5-Halobenzothiophene analogues of melatonin: Synthesis and affinity for mt1 and MT2 receptors in man

Article abstract of DOI:10.1211/146080800128735647

A novel series of melatonin analogues based on the benzothiophene nucleus is described. In these compounds the methoxy group was replaced by electron-attracting groups such halogens (Br and Cl) with the aim of supplementing structure-affinity relationship

Full text of DOI:10.1211/146080800128735647

Pharm. Pharmacol. Commun. 2000, 6: 61±65
Received December 9, 1999
Accepted December 22, 1999
# 2000 Pharm. Pharmacol. Commun.
5-Halobenzothiophene Analogues of Melatonin: Synthesis
and Af®nity for mt1 and MT2 Receptors in Man
V. LECLERC, N. BEAURAIN, P. DEPREUX, C. BENNEJEAN*, P. DELAGRANGE{, J. BOUTIN{
AND D. LESIEUR
Institut de Chimie Pharmaceutique Albert Lespagnol, 3 rue du Professeur Laguesse, B.P. 83,
Â
Â
6 Place de Pleiades, 92415 Courbevoie Cedex, France
59006 Lille Cedex, *ADIR Servier, 1 rue Carle Hebert, 92415 Courbevoie Cedex and {IRI Servier,
Abstract
A novel series of melatonin analogues based on the benzothiophene nucleus is described.
In these compounds the methoxy group was replaced by electron-attracting groups such
halogens (Br and Cl) with the aim of supplementing structure±af®nity relationships on
melatoninergic ligands. Target derivatives were prepared from the corresponding 4-halo-
thiophenol. Some of these derivatives had high af®nity for mt1 and MT2 receptors, almost
as high as that of melatonin.
These results prove that the methoxy group is not an essential requirement for binding to
melatoninergic receptors.
Melatonin (N-acetyl-5-methoxytryptamine), a neu-
rohormone synthesized during the night by the
pineal gland (Lerner et al 1958), acts through the
blood circulation as an internal synchronizer of
circadian rhythms and informs the organism about
the photoperiod. It is also essential in the regulation
of some physiological (endocrinal and neuronal)
processes. The biological activity of melatonin acts
through speci®c receptors, for example the mt1 and
MT2 receptors; these receptors in man were
recently cloned (Reppert et al 1994, 1995, 1996;
Conway et al 1997). These two receptors subtypes
are located in the central nervous system (supra-
chiasmatic nucleus, hypothalamus, hippocampus)
and at the peripheral level (kidney, retina). Because
this distribution of melatonin sites suggests that
each receptor could play a speci®c physiological
role, it is fundamental to conceive and synthesize
new molecules which mimic or antagonize
responses to melatonin and, if possible, selectively
for one subtype, to enable determination of their
respective functional role and to specify their
therapeutic potential.
synthesized in our laboratory (Yous et al 1992;
Depreux et al 1994). This work has enabled us to
specify structure-af®nity and -activity relationships
towards melatonin receptorsÐin particular the
indole nucleus can be replaced by another aromatic
nucleus, for example, benzothiophene, without
seriously affecting the af®nity and the activity of
the ligands. The benzothiophene bioisostere of
melatonin is an agonist with lower af®nity than
melatonin but with the advantage of being meta-
bolically more stable (Depreux et al 1994).
Because the methoxy group seems to be impor-
tant for interaction with and activation of receptors,
we decided to replace this group in the 5-position
by electronically different substituents, with an
equivalent molar volume, but without any possibi-
lity of hydrogen-bonding. This paper describes the
synthesis and pharmacological evaluation of these
benzothiophene compounds substituted by halo-
gens (Br and Cl) in the 5 position (Figure 1).
Material and Methods
Many non-indole analogues of melatonin, for
example the naphthalene, benzofuran and benzo-
thiophene bioisosteres, have been designed and
Chemistry
Melting points were determined by means of a
BuÈchi SMP-535 apparatus. Column chromato-
graphy was performed on silica gel 60 (70-230
mesh, ASTM; Merck) with an appropriate mobile
phase. IR spectra were recorded on Perkin-Elmer
Correspondence: P. Depreux, Institut de Chimie Pharmaceu-
tique Albert Lespagnol, 3 rue du Professeur Laguesse, B.P. 83,
59006 Lille Cedex, France.

62
V. LECLERC ET AL
1Á8 Hz, 1H, H-6), 7Á65 (d, J ˆ 8Á7 Hz, 1H, H-7), 7Á81
(d, J ˆ 1Á8 Hz, 1H, H-4). IR (KBr) n 3080±
2840 cmÀ1.
5-Bromo-3-bromomethylbenzo[b]thiophene (1c)
Dibenzoyl peroxide (DBP; 0Á13 g, 0Á001 mol) was
added to a vigorously stirred solution of 1b (3 g,
0Á013
Figure 1. Structures of 5-halobenzothiophene compounds.
mol) in dry carbon tetrachloride (300 mL). N-bro-
mosuccinimide (NBS; 2Á35 g, 0Á013 mol) was added
in small portions to the boiling mixture which was
then irradiated (500 W). The mixture was heated
under re¯ux for 4 h, then cooled and ®ltered to
remove the succinimide. The solvent was evapo-
rated and 1c was obtained as a solid by triturating
with ether. The solid was recrystallized from
toluene±cyclohexane (4 : 1) to give a yellow pow-
der (56% yield), mp 127±129ꢀC. 1H NMR
(300 MHz, CDCl3) d 5Á05 (s, 2H, CH2), 7Á58 (dd,
J ˆ 8Á7 and 1Á9 Hz, 1H, H-6), 8Á02 (d, J ˆ 8Á7 Hz,
1H, H-7), 8Á04 (s, 1H, H-2), 8Á15 (d, J ˆ 1Á9 Hz, 1H,
H-4).
297 or BruÈcker Vector 22 spectrometers, using KBr
tablets. H NMR spectra were recorded on an AC
1
300 P (300 MHz) spectrometer with d6-DMSO
(dimethylsulphoxide) or CDCl3 as solvent. Chem-
ical shifts are expressed down®eld from the internal
standard tetramethylsilane. Coupling constants (J)
are expressed in Hz. In the data below s ˆ singlet,
d ˆ doublet, t ˆ triplet, m ˆ multiplet, b ˆ broad.
Elemental analysis (C, H, N) was performed by the
CNRS Centre of Analysis (Vernaison, France) and
agree with the proposed structures within 0Á4% of
the theoretical values.
5-Bromo-3-cyanomethyl benzo[b]thiophene (1d)
1c (2 g, 0Á007 mol) was added to a solution of
sodium cyanide (0Á38 g, 0Á008 mol) in dimethyl-
sulphoxide (50 mL). The mixture was heated
slowly to 60ꢀC with vigorous stirring and kept at
this temperature for 1 h. It was then cooled and
poured into ice-water. The resulting precipitate was
puri®ed by recrystallization from 95% ethanol to
give a brown powder (70% yield), mp 139±140ꢀC.
1H NMR (300 MHz, CDCl3) d 3Á88 (s, 2H, CH2),
7Á70±7Á54 (m, 2H, H-2,6), 7Á75 (d, J ˆ 8Á4 Hz, 1H,
H-7), 7Á84 (d, J ˆ 1Á9 Hz, 1H, H-4). IR (KBr) n
2240 cmÀ1.
(4-Bromophenylthio)acetone (1a)
Chloroacetone (5Á35 g, 0Á064 mol) was slowly
added to a cold stirred solution of 4-bromothio-
phenol (10 g, 0Á053 mol) and pyridine (17Á36 g,
0Á212 mol) in ether. The mixture was stirred for 3 h,
then poured into ice-water and extracted with ether.
The organic layer was separated, washed with 1 M
HCl, then with water, dried over magnesium sul-
phate and evaporated under reduced pressure. The
resulting solid was recrystallized from 95% ethanol
to give a white powder (80% yield), mp 63±65ꢀC.
1H NMR (300 MHz, CDCl3) d 2Á28 (s, 3H, CH3),
3Á66 (s, 2H, CH2), 7Á20 (d, J ˆ 6Á6 Hz, 2H, H-2,20),
7Á41 (d, J ˆ 6Á6 Hz, 2H, H-3,30). IR (KBr) n
1700 cmÀ1.
General procedure for preparation of the
N-2-(5-halobenzo[b]thiophen-3-yl)ethylamine
hydrochlorides (1e and 2e) (Figure 2)
The method adopted for the synthesis of N-2-
(5-bromobenzo[b]thiophen-3-yl)ethylamine hydro-
chloride(1e)isdescribed.
5-Bromo-3-methylbenzo[b]thiophene (1b)
1a (5 g, 0Á020 mol) was added to a solution of
phosphorus pentoxide (P2O5; 0Á58 g, 0Á002 mol) in
polyphosphoric acid (PPA; 50 g). The mixture was
heated slowly to 180ꢀC with vigorous stirring, kept
at this temperature for 3 h and, after cooling,
poured into ice-water. After extraction with ether
the organic layer was separated, washed with water,
dried over magnesium sulphate and evaporated,
yielding a residue that was puri®ed by silica gel
column chromatography, with light petroleum as
eluent, to give a white powder (71% yield), mp 38±
A solution of borane±tetrahydrofuran complex
(24 mL, 0Á024 mol; 1 M in tetrahydrofuran (THF)
and 1d (2Á02 g, 0Á008 mol) in tetrahydrofuran
(50 mL) was heated under re¯ux for 3 h under
nitrogen. Hydrochloric acid (6 M, 16 mL,
0Á095 mol) was then added dropwise. Tetra-
hydrofuran was evaporated and the resulting pre-
cipitate was ®ltered and recrystallized from
absolute ethanol affording 1e as a white powder
(70% yield), mp > 260ꢀC. 1H NMR (300 MHz, d6-
DMSO) d 3Á00±3Á19 (m, 4H, 2 CH2), 7Á55 (dd,
J ˆ 8Á3 and 1Á8 Hz, 1H, H-6), 7Á69 (s, 1H, H-2),
1
39ꢀC. H NMR (300 MHz, CDCl3) d 2Á37 (s, 3H,
CH3), 7Á06 (s, 1H, H-2), 7Á40 (dd, J ˆ 8Á7 and

5-HALOBENZOTHIOPHENE ANALOGUES OF MELATONIN
63
Figure 2. Synthesis of N-2-(5-halobenzo[b]thiophen-3-yl)ethylaminehydrochlorides. Phosphroic acid (PPA); Phosphorus pent-
oxide (P2O5); N-bromosuccinimide (NBS); dibenzylperoxide (DBP); borane±tetrahydrofuran (BH3-THF). A and B refer to the
method used for preparation of the compounds.
‡
7Á99-8Á02 (m, 4H, NH3 and H-7), 8Á12 (d,
J ˆ 1Á8 Hz, 1H, H-4). IR (KBr) n 3400±2990 cmÀ1.
Calculated for C10H11BrClNS: C, 41Á04; H, 3Á79;
N, 4Á79; found: C, 41Á03; H, 3Á99; N, 4Á45.
Method B, from acids (derivatives 1k±1m and 2k±
2m). The method adopted for the synthesis of N-
[2-(5-chlorobenzo[b]thiophen-3-yl)ethyl] allylcarbox-
amide (2k) is described.
Potassium carbonate was added slowly to a
solution of N-2-(5-chlorobenzo[b]thiophen-3-yl)-
ethylamine hydrochloride (2e) (0Á74 g, 0Á003 mol)
in water until pH 9. After stirring for 2 h the med-
ium was extracted with ether and the organic layer
dried over magnesium sulphate and evaporated to
obtain the basic amine, which was dissolved in
dichloromethane and cooled to À20ꢀC. Vinylacetic
acid (0Á74 g, 0Á006 mol) and 1-[3-(dimethylami-
no)propyl]-3-ethyl carbodiimide hydrochloride
(1Á15 g, 0Á006 mol) were dissolved in dichlor-
omethane (30 mL); after 30 min the solution was
cooled to À20ꢀC. The previously obtained solution
of the amine in dichloromethane was added drop-
wise to this solution and the reaction mixture was
stirred for a further 30 min at À20ꢀC and then
overnight at room temperature. The dichlor-
omethane was evaporated, the residue was taken up
in ethyl acetate, and the solution was washed with
10% aqueous potassium carbonate and with water.
The organic layer was dried over magnesium sul-
phate and evaporated, yielding a residue which was
recrystallized from cyclohexane±toluene (4 : 1)
affording 2k as a white powder (37% yield), mp
General procedures for preparation of the
N-acylated derivatives
Method A, from acid chlorides (derivatives 1f, 1i,
1o±1q, 2f, 2g, 2i, 2j, 2n, 2o). The method adopted
for the synthesis of N-2-(5-bromobenzo[b]thio-
phen-3-yl)ethyl acetamide (1f) is described.
Potassium carbonate (0Á69 g, 0Á005 mol) was
added to a solution of N-2-(5-bromobenzo[b]-
thiophen-3-yl)ethylamine
hydrochloride
(1e;
0Á88 g, 0Á003 mol) in water (40 mL) and dichloro-
methane (60 mL). The mixture was cooled to 0ꢀC
and acetyl chloride (0Á35 mL, 0Á005 mol) was added
dropwise. The solution was then stirred at room
temperature for 2 h. The organic layer was sepa-
rated, washed with water, dried over magnesium
sulphate and evaporated under reduced pressure
yielding a residue that was recrystallized from
toluene affording 1f as a white powder (55% yield),
1
mp 134±136ꢀC. H NMR (300 MHz, d6-DMSO) d
1Á80 (s, 3H, CH3), 2Á94 (t, J ˆ 7Á1 Hz, 2H, CH2),
3Á34 (m, 2H, CH2-NH), 7Á52 (dd, J ˆ 8Á6 and
2Á0 Hz, 1H, H-6), 7Á55 (s, 1H, H-2), 7Á96 (d,
J ˆ 8Á6 Hz, 1H, H-7), 8Á04 (signal, 1H, NH), 8Á06 (s,
1H, H-4). IR (KBr) n 3230, 3050, 1625 cmÀ1.
Calculated for C12H12BrNOS: C, 48Á33; H, 4Á06;
N, 4Á70; found: C, 48Á65; H, 4Á14; N, 4Á72.
1
76±77ꢀC. H NMR (300 MHz, d6-DMSO) d 2Á87
(d, J ˆ 6Á8 Hz, 2H, CH2-CO), 2Á94 (t, J ˆ 7Á2 Hz,
2H, CH2), 3Á35 (m, 2H, CH2-NH), 5Á05±5Á12 (m,
2H, CH2 ˆ CH), 5Á86 (m, 1H, CH2 ˆ CH), 7Á41 (dd,
J ˆ 8Á4 and 2Á0 Hz, 1H, H-6), 7Á57 (s, 1H, H-2), 7Á94

64
V. LECLERC ET AL
(d, J ˆ 2Á0 Hz, 1H, H-4), 8Á00±8Á03 (m, 2H, H-7
and NH). IR (KBr) n 3292, 3082, 1641 cmÀ1.
Calculated for C14H14ClNOS: C, 60Á10; H, 5Á04; N,
5Á01; found: C, 60Á04; H, 5Á09; N, 5Á00.
MT2 (0Á04 mg mLÀ1) were incubated for 2 h at
37ꢀC in Tris±HCl (50 mM, ®nal volume 0Á25 mL)
containing MgCl2 (5 mM) at pH 7Á40, with different
concentrations of 2-[125I]iodomelatonin (2200
Ci mmolÀ1) from 0Á005 to 1Á5 nM for mt1 and from
0Á02 to 3 nM for MT2 in the absence or presence of
melatonin (10 mM), whichdetermines thenon-speci®c
binding. Competition studies for 2-[125I]iodomela-
tonin binding (radioligand concentration, 0Á025 nM
for mt1 studies and 0Á200 nM for MT2 studies) were
performed in the presence of reference substances
to determine their af®nity, expressed as IC50
(the dose (M) resulting in 50% inhibition), for the
two melatonin receptors subtypes from man.
Method C, from acid anhydrides (derivatives 1h
and 2h). The method adopted for the synthesis of
N-[2-(5-bromobenzo[b]thiophen-3-yl)ethyl] tri¯uor-
oacetamide (1h) is described.
Tri¯uoroacetic anhydride (0Á85 mL, 0Á006 mol)
was added to a solution of N-2-(5-bromo-
benzo[b]thiophen-3-yl)ethylamine hydrochloride
(1e) (0Á88 g, 0Á003 mol) in pyridine (0Á48 mL,
0Á006 mol) and ether (20 mL). The reaction mixture
was stirred at room temperature for 2 h, then acid-
i®ed with 6 M HCl and extracted with ethyl acetate.
The organic layer was washed, dried over magne-
sium sulphate and evaporated yielding a residue
that was recrystallized from toluene affording 1h as
Results and Discussion
1
a white powder (70% yield), mp 144±146ꢀC. H
Chemistry
NMR (300 MHz, d6-DMSO) d 3Á05 (t, J ˆ 7Á2 Hz,
2H, CH2), 3Á50 (m, 2H, CH2-NH), 7Á53 (dd, J ˆ 8Á6
and 1Á6 Hz, 1H, H-6), 7Á59 (s, 1H, H-2), 7Á97 (d,
J ˆ 8Á6 Hz, 1H, H-7), 8Á09 (d, J ˆ 1Á6 Hz, 1H, H-4),
9Á58 (t, J ˆ 5Á6 Hz, 1H, NH). IR (KBr) n 3250,
3000, 1690 cmÀ1. Calculated for C12H9BrF3NOS:
C, 40Á93; H, 2Á58; N, 3Á98; found: C, 41Á09; H, 2Á66;
N, 4Á05.
Key intermediates in the synthesis of the target
compounds described here are the corresponding
amine hydrochlorides (1e and 2e) which were
prepared by successive action of borane-tetra-
hydrofuran complex and hydrochloric acid (Brown
et al 1970) on the synthesized nitrile 1d or the
commercially available nitrile 2d (Figure 2).
The nitrile 1d was obtained by the method
described by Marshall et al (1969). Condensation
of 4-bromothiophenol with chloroacetone in pyr-
idine gave the ketone 1a, which was cyclized in
acidic medium (PPA and P2O5) at 180ꢀC into 5-
bromo-3-methyl benzo[b]thiophene (1b). Com-
pound 1c was obtained by bromination with N-
bromosuccinimide (NBS) in the presence of
dibenzoyl peroxide (DBP). Nucleophilic displace-
ment of bromide with sodium cyanide in DMSO
provided nitrile 1d.
The N-acetylated derivatives (1f, 1h±1i, 1k±1m,
1o±1q, 2f±2o) were prepared (Figure 2) from
appropriate amine hydrochlorides by treatment
with the appropriate acid chloride in the presence
of potassium carbonate as base in a biphasic med-
ium (Method A; Yous et al 1992) according to a
variant of the Schotten±Bauman procedure (Lind-
berg et al 1968) or with acids in the presence of
1-[3-(dimethylamino)propyl]-3-ethyl carbodiimide
hydrochloride and potassium carbonate as base
according to peptide synthesis (Method B) or with
acid anhydrides in pyridine (Method C).
Pharmacology
Cell Culture. Embryonic kidney cell line HEK293
from man (A. D. Strosberg, Paris, France), stably
expressing mt1 or MT2 melatonin receptors, were
grown as monolayers at 37ꢀC (95% O2±5% CO2)
in Dulbecco's modi®ed Eagle's medium glutamax-
1 (Gibco 31966-036; Gibco Laboratories, Grand
Island, NY) supplemented with 10% foetal calf
serum, penicillin, and streptomycin (1%) in the
presence of the selection agent geneticin G-418
(4%) (Gibco 11811-031). The cells were then
washed twice with phosphate-buffered saline
(PBS), harvested in MatriSperse (Becton Dickin-
son, Le Pont-de-Claix, France), pelleted at 4ꢀC at
1000 rev minÀ1, and suspended in PBS. The cells
were homogenized with a Polytron tissue disrupter,
and the resulting homogenate was centrifuged at
20 000 g for 30 min. The pellet was suspended in
buffer, and the protein concentration was measured
by the method of Bradford (1976), with BSA as
standard. The membranes were stored at À80ꢀC at
a concentration of 5 mg mLÀ1.
Pharmacology
Table 1 shows that replacement of the methoxy
group by a halogen does not affect af®nityÐcom-
Binding receptor assays. In saturation experiments,
membrane suspensions of mt1 (0Á04 mg mLÀ1) and

5-HALOBENZOTHIOPHENE ANALOGUES OF MELATONIN
65
Table 1. The physical properties and mt1 and MT2 receptor-binding af®nity of the benzothiophene compounds.
Recrystallization
solvent
IC50 mt1
(M)
IC50 MT2
(M)
Rate selectivity
(mt1=MT2)
Compound
X
±
R
±
Mp (ꢀC)
±
À10
À10
Melatonin
1f
1h
1i
1k
1l
1m
1o
1p
1q
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
±
2Á0 6 10
2Á3 6 10
1Á0 6 10
±
5Á3 6 10
9Á3 6 10
2Á0 6 10
±
0Á40
2Á46
4Á95
±
9Á77
1Á10
0Á20
1Á74
±
À09
À08
À10
À09
Br CH3
Br CF3
Br cyclopropyl
Br CH2CH ˆ CH2
Br CH ˆ CHC6H5
134±136 Toluene
144±146 Toluene
166±168 Toluene
À09
À05
À06
À10
À06
À05
À06
90±91
Toluene
2Á1 6 10
2Á1 6 10
9Á3 6 10
1Á3 6 10
152±153 Toluene±cyclohexanea
> 6 10
Br CH2CH ˆ CHC6H5 130±131 Toluene±cyclohexanea 2Á6 6 10
Br 2-furyl
Br 3,4-Cl2C6H3
Br CH2C6H5
87±88
Toluene±cyclohexanea 2Á1 6 10À06 1Á2 6 10
144±146 Toluene
147±148 Toluene
129±130 Toluene
±
±
±
±
±
À09
À08
À08
À08
À09
À09
À05
À06
À07
À09
À08
À08
À08
À09
À10
À05
À06
À07
À06
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
CH3
(CH2)3Cl
CF3
cyclopropyl
CH ˆ CH2
CH2CH ˆ CH2
CH ˆ CHC6H5
6Á5 6 10
1Á4 6 10
6Á3 6 10
3Á8 6 10
9Á8 6 10
1Á6 6 10
1Á2 6 10
1Á1 6 10
2Á0 6 10
4Á6 6 10
3Á6 6 10
> 6 10
1Á0 6 10
1Á1 6 10
4Á00
1Á15
5Á90
1Á89
2Á13
4Á47
1Á00
3Á20
2Á28
1Á28
83±84
Cyclohexane
132±134 Toluene
161±163 Toluene
111±113 Toluene
76±77
Toluene±cyclohexanea 1Á6 6 10
162±163 Toluene±cyclohexanea
> 6 10
3Á3 6 10
CH2CH ˆ CHC6H5 116±117 Cyclohexane
ꢁ -CH3
79±80
70±71
Toluene±cyclohexanea 2Á6 6 10
2-furyl
Toluene±cyclohexanea 2Á4 6 10À06 1Á9 6 10
aIn the proportions 4 : 1, respectively.
Conway, S., Drew, E. J., Canning, S. J., Barett, P., Jockers, R.,
Ã
pounds 1f and 2f, which can be considered as the
direct 5-halobenzothiophene analogues of mela-
tonin, have slightly less af®nity than melatonin.
Furthermore, most of the 5-bromo compounds (1f,
1h, 1k±1m, 1o) have similar af®nity which is even
higher than that of their 5-chloro analogues (2f, 2h,
2k±2m, 2o).
Strosberg, A. D., Guardiola-Lemaõtre, B., Delagrange, P.,
Morgan, P. J. (1997) Identi®cation of Mel1a melatonin
receptors in the human embryonic kidney cell line HEK
293: evidence of G protein-coupled melatonin receptors
which do not mediate the inhibition of stimulated cyclic
AMP levels. FEBS Lett. 407: 121±126
Depreux, P., Lesieur, D., Ait Mansour, H., Morgan, P., Howell,
H. E., Renard, P., Caignard, D. H., Pfeiffer, B., Delagrange,
Except for the allylic compounds (1k, 2k), which
in each series studied have the best af®nity, repla-
cement of the methyl group of the amidic group by
another substituent results, overall, in a usually
signi®cant reduction of af®nity according to the
nature and steric hindrance of the substituents,
especially when aromatic or conjugated aromatic
groups are present (1l, 1m, 1o, 2l, 2m, 2o).
With regard to selectivity, most of the 5-halo-
benzothiophene derivatives are slightly selective
towards the MT2 binding sites.
These results show that the methoxy group is not
essential requirement for high af®nity. This calls
into question the suggestion that the methoxy
group, which is likely to form a hydrogen-bond
with a receptor residue, is responsible for the
activity and af®nity of melatonin.
Ã
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