Thermal and spectroscopic studies of 2,3,5-trisubstituted and 1,2,3,5-tetrasubstituted indoles as non-competitive antagonists of GluK1/GluK2 receptors
Thermal and spectroscopic studies of
Thermal and spectroscopic studies of 2,3,5-trisubstituted and 1,2,3,5- tetrasubstituted indoles as non-competitive antagonists of GluK1/ GluK2 receptors
Agata Bartyzel 0 2 3 4 5 6
Dariusz Matosiuk 0 1 2 3 4 5 6
Agnieszka A. Kaczor 0 1 2 3 4 5 6
Halina Głuchowska 0 2 3 4 5 6
Monika Pitucha 0 2 3 4 5 6
Tomasz M. Wr o´bel 0 1 2 3 4 5 6
0 Department of Synthesis and Chemical Technology of Pharmaceutical Substances with Computer Modeling Laboratory, Faculty of Pharmacy with Division of Medical Analytics, Medical University , 4a Chodzki St., 20093 Lublin , Poland
1 , 20031 Lublin , Poland
2 Department of General and Coordination Chemistry, Maria Curie-Skłodowska University , Maria Curie-Sklodowska Sq
3 & Agata Bartyzel
4 Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen , Universitetsparken 2, 2100 Copenhagen Ø , Denmark
5 Department of Organic Chemistry, Faculty of Pharmacy with Division of Medical Analytics, Medical University , 4a Chodzki St., 20093 Lublin , Poland
6 Department of Pharmaceutical Chemistry, School of Pharmacy, University of Eastern Finland , Yliopistonranta 1, P.O. Box 1627, FI-70211 Kuopio , Finland
This paper reports the thermal stability and thermal degradation of six derivatives of indole by means of TG-DSC (in air) and TG-FTIR (in nitrogen) techniques. The compounds were also characterized by infrared spectroscopy. In addition, IR spectra were calculated and compared with the experimental data. In particular, the potential energy distribution analysis was performed to assign IR signals. The studied compounds are characterized by good thermal stability in oxidizing and inert atmospheres which is important for potential medical application. Thermogravimetric measurements in air atmosphere showed that the decomposition of compounds proceeds in two or three main stages. The thermal degradation of compounds is preceded by the melting process. The pyrolysis of samples is a one-step process. Together with the analyses performed in nitrogen, the FTIR spectra of the evolved gas phase products were recorded. On the FTIR spectra of gaseous products, only the bands of water, carbon dioxide and carbon oxide molecules are present. In the case of indole derivatives containing the p-chlorobenzyl substituent in position 1, the bands of anisole, p-chlorotoluene and chlorobenzene also appear.
Indole derivatives; Thermal behaviour; IR spectra; PED analysis; Theoretical computations; Non-competitive antagonists of GluK1/GluK2 receptors
Indole is a well-known and important privileged structure
scaffold found in many natural and synthetic compounds.
This core is recognized as one of the foremost biologically
significant moieties in nature. The indoles are characterized
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10973-018-7146-6) contains supplementary
material, which is available to authorized users.
by excellent binding affinity for various receptors [
The indole-based compounds can also exhibit various
biological activities such as anti-inflammatory, analgesic,
antifungal, antimicrobial, insecticidal, antioxidant,
antiviral, antidepressant, antiarrhythmic, antihistaminic and
antidiabetic ones. [
]. According to the literature, there
are over ten thousand biologically active compounds
containing indole core. More than 200 of them were approved
as commercially available drugs or are undergoing clinical
], e.g., vinblastine (anticancer), indomethacin
(anti-inflammatory), arbidol (antiviral), delavirdine
(antiHIV), zafirlukast (anti-asthmatic), pravadoline (analgesic),
bucindolol (b-blocker) and roxindole (schizophrenia)
]. The indoles are also used as dyes, pigments, plastics,
fungicides, vitamin supplements, flavour enhancers, and
The main aim of this research was the thermal and
spectroscopic characterization of six indole derivatives,
i.e., 5-methoxy-2-(4-methoxyphenyl)-3-methylindole (1),
1-ethyl-2-phenyl-5-methoxy3-methylindole (5) and
11-ethyl-8-methoxy-6,11-dihydro5H-benzo[a]carbazole (6) (Scheme 1). The investigated
compounds were synthesized in the Fisher indolization
reaction from the respective arylhydrazine hydrochloride
and appropriate ketone in anhydrous boiling ethanol
saturated with HCl followed by alkylation with alkyl halide (with
application of sodium hydride). These compounds are a
noncompetitive antagonists of kainate GluK1/GluK2 (GluK—
glutamatergic kainate receptors) receptors with low
micromolar activity, and compound 2 is the most promising from
the series [
]. Non-competitive antagonists of kainate
] can be considered promising compounds
for the treatment of neurodegenerative diseases [
well as epilepsy . In particular, a non-competitive mode
of action can result in a better safety profile as reported for
non-competitive antagonists of
a-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid (AMPA) receptors [
In the previous studies, we also constructed 3D models of
GluK1 and GluK2 receptors and suggested that
indolederived non-competitive antagonists can bind in the receptor
transduction domain [
11, 14, 15, 20, 21
]. In this study, in
order to investigate thermal behaviour of indole derivatives
the TG-DSC and TG-FTIR methods were applied. The
compounds were also characterized by infrared
spectroscopy. In addition, infrared (IR) spectra of the studied
indole derivatives were calculated and compared with the
experimental data. Theoretical vibrational spectra of the
compounds were interpreted in terms of potential energy
distribution (PED) analysis. Shortly, in order to describe the
vibration of a N-atomic molecule using the PED analysis, the
construction of the set of 3N-6 local, linearly independent,
internal coordinates is required [
]. This set represents
stretching, bending and deformation motions of the
functional groups or the chosen fragments of the molecule [
The availability of such a coordination set instead of the
normal modes causes that the potential energy distribution
matrix, the PED matrix, ceases to be diagonal, but the energy
distribution originating from the motions of particular
functional groups is understandable for the interpreter [
The rationale of the performed research results from the
necessity of thermal stability of the compounds under
investigations which is important for their potential
Synthesis and short description of compounds
The investigated compounds were synthesized according to
the following procedure: the mixture of 0.05 mol of
arylhydrazine hydrochloride, 0.05 mol of ketone, 100 cm3 of
anhydrous ethanol and 10 cm3 of ethanol saturated with
HCl was refluxed for 4 h. The reaction mixture was left
overnight at room temperature. The obtained product was
Scheme 1 The studied indoles
filtered and purified by crystallization from ethanol and
washed with n-hexane. In the next step, 0.01 mol of the
indole derivative was dissolved in 30 cm3 of anhydrous
N,N-dimethylformamide (DMF). The reaction mixture was
cooled to 0 C, and 0.8 g of sodium hydride was added
(50% oil suspension). After 30 min of mixing, a solution of
0.012 mol of alkyl halide in 20 cm3 of anhydrous DMF
was added dropwise. The reaction was allowed to continue
at room temperature for 3 h. The mixture was filtered, and
30–40 cm3 of water was added to the filtrate. The obtained
precipitate was filtered off and purified by crystallization
from ethanol and washed with n-hexane. The detailed
physicochemical and spectral properties of the investigated
compounds were described in the previous paper [
The thermal behaviour of compounds was studied in air
and nitrogen atmospheres. The thermal stability and
decomposition in oxidizing atmosphere were determined
using the Setaram Setsys 16/18 derivatograph. The TG and
DSC curves were recorded in temperature range between
30 and 800 C. The samples (6.18–7.56 mg) were heated
in a ceramic crucible at the heating rate of 10 C min-1 in
flowing air (v = 0.75 dm3 h-1). The temperature and heat
flow of the instrument were calibrated by the melting point
Atm atmosphere of analysis, Tpeak DTG peak temperature (maximum
change of mass)
Methods and physical measurements
and enthalpy of indium standard. The studies in inert
atmosphere were carried out using the TGA Q5000
analyser (TA Instruments, New Castle, Delaware, USA). The
compounds (15.14–29.69 mg) were heated in an open
platinum crucible from ambient temperature
(* 23–25 C) to 700 C in flowing nitrogen (25 cm3 min-1).
Simultaneously with the TG analysis in nitrogen, the infrared
spectra of gaseous products were recorded using the Nicolet
6700 FTIR spectrophotometer (Thermo Scientific) in the
spectral range of 600–4000 cm-1 with a resolution of 4 cm-1
and 6 scans per spectrum. ATR-FTIR spectra were collected
using the Thermo Scientific Nicolet 6700 FTIR spectrometer
equipped with a Smart iTR diamond ATR accessory in the
range from 4000 to 500 cm-1. The samples were placed
directly on the diamond crystal of ATR accessory, and the
spectra were obtained from 16 scans taken at a resolution of
4 cm-1 and normalized.
Energy and geometry of compounds 1–6 were optimized
with the B3LYP DFT method (DFT—density functional
theory) and the 6-311G??(2df, 2pd) basis set of
Gaussian09 software [
]. Gaussian09 was also used to
calculate IR spectra. The computed IR spectra were corrected
using the scaling factor of 0.962 as recommended for this
level of theory [
]. Moreover, the computed vibrational
frequencies have been unambiguously assigned by means
of the potential energy distribution (PED) analysis of all
the fundamental vibration modes by using VEDA 4
] as described previously [
Results and discussion
Thermal behaviour of 1–6
The TG-DSC curves providing information about the
thermal properties of 1–6 are shown in Fig. 1. As it can be
seen in the TG curves, the compounds are characterized by
good thermal stability, which is a very important parameter
for their potential application as drugs. The first changes
are recorded on the DSC curves as endothermic peaks at
temperature 100–140 C. These peaks are not accompanied
by a mass loss and can be attributed to the melting process.
They are roughly similar to the values reported earlier and
determined using a Boetius apparatus [
]. The melting
point onset temperature (Tonset), peak temperature (Tpeak)
corresponding to each peak, and enthalpy of fusion taken
from the DSC curves for all compounds are listed in
Table 1. The melting peaks are sharp which indicates that
the compounds were probably synthesised as pure,
crystalline substances [
]. Generally, the substitution of
the pyrrole hydrogen atom in position 1 leads to a
decreased melting point. The exception is compound 4
where the melting point is comparable to that of compound
1. This can be a result of a lack of a substituent in the
5-position of the indole core.
The decompositions of the compounds in air are two- or
three-step processes that are noted on the TG curves (see
Fig. 1). For all compounds, the major mass loss
(56.98–95.93%) occurs in the first stage which starts at
183–251 C (Table 2). Taking into account the initial
temperature of the decomposition processes, the following
relative thermal stability order: 5 = 4 \ 2 \ 6 \ 1 \ 3 can
be established. As can be observed, the substitution of
hydrogen attached to nitrogen atom with the ethyl group
decreases the combustion temperature of compounds while
the p-chlorobenzyl substituent stabilizes the molecule and
increases the decomposition temperature of 3 in air
compared to 1. In the case of compounds 1 and 3, probably
during the first stage the substituents of the indole core are
broken off and combusted. The theoretical values of the
indole residue (C8H7N) are 43.82 and 29.95% while the
experimental ones are equal to 43.03 and 29.60% for
compounds 1 and 3, respectively. The formed products are
unstable and immediately undergo complete destruction
and combustion accompanied by a significant exothermic
effect. The samples are fully decomposed at 612–652 C. It
is worth mentioning that for compound 1 on the DTG curve
two maxima are visible during this stage, but this is not
clearly indicated on the TG curve. The remaining
compounds are almost completely decomposed during this step
(more than 85% of overall mass is lost), and the formed
residues are combusted during one (5 and 6) or two (2 and
Table 3 Experimental and
computed IR frequencies and
PED assignment of signals for
compound 2 (the most
promising compound within the
4) steps. Compounds 4, 5 and 6 are fully combusted at a
temperature of 570–603 C. In the case of 2, it was found
that under the measuring conditions a small amount of
unburnt organic matrix remains (1.49%).
Thermal behaviour of 1–6 was also studied in nitrogen
atmosphere (see Fig. 2). The order of thermal stabilities in
inert and oxidizing atmospheres is similar; taking into
account the initial temperature of the decomposition
processes under nitrogen stream, the following relative
thermal stability order: 5 & 4 \ 2 \ 6 \ 1 \ 3 can be
established. In contrast to the thermal decomposition in air,
the pyrolysis processes start at a lower temperature (about
12–22 C) than the combustion ones. Thermal
decompositions of compounds in nitrogen proceed in one major
mass loss step and similar to the combustion, the
compounds are almost completely burnt (for compound 2 the
residue after the pyrolysis is 1.54%). The total pyrolysis of
the compounds can be associated with the presence of
methoxy groups as observed for other compounds
containing such substituent [
]. Simultaneously with the
TG analysis in nitrogen, the FTIR spectra of gaseous
products were recorded. The FTIR spectra of gaseous
products evolved during the decomposition of compounds
1 and 3 are given in Fig. 3. In the spectra of gaseous
products, except for the compound 3, only the bands
characteristic of water, carbon monoxide and carbon
dioxide were present. The peaks at 2240–2400 cm-1 are
assigned to stretching vibration m(C–O) of carbon dioxide
molecule. In addition, a band at 669 cm-1 is observed due
to the deformation vibration of CO2. The double peaks in
the range 2050–2275 cm-1 correspond to the vibrations of
CO molecule. The characteristic bands of water molecules
are observed in the range 3450–4000 and 1300–1950 cm-1
for stretching and deformation vibrations, respectively
31, 32, 34
]. Probably other compounds such as indole can
be condensed in the transfer line and do not reach the
detector. In the case of compound 3, at a temperature
between 340 and 420 C of the pyrolysis process on the
FTIR spectra of evolved gases, several bands in the range
2700–3100 and 750–1800 cm-1 were recorded. These
peaks are probably due to the presence of anisole,
pchlorotoluene and chlorobenzene (see Fig. 4).
The FTIR spectra of 1–6 are given in Figs. S1–S6. The
observed and calculated frequencies in the infrared spectra
of studied compounds together with their PED assignment
of signals are presented in Tables 3 and S1–S5
(Supplementary material). The scaled computed IR spectra are in
accordance with the experimental values. The strong, sharp
peak at 3379 cm-1 in the FTIR spectrum of 1 is
characteristic of m(N–H) vibrations of the pyrrole ring. This is
consistent with the literature data; the indoles unsubstituted
in the l-position (N) give a sharp absorption peak in the
range 3500–3300 cm-1 [
]. This band was not
recorded in the spectra of other studied compounds due to
the substitution of hydrogen atom in position 1 by ethyl or
4-chlorobenzyl group. The peaks at 3100–2990 cm-1 can
be assigned to the C–H stretching vibrations of the
aromatic bonds. The symmetric and asymmetric C–H
stretching bands of the methyl/ethyl groups are observed in
2990–2800 cm-1. The other important peaks are observed
at the range 1620–1400 cm-1, and they are mainly due to
the stretching vibrations m(CC) of indole and benzene rings.
The varying intensity bands observed in the FTIR spectra
at 1373–1342 and 1287–1283 cm-1 can be assigned to the
C–N stretching modes of pyrrolic ring. These assignments
are in agreement with the literature [
35, 39, 41
correlate with the theoretical calculations. The remaining
bands characteristic of 1–6 with their detailed description is
given in Tables 3 and S1–S5.
Thermal analysis of six indoles derivatives was performed.
The investigated compounds are stable at room
temperature which is important for their medical application. The
DSC melting peaks of compounds are sharp indicating that
they are probably crystalline, pure substances. In air
atmosphere, the decomposition process of 1–6 occurs in
two or three stages where the main mass loss occurs during
the first one. On the basis of the TG-DSC analysis, it can be
concluded that the substitution of hydrogen atom by the
ethyl group on the pyrrole ring in 1 position leads to
decrease in the thermal stability of the studied indoles.
Changing the substituent to the p-chlorobenzyl group
causes the compound stabilization and, consequently, an
increase in the decomposition temperature of 3 compared
to that of the other compounds. The performed TG-FTIR
analysis showed there are no residual solvents in the
structure of studied compounds.
Acknowledgements The paper was developed using the equipment
purchased within the projects ‘‘The equipment of innovative
laboratories doing research on new medicines used in the therapy of
civilization and neoplastic diseases’’ and ‘‘Enhancement of the Research
and Development Potential of the UMCS Faculty of Chemistry and
the Faculty of Biology and Earth Sciences’’ within the Operational
Program Development of Eastern Poland 2007–2013, Priority Axis I
Modern Economy, operations I.3 Innovation promotion. Calculations
with Gaussian 09 were performed under a computational grant by
Interdisciplinary Center for Mathematical and Computational
Modeling (ICM), Warsaw, Poland, grant number G30-18 and under
resources and licenses by CSC, Finland.
Open Access This article is distributed under the terms of the
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tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
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