binding pocket, in contrast to all other derivatives (Fig.
6). The four derivatives can be categorized by docking affinity: Benzoxazole and benzothiazole 14
and 15 both showed a binding affinity of -6.5 kcal/mol, stabilized by pi-pi stacking with His263 and
pi-sigma interactions with Val283. Benzimidazole 16 and purine 17 showed more favorable
docking scores (-6.7 kcal/mol), due to the presence of the N-H group, which serves as a hydrogen
bond-donor. Both 16 and 17 interact with Val283 and His263; however, benzimidazole 16 is further
stabilized by hydrogen bonding with Asn260, while purine 17 is stabilized by hydrogen bonding
with Met280.

Fig. 6. A) Docking conformations of fused rings 14 (green), 15 (blue-green), and 16 (light blue)
(16. B) Docking conformations of 14-16, with purine 17 (orange).
Based on the favorable binding affinity data for compounds 4-17, in vitro tyrosinase
inhibitory experiments were explored. However, even the most polar compounds gave poor
solubility in the assay medium, and thus those studies were not completed.
3. Conclusions
In summary, fourteen new heterocyclic sulfide derivatives of thymol (4-17) have been
synthesized in three steps and evaluated for their ability to reduce Cu(II) to Cu(I) using the CUPRAC
assay. Eight compounds (5-8, 10, 15-17) showed greater or equal reducing capacity relative to trolox.
For the tyrosinase docking studies, all the derivatives showed lower binding energy than kojic acid
and thymol, implying greater affinity toward the binding site. The oxadiazole derivatives (9-12),

containing an appended aromatic ring, have the largest binding affinities for the active site. This study
reveals that the structure of the heterocyclic moiety has an influence on the Cu(II) reducing capacity
of the heterocyclic sulfides, and that they are effective Cu(II) antioxidants. However, there is no
conclusive correlation of Cu(II) reducing capacity and tyrosinase binding affinity. Furthermore, the
sulfur atom does not appear to play a significant role in the binding affinity of the derivatives to the
tyrosinase enzyme.

4. Experimental section
4.1 Materials and instrumentation
The chemicals and solvents were obtained from TCI Chemicals, Sigma-Aldrich, Fisher Scientific.
Dichloromethane and acetonitrile were dried according to standard procedures. Column
chromatography was performed using Teledyne Isco Rf-Gold prepacked silica gel columns (20-40
m). Melting point data were acquired on a Thomas-Hoover capillary melting point apparatus.
Ultraviolet-Visible and Infrared spectra were recorded using a Varian Cary 4000 spectrometer and a
Thermo Scientific Nicolet iS50 FT-IR spectrometer, respectively. NMR data were recorded using a
JEOL ECZ 400S spectrometer (1H NMR, 400 MHz; 13C NMR, 100 MHz), using CDCl3 or DMSO-
d6 as solvents and TMS as internal standard. HR-ESIMS data were acquired on a Waters SYNAPT
G2-S QTOFMS system.
4.2 Synthesis
4.2.1 Synthesis of methyl thymol, 1-isopropyl-2-methoxy-4-methylbenzene (2)
A mixture of thymol 1, (8.0 g, 53 mmol), iodomethane (5.0 mL, 80 mmol), and cesium
carbonate (26.0 g, 80 mmol) in anhydrous DMF (30 mL) was heated for 17 h. After cooling, the
mixture was poured into water (150 mL) and extracted with EtOAc (3 × 50 mL). The EtOAc solution
was washed with brine (60 mL), dried with Na2SO4, filtered and concentrated in vacuo. Column
chromatography of the residue afforded the methyl ether (2) as a colorless oil (7.69 g, 88%). The
NMR data for compound 2 was in good agreement with literature data [17].
4.2.2 Synthesis of 2-chloro-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (3)

Chloroacetyl chloride (4.1 mL, 47 mmol) was added slowly via syringe to an ice-cooled
mixture of methyl thymol, 2, (6.05 g, 37 mmol) and AlCl3 (6.44 g, 48 mmol) in anhydrous CH2Cl2
(180 mL) under nitrogen, with stirring. The mixture was allowed to warm to room temperature and
stirring was continued for 72 hours before pouring into crushed ice (~400 mL). After separation of
the layers, the CH2Cl2 solution was washed with10% aqueous NaHCO3 (3 × 100 mL), followed by
saturated NaCl (100 mL). The solution was dried over anhydrous Na2SO4, filtered and concentrated
in vacuo. The residue was purified by silica gel column chromatography, eluting with 5% ethyl
acetate-hexanes to afford compound 3 (4.25 g).
Off white solid (48%); M.p. 50-53 °C; IR (ATR), cm-1: 1691 (C=O).
1H NMR (CDCl3):  7.53 (1H, s, CH=C-C=O), 6.70 (1H, s, CH=C-O), 4.65
O
(2H, s, CH2Cl), 3.87 (3H, s, CH3O), 3.26 (1H, septet, J = 6.8 Hz, CH[CH3]2),
Cl
2.56 (3H, s, CH3Ar), 1.20 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):
O
 192.3 (C=O), 160.0 (C-O), 140.9 (C-CH3), 134.3 (C-CH[CH3]2), 128.0 (CH=C-C=O), 126.2 (C-
C=O), 114.1 (CH=C-O), 55.5 (CH3O), 47.9 (CH2Cl), 26.6 (CH[CH3]2), 22.6 ([CH3]2CH), 22.4
(CH3Ar).
HRMS (ESI): m/z 241.0984 [M+H] +; calcd. for C13H18O2Cl, 241.0989
4.2.3 General synthesis of -keto sulfide derivatives of methyl thymol (4-17)
KI (1.2 equiv) was added to a stirred solution of compound 3 (150 mg, 0.62 mmol) in dry
acetonitrile (20 mL) at room temperature. After 5 minutes, the aromatic thiol (1.05 equiv) was added
to the resulting cloudy mixture, followed by K2CO3 (1.5 equiv). The mixture was stirred at room
temperature for 4 to 24 hours, followed by removal of the acetonitrile in vacuo. Column
chromatography of the residue using ethyl acetate mixtures (100% hexanes to 60% ethyl acetate-

hexanes; 12g column; flow rate 4 mL/min) gave the target compounds in 45-96% yields, based on
compound 3. For benzimidazole, amino thiadiazole, and amino triazole, only one equivalent of
K2CO3 was used and the solvent included 20% of water. The mercapto-oxadiazole nucleophiles used
in the synthesis of compounds 10-12 were obtained by treatment of the corresponding pyridine
hydrazides with carbon disulfide under basic conditions, as previously reported [24].
2-((1H-1,2,4-triazol-3-yl)thio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (4)
White solid (88%); M.p. 147-148 °C; IR (ATR), cm-1: 1670 (C=O).
1H NMR (CDCl3):   (1H, s, CH=N), 7.64 (1H, s, CH=C-C=O), 6.67
O
NH
N
(1H, s, CH=C-O), 4.57 (2H, s, CH2S), 3.86 (3H, s, CH3O), 3.24 (1H,
septet, J = 6.8 Hz, CH[CH3]2), 2.51 (3H, s, CH3Ar), 1.19 (6H, d, J = 6.8
N
S
O
Hz, ([CH3]2CH); 13C NMR (CDCl3):  195.8 (C=O), 160.1 (C-O), 140.9 (C-CH3), 134.4 (C-
CH[CH3]2), 128.5 (CH=C-C=O), 127.2 (C-C=O), 114.1 (CH=C-O), 55.6 (CH3O), 41.4 (CH2S), 26.8
(CH[CH3]2), 22.5 (CH3Ar), 22.5 ([CH3]2CH). Triazole carbons did not show on the spectrum.
HRMS (ESI): m/z 306.1267 [M+ NH] +; calcd. for C15H20N3NO2S, 306.1276.
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-phenyl-4H-1,2,4-triazol-3-yl)thio)ethan-1-one (5)
White solid (45 %); M.p. 212-214 °C; IR (ATR), cm-1: 1641 (C=O).
1H NMR (DMSO-d6):  7.89 (2H, d, J = 8.0 Hz, CH=CH,
O
N
N
ortho),7.72 (1H, s, CH=C-C=O), 7.44-7.45 (3H, m, CH=CH-
CH),6.86 (1H, s, CH=C-O), 4.71 (2H, s, CH2S), 3.82 (3H, s,
N
S
H
O
CH3O), 3.17 (1H, septet, J = 7.2 Hz, CH[CH3]2), 2.37 (3H, s, CH3Ar), 1.14 (6H, d, J = 7.2 Hz,

([CH3]2CH); 13C NMR (DMSO-d6):  196.2 (C=O), 159.3 (C-O), 139.0 (C-CH3), 133.6 (C-
CH[CH3]2), 130.4 (C-C=N), 129.5 (CH=CH-CH), 128.8 (C-C=O), 128.3 (CH=C-C=O), 126.4
(CH=CH-CH), 114.5 (CH=C-O), 56.2 (CH3O), 41.6 (CH2S), 26.6 (CH[CH3]2), 22.9 ([CH3]2CH),
21.7 (CH3Ar). ). The triazole carbons did not show on the spectrum.
HRMS (ESI): m/z 382.1593 [M+H] +; calcd. for C21H24N3O2S, 382.1583
2-((5-amino-4H-1,2,4-triazol-3-yl)thio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (6)
White solid (94%); M.p. 166-169 °C; IR (ATR), cm-1: 1663 (C=O).
1H NMR (DMSO-d6):   1H, s, NH), 7.66 (1H, s, CH=C-
O
N
N
C=O), 6.84 (1H, s, CH=C-O), 6.01 (2H, s, NH2), 4.46 (2H, s,
CH2S), 3.81 (3H, s, CH3O), 3.16 (1H, septet, J = 7.2 Hz,
NH2
N
S
H
O
CH[CH3]2), 2.37 (3H, s, CH3Ar), 1.14 (6H, d, J = 7.2 Hz, ([CH3]2CH); 13C NMR (DMSO-d6):  196.6
(C=O), 159.2 (C-O), 157.9 (C-NH2), 155.9 (C-S), 139.0 (C-CH3), 133.5 (C-CH[CH3]2), 128.7
(CH=C-C=O), 128.4 (C-C=O), 114.5 (CH=C-O), 56.1 (CH3O), 40.9 (CH2S), 26.6 (CH[CH3]2), 22.9
([CH3]2CH), 21.8 (CH3Ar).
HRMS (ESI): m/z 321.1392 [M+H] +; calcd. for C15H21N4O2S, 321.1385
2-((5-amino-1,3,4-thiadiazol-2-yl)thio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (7)
Off-white solid (91%); M.p. 118-119 °C; IR (ATR), cm-1: 1662 (C=O).
1H NMR (DMSO-d6):  7.68 (1H, s, CH=C-C=O), 7.25 (2H, s, NH2),
O
N
N
NH2
6.86 (1H, s, CH=C-O), 4.66 (2H, s, CH2S), 3.82 (3H, s, CH3O), 3.17 (1H,
S
S
O
septet, J = 6.8 Hz, CH[CH3]2), 2.39 (3H, s, CH3Ar), 1.14 (6H, d, J = 6.8

Hz, ([CH3]2CH); 13C NMR (DMSO-d6):  195.6 (C=O), 170.3 (C-NH2), 159.5 (C-O), 150.0 (C-S),
139.5 (C-CH3), 133.7 (C-CH[CH3]2), 128.7 (CH=C-C=O), 128.2 (C-C=O), 114.5 (CH=C-O), 56.2
(CH3O), 44.1 (CH2S), 26.6 (CH[CH3]2), 22.9 ([CH3]2CH), 22.0 (CH3Ar).
HRMS (ESI): m/z 338.0986 [M+H] +; calcd. for C15H20N3O2S2, 338.0997
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-methyl-1,3,4-thiadiazol-2-yl)thio)ethan-1-one (8)
White solid (87%); M.p. 102-105 °C; IR (ATR), cm-1: 1662 (C=O).
1H NMR (CDCl3):  7.69 (1H, s, CH=C-C=O), 6.69 (1H, s, CH=C-
O
N
S
N
O), 4.91 (2H, s, CH2S), 3.87 (3H, s, CH3O), 3.26 (1H, septet, J =
6.8 Hz, CH[CH3]2), 2.71 (3H, s, CH3C=N), 2.55 (3H, s, CH3Ar),
S
O
1.22 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):  193.7 (C=O), 165.3 (CH3C=N), 164.8 (C-
S), 160.1 (C-O), 140.7 (C-CH3), 134.3 (C-CH[CH3]2), 128.4 (CH=C-C=O), 127.3 (C-C=O), 114.9
(CH=C-O), 55.5 (CH3O), 44.0 (CH2S), 26.8 (CH[CH3]2), 22.6 ([CH3]2CH), 22.5 (CH3Ar), 15.7
(CH3C=N).
HRMS (ESI): m/z 337.1051 [M+H] +; calcd. for C16H21N2O2S2, 337.1038.
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-phenyl-1,3,4-oxadiazol-2-yl)thio)ethan-1-one (9)
White solid (95%); M.p. 156-159 °C; IR (ATR), cm-1: 1665 (C=O).
1H NMR (CDCl3):  8.01 (2H, d, J = 6.4 Hz, CH=CH, ortho), 7.71
O
N
N
(1H, s, CH=C-C=O), 7.49-7.52 (3H, m, CH=CH-CH), 6.71 (1H, s,
CH=C-O), 4.95 (2H, s, CH2S), 3.89 (3H, s, CH3O), 3.28 (1H,
O
S
O
septet, J = 7.2 Hz, CH[CH3]2), 2.59 (3H, s, CH3Ar), 1.23 (6H, d, J = 7.2 Hz, ([CH3]2CH); 13C NMR

(CDCl3):  192.9 (C=O), 165.9 (N=C-C), 164.3 (C-S), 160.3 (C-O), 141.0 (C-CH3), 134.6 (C-
CH[CH3]2), 131.8 (CH=CH-CH, para), 129.1 (CH=CH, meta), 128.5 (CH=C-C=O), 126.8 (C-C=O),
126.8 (CH=CH, ortho), 123.7 (C-C=N), 114.2 (CH=C-O), 55.6 (CH3O), 43.9 (CH2S), 26.9
(CH[CH3]2), 22.7 (CH3Ar), 22.6 ([CH3]2CH).
HRMS (ESI): m/z 383.1418 [M+H] +; calcd. for C21H23N2O3S, 383.1423
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl)thio)ethan-1-
one (10)
White solid (84%); M.p. 159-160 °C; IR (ATR), cm-1: 1668 (C=O).
1H NMR (CDCl3):  8.74 (1H, d, J = 4.8 Hz, CH=N), 8.16 (1H, d,
O
N
N
N
J = 8.0 Hz, CH=C-N), 7.85 (1H, t, J = 8.0 Hz, CH=CH-CH=N),
7.69 (1H, s, CH=C-C=O), 7.44 (1H, dd, J = 8.0, 4.8 Hz, CH=CH-
O
S
O
CH=N), 6.69 (1H, s, CH=C-O), 4.97 (2H, s, CH2S), 3.86 (3H, s, CH3O), 3.26 (1H, septet, J = 6.8 Hz,
CH[CH3]2), 2.56 (3H, s, CH3Ar), 1.22 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):  192.8
(C=O), 165.9 (N=C-C), 165.1 (C-S), 160.3 (C-O), 150.4 (CH=N), 143.3 (C-C=N), 141.0 (C-CH3),
137.3 (CH=CH-CH=N), 134.6 (C-CH[CH3]2), 128.6 (CH=C-C=O), 126.7 (C-C=O), 125.9 (CH=C-
N), 122.9 (CH=CH-CH=N), 114.2 (CH=C-O), 55.6 (CH3O), 44.0 (CH2S), 26.9 (CH[CH3]2), 22.7
(CH3Ar), 22.6 ([CH3]2CH).
HRMS (ESI): m/z 384.1366 [M+H] +; calcd. for C20H22N3O3S, 384.1376.
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-(pyridin-3-yl)-1,3,4-oxadiazol-2-yl)thio)ethan-1-
one (11)

White solid (46%); M.p. 169-172 °C; IR (ATR), cm-1: 1657 (C=O).
1H NMR (CDCl3):  9.22 (1H, s, N=CH-C), 8.75 (1H, d, J = 4.8
O
N
N
N
Hz, CH=CH-N), 8.28 (1H, d, J = 8.0 Hz, C=CH-CH), 7.69 (1H, s,
CH=C-C=O), 7.44 (1H, dd, J = 8.0, 4.8 Hz, CH=CH-N), 6.70 (1H,
O
S
O
s, CH=C-O), 4.96 (2H, s, CH2S), 3.87 (3H, s, CH3O), 3.27 (1H, septet, J = 6.8 Hz, CH[CH3]2), 2.57
(3H, s, CH3Ar), 1.23 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):  192.6 (C=O), 165.3 (N=C-
C), 163.8 (C-S), 160.4 (C-O), 152.4 (C-CH=N), 147.7 (CH=CH-N), 141.1 (C-CH3), 134.6 (C-
CH[CH3]2), 134.0 (C=CH-CH), 128.5 (CH=C-C=O), 126.7 (C-C=O), 123.9 (CH-CH=CH), 120.2 (C-
CH=N), 114.2 (CH=C-O), 55.6 (CH3O), 43.9 (CH2S), 26.9 (CH[CH3]2), 22.7 (CH3Ar), 22.6
([CH3]2CH).
HRMS (ESI): m/z 384.1373 [M+H] +; calcd. for C20H22N3O3S, 384.1376.
1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-((5-(pyridin-4-yl)-1,3,4-oxadiazol-2-yl)thio)ethan-1-
one (12)
White solid (51%); M.p. 136-118 °C; IR (ATR), cm-1: 1661 (C=O).
1H NMR (CDCl3):  8.79 (2H, d, J = 4.8 Hz, N-CH=CH), 7.85
O
N
N
(2H, d, J = 4.8 Hz, N-CH=CH), 7.70 (1H, s, CH=C-C=O), 6.71
(1H, s, CH=C-O), 4.97 (2H, s, CH2S), 3.89 (3H, s, CH3O), 3.28
N
O
S
O
(1H, septet, J = 6.8 Hz, CH[CH3]2), 2.58 (3H, s, CH3Ar), 1.24 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C
NMR (CDCl3):  192.5 (C=O), 166.0 (N=C-C), 164.1 (C-S), 160.4 (C-O), 151.0 (N-CH=CH), 141.1
(C-CH3), 134.7 (C-CH[CH3]2), 130.7 (C-C=N), 128.5 (CH=C-C=O), 126.6 (C-C=O), 120.2 (N=CH-
CH), 114.2 (CH=C-O), 55.6 (CH3O), 43.9 (CH2S), 26.9 (CH[CH3]2), 22.7 (CH3Ar), 22.6 ([CH3]2CH).
HRMS (ESI): m/z 384.1379 [M+H] +; calcd. for C20H22N3O3S, 384.1376.

1-(5-isopropyl-4-methoxy-2-methylphenyl)-2-(thiazol-2-ylthio)ethan-1-one (13)
Yellow solid (91%); M.p. 110-112 °C; IR (ATR), cm-1: 1668 (C=O).
1H NMR (CDCl3):  7.67 (1H, s, CH=C-C=O), 7.63 (1H, d, J = 3.2 Hz,
O
N
CH=CH-N), 7.19 (1H, d, J = 3.2 Hz, CH=CH-S), 6.68 (1H, s, CH=C-O),
4.72 (2H, s, CH2S), 3.86 (3H, s, CH3O), 3.26 (1H, septet, J = 7.2 Hz,
S
S
O
CH[CH3]2), 2.53 (3H, s, CH3Ar), 1.21 (6H, d, J = 7.2 Hz, ([CH3]2CH); 13C NMR (CDCl3):  194.5
(C=O), 163.7 (C-S), 159.8 (C-O), 142.6 (CH=CH-N), 140.4 (C-CH3), 134.2 (C-CH[CH3]2), 128.3
(CH=C-C=O), 127.7 (C-C=O), 119.3 (CH=CH-S), 114.0 (CH=C-O), 55.5 (CH3O), 43.6 (CH2S), 26.8
(CH[CH3]2), 22.6 ([CH3]2CH), 22.3 (CH3Ar).
HRMS (ESI): m/z 322.0937 [M+H] +; calcd. for C16H20NO2S2, 322.0935.
2-(benzo[d]thiazol-2-ylthio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (14)
White solid (96%); 129-130 °C; IR (ATR), cm-1: 1661 (C=O).
1H NMR (CDCl3):  7.82 (1H, d, J = 8.0 Hz, CH=C-N), 7.75 (1H, s,
O
N
CH=C-C=O), 7.74 (1H, d, J = 8.0 Hz, CH=C-S), 7.39 (1H, t, J = 8.0
S
S
O
Hz, CH-CH=C-N), 7.28 (1H, t, J = 8.0, CH-CH=C-S), 6.70 (1H, s,
CH=C-O), 4.86 (2H, s, CH2S), 3.87 (3H, s, CH3O), 3.27 (1H, septet, J = 6.8 Hz, CH[CH3]2), 2.55
(3H, s, CH3Ar), 1.20 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):  194.3 (C=O), 165.8 (C-
S), 159.8 (C-O), 153.0 (C-N), 140.4 (C-CH3), 135.6 (C-S), 134.3 (C-CH[CH3]2), 128.3 (CH=C-C=O),
127.8 (C-C=O), 126.1 (CH-CH=C-N), 124.4 (CH-CH=C-S), 121.6 (CH=C-N), 121.1 (CH=C-S),
114.0 (CH=C-O), 55.5 (CH3O), 42.8 (CH2S), 26.8 (CH[CH3]2), 22.6 ([CH3]2CH), 22.3 (CH3Ar).
HRMS (ESI): m/z 372.1967 [M+H] +; calcd. for C20H22NO2S2, 372.1086

2-(benzo[d]oxazol-2-ylthio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (15)
Off-white solid (72%); M.p. 135-138 °C; IR (ATR), cm-1: 1660 (C=O).
1H NMR (CDCl3):  7.77 (1H, s, CH=C-C=O), 7.57 (1H, d, J = 8.0 Hz,
O
N
CH=C-N), 7.43 (1H, d, J = 8.0 Hz, CH=C-O), 7.27 (1H, t, J = 8.0 Hz,
O
S
O
CH-CH=C-N), 7.25 (1H, t, J = 8.0, CH-CH=C-O), 6.70 (1H, s, CH=C-
O), 4.86 (2H, s, CH2S), 3.88 (3H, s, CH3O), 3.28 (1H, septet, J = 6.8 Hz, CH[CH3]2), 2.56 (3H, s,
CH3Ar), 1.24 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C NMR (CDCl3):  193.7 (C=O), 164.5 (C-S), 160.1
(C-O), 152.1 (C-O-C=N), 141.9 (C-N), 140.7 (C-CH3), 134.4 (C-CH[CH3]2), 128.5 (CH=C-C=O),
127.4 (C-C=O), 124.4 (CH-CH=C-N), 124.0 (CH-CH=C-O), 118.5 (CH=C-N), 114.1 (CH=C-O),
110.1 (CH=C-O-C=N), 55.6 (CH3O), 42.8 (CH2S), 26.9 (CH[CH3]2), 22.6 ([CH3]2CH), 22.4
(CH3Ar).
HRMS (ESI): m/z 356.1326 [M+H] +; calcd. for C20H22NO3S, 356.1314.
2-((1H-benzo[d]imidazol-2-yl)thio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (16)
Yellow solid (60%); M.p. 158 °C (decomposed); IR (ATR), cm-1: 1655 (C=O).
1H NMR (DMSO-d6):  7.82 (1H, s, CH=C-C=O), 7.38 (2H, dd, J =
O
N
6.0, 3.2 Hz, CH=C-N), 7.08 (2H, dd, J = 6.0, 3.2 Hz, CH-CH=C-N),
N
S
H
O
6.86 (1H, s, CH=C-O), 4.85 (2H, s, CH2S), 3.82 (3H, s, CH3O), 3.18
(1H, septet, J = 6.8 Hz, CH[CH3]2), 2.37 (3H, s, CH3Ar), 1.13 (6H, d, J = 6.8 Hz, ([CH3]2CH); 13C
NMR (DMSO-d6):  195.9 (C=O), 159.4 (C-O), 150.3 (C-S), 139.1 (C-CH3), 133.6 (C-CH[CH3]2),
128.8 (C-C=O), 128.5 (CH=C-C=O), 122.0 (broad, imidazole aromatic), 114.5 (CH=C-O), 56.2
(CH3O), 41.5 (CH2S), 26.6 (CH[CH3]2), 22.9 ([CH3]2CH), 21.8 (CH3Ar).

HRMS (ESI): m/z 355.1478 [M+H] +; calcd. for C20H23N2O2S, 355.1474
2-((9H-purin-6-yl)thio)-1-(5-isopropyl-4-methoxy-2-methylphenyl)ethan-1-one (17)
Pale yellow solid (45%); M.p: 181-184 C; IR (ATR): 1661 (C=O ketone),
1H NMR (DMSO-d6):  8.55 (1H, s, N-CH=N), 8.41 (1H, s, NH-
O
N
N
CH=N), 7.81 (1H, s, CH=C-C=O), 6.86 (1H, s, CH=C-O), 4.84 (2H,
S
NH
N
O
s, CH2S), 3.82 (3H, s, CH3O), 3.19 (1H, septet, J = 6.8 Hz, CH[CH3]2),
13
2.36 (3H, s, CH3Ar), 1.12 (6H, d, J = 6.8 Hz, ([CH3]2CH); C NMR (DMSO-d6):  195.9 (C=O),
159.2 (C-O), 158.4 (C-S), 151.7 (N-CH=N), 149.8 (N-C-NH), 143.6 (NH-CH=N), 138.8 (C-CH3),
133.6 (C-CH[CH3]2), 130.7 (C-C-S), 129.2 (CH=C-C=O), 128.1 (C-C=O), 114.4 (CH=C-O), 56.2
(CH3O), 38.3 (CH2S), 26.6 (CH[CH3]2), 22.9 ([CH3]2CH), 21.6 (CH3Ar).
HRMS (ESI): m/z 357.1387 [M+H] +; calcd. for C18H21N4O2S, 357.1385
4.3 CUPRAC assay [16,18]
Test solutions for the CUPRAC assay were prepared as follows: 6 tubes each containing 1 mL
each of MilliQ H2O, 7.5 mM neocuproine in absolute ethanol, 1 M aqueous NH4Ac in and 10 mM
aqueous CuCl2 solutions. Additional reagents were placed in the tubes as follows, using 5 mM stock
solutions of test samples (TS), prepared in 95% ethanol or 85:15 ethanol:DMSO: Tube 1, Blank (100
L H2O); Tube 2 (10 µL TS, 90 L H2O); Tube 3 (20 µL TS, 80 L H2O); Tube 4 (30 µL TS, 70 L
H2O); Tube 5 (40 µL TS, 60 L H2O); Tube 6 (50 µL TS, 50 L H2O) to give final concentrations of
test samples ranging from 12.2-60.9 M. The samples were vortexed for 30 seconds, and allowed to
sit at room temperature for 30 minutes. During this period, the blue green color of the test solution
was changed to various shades of yellow and orange, depending on the reducing power of each

compound. trolox, ascorbic acid, and carvacrol were used as positive controls. The measurements
were performed in triplicate for each concentration, and average absorbance values were used to
generate the calibration plots. The Trolox Equivalent Antioxidant Capacity (TEAC) coefficient for
this assay was determined by relating the molar absorptivity,  (obtained from the slopes of the
calibration plots), of the test samples to that of trolox as follows:  Test samples/ Trolox.
Note: For the benzoxazole derivative (15), the calibration plot was linear up to 36.6 M (30 µL TS),
thus the data from tubes 5 (40 µL TS), and 6 (50 µL TS) was not used in determining the slope.
4.4 Tyrosinase docking studies
To determine whether the heterocyclic sulfide derivatives possessed tyrosinase inhibitory
activity, compounds 4-17, thymol and kojic acid were docked onto mushroom tyrosinase (PDB ID:
2Y9X) using Autodock Vina. To prepare tyrosinase 2Y9X for docking, all water molecules and
tropolone ligands were removed, and the protein was saved as a rigid .pdbqt file. This .pdbqt file was
opened in a text editor to manually change the charge of each copper atom to 2.00. The binding site
was centered around the copper-containing active site of the B chain, encompassing a volume 15 Å
× 15 Å × 15 Å. All ligands were sketched in Avogadro, and their geometries were optimized using a
steepest-descent algorithm. The ligand files were saved as .pdb before torsions were selected in
Python Molecule Viewer and exported as .pdbqt files. The prepared ligands were docked to tyrosinase
over three consecutive runs, using an exhaustiveness value of 8, and the most stable conformation
was recorded (Table 1). The lowest-energy docking poses were visually inspected for interactions
between the active-site copper atoms and ligand sulfur.

Declaration of Competing Interests
The authors declare no competing conflicts of interest.
Acknowledgments
We gratefully acknowledge the financial support provided Susquehanna University this work.
The JEOL ECZ 400S NMR spectrometer was acquired by a grant to Susquehanna University from
the National Science Foundation (NSF:MRI CHE 1625340). The Thermo Scientific Nicolet iS50 FT-
IR spectrometer was funded by the Sherman Fairchild Foundation. The Waters SYNAPT G2-S
QTOFMS system used for HRMS data was obtained by a grant (# P20GM103430) from National
Center for Research Resources (NCRR).

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