Ring-substituted 8-hydroxyquinoline-2-carboxanilides as photosystem II inhibitors

Article abstract of DOI:10.1016/j.bmcl.2016.07.021

Ring-substituted 8-hydroxyquinoline-2-carboxanilides inhibited photosynthetic electron transport (PET) through photosystem (PS) II. Their inhibitory efficiency depended on the compound lipophilicity, the electronic properties of the substituent R and the

Full text of DOI:10.1016/j.bmcl.2016.07.021

Bioorganic & Medicinal Chemistry Letters 26 (2016) 3862–3865
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Ring-substituted 8-hydroxyquinoline-2-carboxanilides as
photosystem II inhibitors
b
c
d
Josef Jampilek a, , Katarina Kralova , Matus Pesko , Jiri Kos
⇑
a Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, Odbojarov 10, 832 32 Bratislava, Slovakia
b Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska dolina Ch-2, 842 15 Bratislava, Slovakia
c Department of Environmental Ecology, Faculty of Natural Sciences, Comenius University, Mlynska dolina Ch-2, 842 15 Bratislava, Slovakia
d Department of Chemical Drugs, Faculty of Pharmacy, University of Veterinary and Pharmaceutical Sciences, Palackeho 1/3, 612 42 Brno, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Ring-substituted 8-hydroxyquinoline-2-carboxanilides inhibited photosynthetic electron transport (PET)
through photosystem (PS) II. Their inhibitory efficiency depended on the compound lipophilicity, the
electronic properties of the substituent R and the position of the substituent R on the benzene ring.
Received 2 June 2016
Revised 6 July 2016
Accepted 7 July 2016
Available online 7 July 2016
The most effective inhibitors showing IC50 values in the range 2.3–3.6 l
M were substituted in C0(3) by
F, CH3, Cl and Br. The dependence of the PET-inhibiting activity on the lipophilicity of the compounds
was quasi-parabolic for 3-substituted derivatives, while for C0(2) ones a slight increase and for C0(4) deriva-
tives a sharp decrease of the activity were observed with increasing lipophilicity. In addition, the depen-
Keywords:
8-Hydroxyquinolines
Hill reaction
Photosystem II
Structure–activity relationships
dence of PET-inhibiting activity on electronic Hammett’s
r
parameter of the substituent R was observed
param-
with optimum
r
value 0.06 for C0(4) and 0.34 for C0(3) substituted derivatives, while the value of
r
eter did not significantly influence the PET-inhibiting activity of C0(2) substituted compounds. Interactions
of the studied compounds with chlorophyll a and aromatic amino acids present in the pigment–protein
complexes mainly in PS II were documented by fluorescence spectroscopy. The section between P680 and
plastoquinone QB occurring on the acceptor side of PS II can be suggested as the site of action of the
compounds.
Ó 2016 Elsevier Ltd. All rights reserved.
8-Hydroxyquinoline scaffold represents an important type of
‘privileged structure’ possessing a rich diversity of biological prop-
erties.1 Some quinoline derivatives and their analogues/isosteres
also show noteworthy herbicidal activities. Quinclorac belongs to
the class of highly selective auxin herbicides and is used primarily
to control weeds in rice crops.2 Some other quinoline derivatives
could be used to control undesirable plant growth.3–5 Also several
recently described hydroxyquinoline-carboxamides reduced
chlorophyll content in Chlorella vulgaris with IC50 values about
The most potent inhibitors were found to be 2-methyl-3-alkyl-,
1-hydroxy-2-methyl-3-alkyl- and 1-hydroxy-2-alkyl-(1H)-quino-
12
lones with n-alkyl side chains varying from C5 to C17
ment experiments with
.
Displace-
[
14C]atrazine indicated that the
quinolones share an identical binding site with other PS II commer-
cial herbicides. Maximal inhibitory potency was achieved at the
carbon chain length of 12–14 Å, and a further increase of the chain
length resulted in a decreased activity. PET inhibition in spinach
chloroplasts was also observed for several other quinoline deriva-
tives.6,13–15
5.9–10.9 l
M.6 On the other hand, some quinoline derivatives orig-
inating from anthropogenic activities are environmental contami-
Ring-substituted 8-hydroxyquinoline-2-carboxanilides were
prepared according to Scheme 1 and published recently.16 Based
on the above-mentioned observations,6,13–15 they were tested for
their PET-inhibiting activity in spinach (Spinacia oleracea L.) chloro-
plasts, where PET through PS II, from H2O to plastoquinone QB, was
monitored spectrophotometrically as photoreduction of the artifi-
cial electron acceptor 2,6-dichlorophenol indophenol (DCPIP)
according to Kralova et al.17 The studied compounds strongly
inhibited photoreduction of DCPIP in spinach chloroplasts, and
their activity was compared with the commercial PET inhibitor
diuron. DMSO solutions of the studied compounds were added to
nants that are toxic to living organisms.7–9
Quinoline derivatives were found to inhibit photosynthetic
electron transport (PET) in plant chloroplasts. 2-Heptyl-1-
hydroxy-4(1H)-quinolone and 4-hydroxy-2-nonylquinoline-N-
oxide are potent inhibitors of PET in isolated thylakoids acting in
photosystem (PS) II at the QB-site before the site of diuron and,
in addition, at the cytochrome b6f-complex.10,11 Also quinolone-
N-oxides inhibited PET in PS II and in the cytochrome b6f-complex.
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.bmcl.2016.07.021
0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.

J. Jampilek et al. / Bioorg. Med. Chem. Lett. 26 (2016) 3862–3865
3863
Table 1
H2N
H
N
Predicted values of lipophilicity log P, experimentally determined values of lipophilic-
ity log k and electronic Hammett’s parameters of substituents R, IC50 values related
to PET inhibition in spinach chloroplasts in comparison with diuron (DCMU, 3-(3,4-
a
+
R
r
N
COOH
N
R
OH
R = H ( ), OCH3
OH
5a c
O
6a c
dichlorophenyl)-1,1-dimethylurea) standard
1
2a c
- ), CH3
3a c
(
4a c
7a c
- )
(
- ), F ( - ), Cl ( - ), Br ( - ), CF3
(
Compd
R1
log k
log Pa
ra
PET IC50 [lM]
Scheme 1. Synthesis of ring-substituted 8-hydroxyquinoline-2-carboxanilides 1–
7c: (a) PCl3, chlorobenzene, microwave-assisted synthesis.16
1
H
0.7600
0.7935
0.8164
0.7129
0.6944
0.9686
0.9521
0.6806
0.9420
0.8598
0.9566
1.1718
1.1543
1.0536
1.2357
1.2347
0.9147
1.3206
1.3653
—
2.55
2.67
2.61
2.51
2.90
2.90
2.90
2.59
2.76
2.59
3.07
3.28
3.05
3.16
3.31
3.19
3.36
3.44
3.27
0
16.6
134.6
16.0
b
81.6
2.7
150.0
32.1
2.3
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
7a
7b
7c
2-OCH3
3-OCH3
4-OCH3
2-CH3
3-CH3
4-CH3
2-F
3-F
4-F
2-Cl
3-Cl
4-Cl
2-Br
3-Br
4-Br
2-CF3
3-CF3
4-CF3
—
ꢀ0.28
0.12
ꢀ0.27
ꢀ0.17
ꢀ0.07
ꢀ0.17
0.06
0.34
0.06
0.22
0.37
0.23
0.22
0.39
0.23
0.51
0.43
0.51
chloroplasts due to the limited solubility of the compounds in
water. The applied DMSO concentration (up to 4 vol %) did not
affect the photochemical activity of spinach chloroplasts. Relation-
ships between the structure and the PET inhibition of the studied
compounds are discussed.
The PET-inhibiting activity was expressed by the negative loga-
rithm of IC50 value (compound concentration in mol/L causing 50%
inhibition of PET). The activity of the most potent compound 4b
5.6
46.0
3.6
42.5
38.9
3.4
72.7
51.2
21.3
477.6
1.9
(R = 3-F; IC50 = 2.3
DCMU (IC50 = 1.9
pounds 3b (R = 3-CH3; IC50 = 2.7
and 5b (R = 3-Cl; IC50 = 3.6 M) were very effective PET inhibitors.
l
l
M) was comparable with that of the standard
M). However, also other 3-substituted com-
M), 6b (R = 3-Br; IC50 = 3.4 M)
l
l
l
The dependences of log(1/IC50) on the lipophilicity of the com-
pounds expressed as log k16 are presented in Figure 1. The
lipophilicity of the compounds is additionally expressed and listed
in Table 1 as log P values predicted by sw. ACD/Percepta ver. 2012
for comparison. Below discussed relationships are valid both for
log k and log P values, therefore the dependence of log(1/IC50) on
log P values is not illustrated. Based on the obtained results it
can be stated that the C0(3) substituted compounds expressed the
highest PET-inhibiting activity, and the dependence of their PET-
inhibiting activity on lipophilicity was quasi-parabolic (Fig 1).
The lipophilicity of the most active compounds (IC50 range 2.3–
DCMU
a
Predicted using sw. ACD/Percepta ver. 2012.
Not determined due to precipitation during experiment.
b
4b and 7c were the most active compounds among C0(3) and C0(4)
substituted derivatives. On the other hand, the value of
r parame-
ter did not significantly influence the PET-inhibiting activity of C0(2)
substituted compounds.
From the above-mentioned results it is evident that beside
lipophilicity and electronic properties, the PET-inhibiting activity
of the studied compounds is significantly affected by the position
of substituents R on the phenyl ring. For example, the lower PET-
inhibiting activity of C0(2) substituted derivatives as compared to
C0(3) and C0(4) substituted ones was observed previously for several
esters of 2-, 3- and 4-substituted alkoxyphenylcarbamic acids.18,19
In these series, the lower inhibitory activity of 2-alkoxy substituted
derivatives in comparison with their 3- and 4-substituted ana-
logues can be explained by a secondary steric effect, which is
induced due to interactions between the alkoxy substituent and
the carbamate group.20 The lower activity of the tested C0(2) substi-
tuted 8-hydroxyquinoline-2-carboxanilides could be connected
with intramolecular interactions of the substituent R with the
NH group resulting in reduced interaction of these compounds
with photosynthetic proteins embedded in thylakoid membranes.
On the other hand, the strong activity decrease with increasing
lipophilicity of 4-substituted compounds could be caused by the
limited solubility of more lipophilic compounds. Summarizing, it
could be concluded that for PET-inhibiting activity, sufficient (but
not too high) lipophilicity enabling easier penetration of the com-
pounds into the lipids of photosynthetic membranes is necessary.
On the other hand, the increasing electronegativity of halogen sub-
stituents in positions C0(3) and C0(4) was reflected in a gradual activ-
ity decrease.
3.6 lM) varied in the range from 0.9420 to 1.2357 for log k (from
2.76 to 3.31 for log P). On the other hand, while the PET-inhibiting
activity of C0(2) substituted compounds increased slightly with
increasing compound lipophilicity, the activity of C0(4) substituted
compounds showed a strong decrease (Fig. 1).
Electronic properties of individual anilide substituents
expressed as Hammett’s
r constants (predicted by sw. ACD/
Percepta ver. 2012, see Table 1) were determined as another
parameter that could influence PET-inhibiting activity. For C0(3)
and C0(4) substituted derivatives increasing
r
values of halogen
substituents caused a gradual decrease of activity expressed as
IC50, namely from 2.3 M (4b, R = 3-F; = 0.34) to 21.3 M (7b,
R = 3-CF3; = 0.43) and from 5.6 M (4c, R = 4-F; = 0.06) to
l
r
l
r
l
r
477.6
lM (7c, R = 4-CF3;
r
= 0.51), respectively, whereby anilides
6.0
3-F
3-Br
5.5
5.0
4.5
4.0
3.5
3.0
3-CH3
3-Cl
4-F
H
3-OCH3
2-F
2-Br
3-CF3
The detection of PET through PS II (from the intermediate ZÅ sit-
uated on the donor side of PS II to QB located on the acceptor side of
PS II) was performed according to Xiao et al.21 and Sersen at al.22
using the artificial electron donor 1,5-diphenylcarbazide (DPC) act-
ing in the ZÅ/DÅ intermediate.23 The application of DPC to chloro-
plasts, the activity of which was inhibited by compounds 4b
(R = 3-F) or 6b (R = 3-Br) to 85%, resulted in a gradual restoration
of PET with increasing DPC concentration. The complete restora-
tion of PET occurred only when the concentration of DPC was
higher by more than one order of magnitude than the concentra-
tion of the applied inhibitor. Therefore it could be assumed that
4-Br
4-Cl
2-Cl
2-CF3
2-CH3
2-OCH3
4-CH3
4-CF3
1.3
0.6
0.7
0.8
0.9
1.0
log k
ortho meta
1.1
1.2
1.4
H
para
Figure 1. Relationships between PET inhibition log(1/IC50) [M] in isolated spinach
chloroplasts and lipophilicity expressed as log k. (Compound 3c (4-CH3) not
included in SAR discussion is marked by empty symbol.).

3864
J. Jampilek et al. / Bioorg. Med. Chem. Lett. 26 (2016) 3862–3865
Figure 2. Fluorescence emission spectra of chlorophyll a in suspension of spinach chloroplasts without and with compound 4b (R = 3-F; c = 0, 55, 110 and 220
lM, the curves
from top to bottom; excitation wave length k = 436 nm) (A) and fluorescence emission spectra of aromatic amino acids in suspension of spinach chloroplasts without and
with compound 4b (c = 0, 11, 22, 44 and 66 lM, the curves from top to bottom; excitation wave length k = 275 nm) (B). Chlorophyll concentration in chloroplast suspension:
10 mg/L.
the section between P680 (primary donor of PS II) and plasto-
quinone QB occurring on the acceptor side of PS II was damaged
by these PET inhibitors. The complete restoration of the photo-
chemical activity of chloroplasts treated with 4b or 6b at higher
DPC concentrations could be connected with the replacement of
these inhibitors from their binding site by sym-diphenylcarbazide
due to their direct interaction with the herbicide-binding niche,
similarly as it was demonstrated for atrazine24 or metribuzin.25
Inhibition of electron transport in PS II at the QB site before the site
of diuron by 4-hydroxyquinoline-N-oxides10 or quinolones and
quinolone N-oxides12 was reported previously.
antitubercular activity of compounds mainly against M. tuberculo-
sis was comparable, independently of compound lipophilicity, elec-
tronic properties of R substituent or its position of substitution.16
Similarly to the present results in relation to PET-inhibiting activ-
ity, compounds with potency against all three mycobacterial
strains were obtained by the substitution of C0(3) position of ani-
line; however, also the hydroxyl moiety in C(8) of quinoline seems
to play a significant role in antimycobacterial activity, because its
absence in quinoline-2-carboxanilides led to an activity decrease.32
An essential contribution of the hydroxyl moiety for amplifying
antimycobacterial potency was observed previously also for
1-hydroxynaphthalene-2-carboxanilides29 and 6-hydroxynaph-
The effect of the studied compounds on the fluorescence of
chlorophyll a (Chla) and aromatic amino acids (AAA) in spinach
thalene-2-carboxanilides33
carboxanilides.32
contrary
to
naphthalene-2-
chloroplasts was investigated as well, applying
a published
method.26 The DMSO concentration in all samples was the same
as in the control (10% (v/v)). The studied 8-hydroxyquinoline-2-
carboxanilides affected the chlorophyll a (Chla) fluorescence in spi-
nach chloroplasts. As shown in Figure 2A, the intensity of the Chla
emission band at 686 nm belonging to the pigment–protein com-
plexes in PS II decreased in the presence of compound 4b, indicat-
ing a perturbation of the Chla–protein complexes in the thylakoid
Acknowledgment
This study was supported by IGA VFU Brno 322/2015/FaF.
References and notes
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