Synthesis and analytical evaluation by voltammetric studies of some new indole-3-propionamide derivatives

Article abstract of DOI:10.1016/S0014-827X(01)01150-8

Some biologically important and melatonin-related indole-3-propionamide derivatives were synthesized. The compounds synthesized were analyzed and characterized first by NMR and mass spectrometry and then investigated by analytical voltammetric techniques. Based on this study a simple, rapid and sensitive voltammetric method was developed for the determination of the indole derivatives that are readily oxidized at the carbon-based electrodes. The oxidative behavior of the indole derivatives was studied as a function of pH at a glassy carbon electrode. The characteristics of the corresponding electrode reaction were discussed.

Full text of DOI:10.1016/S0014-827X(01)01150-8

www.elsevier.com/locate/farmac
Il Farmaco 56 (2001) 835–840
Synthesis and analytical evaluation by voltammetric studies of
some new indole-3-propionamide derivatives
a,
a
b
c
8
Sibel Su¨zen *, Zeynep Ates¸-Alago¨z , B. Tolga Demircigil , Sibel A. Ozkan
a Department of Pharmaceutical Chemistry, Faculty of Pharmacy (Eczacılık), Ankara Uni6ersity, 06100 Tandog˘an, Ankara, Turkey
b Department of Analytical Chemistry, Faculty of Pharmacy, Gazi Uni6ersity, 06330 Etiler, Ankara, Turkey
c Department of Analytical Chemistry, Faculty of Pharmacy (Eczacılık), Ankara Uni6ersity, 06100 Tandog˘an, Ankara, Turkey
Received 20 January 2001; accepted 12 June 2001
Abstract
Some biologically important and melatonin-related indole-3-propionamide derivatives were synthesized. The compounds
synthesized were analyzed and characterized first by NMR and mass spectrometry and then investigated by analytical
voltammetric techniques. Based on this study a simple, rapid and sensitive voltammetric method was developed for the
determination of the indole derivatives that are readily oxidized at the carbon-based electrodes. The oxidative behavior of the
indole derivatives was studied as a function of pH at a glassy carbon electrode. The characteristics of the corresponding electrode
reaction were discussed. © 2001 Elsevier Science S.A. All rights reserved.
Keywords: Indole-3-propionic derivatives; Synthesis; Voltammetry; Analysis
1. Introduction
Voltammetric techniques in general, and differential
pulse voltammetry in particular, are considered to be
useful tools for the determination of indole derivatives
[8,9,11].
Indole derivatives such as melatonin, melatonin-re-
lated indole structures, and indole-3-propionic (IPA)
derivatives constitute an important class of antioxidant
agents [1–7] potentially useful against Alzheimer’s dis-
ease. Indole and its derivatives are the well-known
electro active compounds that are readily oxidized at
carbon based electrodes, i.e. glassy carbon electrode
and therefore analytical procedures, based on liquid
chromatography with electrochemical detection [8] and
voltammetry [9,10] have been developed for the deter-
mination of important indoles such as tryptamine and
serotonin in complex mixtures.
This paper reports the electrochemical behavior of
synthesized indole derivatives. In order to understand
the electrochemical process that occurs on glassy car-
bon electrode, both pH and scan rate studies were
carried out. The main purpose of this work was to
establish experimental conditions for the electrochemi-
cal oxidation and determination of synthesized indole
derivatives using a glassy carbon electrode. For the
quantitative determination, differential pulse voltam-
metric method was applied to the indole derivatives.
Practical application of electrochemistry includes the
determination of electrode oxidation mechanisms. Ow-
ing to the existing resemblance between electrochemical
and biological reactions, it can be assumed that the
oxidation mechanisms taking place at the electrode and
in the body share similar principles.
2. Experimental
2.1. Synthesis and 6oltammetric measurements of
indole-3-propionamide deri6ati6es
2.1.1. Apparatus and chemicals
Uncorrected melting points (m.p. (dec.)) were deter-
mined with a Bu¨chi SMP-20 apparatus. The H NMR
* Corresponding author.
E-mail address: sibel@pharmacy.ankara.edu.tr (S. Su¨zen).
1
0014-827X/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved.
PII: S0014-827X(01)01150-8

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S. Su¨zen et al. / Il Farmaco 56 (2001) 835–840
spectra were determined with a Bruker GmbH DPX-
400, 400 MHz using CDCl3 as solvent and TMS as an
internal standard (chemical shifts in l, ppm). Mass
spectra were determined on a VG Platform II spectrom-
eter by using Electron Ionization (Ionization Energy 70
eV). Chromatography was carried out using the flash
method and Merck silica gel 60 (230–400 mesh
ASTM). All the amines and indole-3-propionic acid
were purchased from Aldrich. Petroleum spirit 60–
80 °C was used.
The electrochemical measurements were made with
BAS 100 W electrochemical analyzer (Bioanalytical
System, USA). Voltammetric measurements utilized a
glassy carbon working electrode (f 3 mm, BAS), a
platinum wire auxiliary electrode and Ag/AgCl (NaCl
3M, BAS) reference electrode. Before each experiment,
the glassy carbon electrode was polished manually with
alumina (f 0.01 mm,) in the presence of double distilled
water on a smooth polishing cloth.
Stock solutions under voltammetric investigation
were prepared daily by direct dissolution in methanol.
The working solutions were prepared by the dilution of
the stock solution with selected supporting electrolytes.
Three different supporting electrolytes, namely sulfuric
acid (0.5 M), phosphate buffer (0.2 M, pH 5–7) and
Britton–Robinson buffer (0.04 M, pH 2–11) were pre-
pared in double distilled water.
gaseous HCl and refluxing at 70 °C for 15 h. The
methanol was evaporated to dryness on a rotary evapo-
rator and then the crude product was extracted by ethyl
acetate and purified by column chromatography
(CHCl3–hexan). The yield was 65%. All the amide
derivatives (3a–3g) (Fig. 1) were synthesized by using
the appropriate amine and compound 2.
Compound 2 (1 mol) was dissolved in the appropri-
ate amine (5 ml) and the mixture was refluxed by
heating under nitrogen. The reaction was followed by
TLC until no starting material could be detected. The
excess amine was evaporated to dryness on a rotary
evaporator using 40 °C water bath. The crude product
was then purified by column chromatography and char-
acterized by NMR and mass analyses.
Compounds 3a and 3b were prepared as described in
Ref. [10].
2.1.2.1. Synthesis of 3-(N-pentyl-propanamide-3-yl)-in-
dole (3c). Compound 2 was treated with pentyl amine
1
according the general procedure. H NMR: l 0.79 (3H,
t, CH3), 1.19 (2H, sext, CH2ꢀCH3), 1.26 (2H, m,
CH2ꢀCH2ꢀCH3), 1.30 (2H, m, NHꢀCH2ꢀCH2), 2.49
(2H, t, NHꢀCH2), 3.05 (2H, t, CH2ꢀCO), 3.10 (2H, t,
CH2ꢀCH2ꢀCO), 5.25 (1H, brs, NHꢀCO), 6.95 (1H, d,
indol-H2), 7.05 (1H, t, indol-H6), 7.13 (1H, t, indol-H7),
7.28 (1H, d, indol-H5), 7.53 (1H, d, indol-H8), 7.74 (1H,
brs, indol-NH). EI MS; m/z (%): 259 (12.3) [M+1]+
,
’
258 (56.4) [M]+ , 144 (77.2), 142 (100), 130 (100), 114
(29.4), 101 (13.1), 90 (5.3), 88 (15.9), 84 (20.6), 82
(28.2), 71 (38.8), 58 (47.8), 54 (40.3), 43 (53.3), 41
(60.7), 27 (36.8).
’
2.1.2. General procedure for the synthesis of
indole-3-propionamide deri6ati6es
Compound 2 (m.p. (dec.) 79 °C) was prepared by the
esterification of compound 1 in dry methanol using
Fig. 1. Synthesis of the indol-3-propionamide derivatives.

S. Su¨zen et al. / Il Farmaco 56 (2001) 835–840
837
Table 1
Indole 3-propionamide derivatives
* n-Hexane.
2.1.2.2. Synthesis of 3-[N%-(N,N-dimethylaminoethyl)-
propanamide-3-yl]-indole (3d). Compound 2 was treated
with 2-dimethylamino-1-ethyl amine according the gen-
2.39 (6H, m, CH2ꢀN(CH2)2), 2.50 (2H, t, NHꢀCH2),
3.04 (2H, t, CH2ꢀCO), 3.18 (2H, q, CH2ꢀCH2ꢀCO),
6.02 (1H, brs, NHꢀCO), 6.92 (1H, d, indol-H2), 7.03
(1H, t, indol-H6), 7.10 (1H, t, indol-H7), 7.25 (1H, d,
indol-H5), 7.52 (1H, d, indol-H8), 8.23 (1H, brs, indol-
1
eral procedure. H NMR: l 2.06 (6H, s, 2CH3), 2.48
(2H, t, CH2ꢀN), 2.51 (2H, t, NHꢀCH2), 3.05 (2H, t,
CH2ꢀCO), 3.10 (2H, q, CH2ꢀCH2ꢀCO), 5.97 (1H, brs,
NHꢀCO), 6.94 (1H, d, indol-H2), 7.04 (1H, t, indol-H6),
7.11 (1H, t, indol-H7), 7.26 (1H, d, indol-H5), 7.53 (1H,
NH). EI MS; m/z (%): 288 (13.0) [M+1]+ , 287 (5.5)
’
[M]+ , 144 (1.5), 143 (2.9), 130 (9.7), 115 (2.0), 100
’
(5.6), 99 (13.0), 86 (100), 72 (1.5), 71 (1.5), 58 (8.9), 42
(2.8), 34 (10.0), 30 (18.8), 27 (32.8).
d, indol-H8), 8.0 (1H, brs, indol-NH). EI MS; m/z (%):
+
260 (5.45) [M+1]+ , 259 (6.5) [M] , 188 (3.1), 144
(3.0), 143 (6.0), 130 (17.3), 115 (4.7), 105 (3.6), 103
(2.4), 89 (2.1), 84 (4.2), 82 (3.2), 77 (5.2), 72 (19.5), 71
(53.1), 58 (100), 42 (9.8), 32 (27.8).
’
’
2.1.2.5. Synthesis of 3-[N%-(N,N-diethylaminopropyl)-
propanamide-3-yl]-indole (3g). Compound 2 was treated
with 3-diethylamino-1-propyl amine according the gen-
1
eral procedure. H NMR: l 0.86 (6H, t, 2CH3), 1.46
(2H,
m,
CH2ꢀCH2ꢀN),
2.30–2.36
(6H,
m,
2.1.2.3. Synthesis of 3-[N%-(N,N-dimethylaminopropyl)-
propanamide-3-yl]-indole (3e). Compound 2 was treated
with 3-dimethylamino-1-propylamine according the
CH2ꢀN(CH2)2), 2.45 (2H, t, NHꢀCH2), 3.04 (2H, t,
CH2ꢀCO), 3.22 (2H, q, CH2ꢀCH2ꢀCO), 6.91 (1H, brs,
indol-H2), 7.02 (1H, t, indol-H6), 7.09 (1H, t, indol-H7),
7.25 (1H, d, indol-H5), 7.51 (1H, d, indol-H8), 8.41 (1H,
general procedure. 1H NMR:
l 1.19 (2H, m,
CH2ꢀCH2ꢀN), 2.16 (6H, s, 2CH3), 2.48 (2H, t, CH2ꢀN),
2.50 (2H, t, NHꢀCH2), 3.04 (2H, t, CH2ꢀCO), 3.20 (2H,
q, CH2ꢀCH2ꢀCO), 6.65 (1H, brs, NHꢀCO), 6.94 (1H,
d, indol-H2), 7.04 (1H, t, indol-H6), 7.11 (1H, t, indol-
H7), 7.27 (1H, d, indol-H5), 7.52 (1H, d, indol-H8), 8.13
brs, indol-NH). COꢀNH not observed. EI MS; m/z
(%): 302 (7.1) [M+1]+ , 301 (8.0) [M] , 272 (6.7), 229
(6.6), 144 (3.8), 142 (17.9), 130 (18.3), 129 (33.4), 114
(12.0), 111 (8.4), 99 (12.1), 85 (100), 71 (48.6), 57 (34.0),
43 (13.9), 41 (33.5), 30 (21.0), 27 (29.2).
+
’
’
(1H, brs, indol-NH). EI MS; m/z (%): 273 (14.5) [M+
+
1]+ , 272 (32.4) [M] , 142 (39.1), 130 (18.3), 128
(52.1), 114 (23.2), 101 (19.1), 97 (6.0), 89 (12.3), 83
(71.3), 75 (8.2), 72 (13.1), 69 (32.5), 57 (100), 42 (100),
34 (10.0), 30 (18.8), 27 (32.8).
’
’
3. Results and discussion
All synthesized compounds were analytically charac-
terized and are represented by the general formula in
Table 1.
The voltammetric oxidation of the synthesized indole
derivatives at the glassy carbon electrode was investi-
2.1.2.4. Synthesis of 3-[N%-(N,N-diethylaminoethyl)-
propanamide-3-yl]-indole (3f). Compound 2 was treated
with 2-diethylamino-1-ethyl amine according the gen-
1
eral procedure. H NMR: l 0.85 (6H, t, 2CH3), 2.33–

838
S. Su¨zen et al. / Il Farmaco 56 (2001) 835–840
gated in the pH range of 1.5–11.0 and different sup-
porting electrolytes. Current–voltage curves of all the
investigated indole derivatives showed one well-defined
oxidation peak at pH= or \4. It was observed that
this single peak split into two overlapping peaks as the
acidity was increased (Fig. 2).
rent of all indole derivatives (3a–3g) were evaluated.
The linear increase in the oxidation peak current with
the square root of the scan rate with similar slopes and
correlation coefficients, showed the diffusion control
process (Table 2). Positive shift in the peak potential
was observed, which also confirms the irreversibility of
the process, with a simultaneous increase in diffusion
current when the scan rate was increased. A plot of
The effects of potential scan rate between 10 and
1000 mV s−1 on the peak potential and the peak cur-
Fig. 2. Cyclic voltammogram of 1×10−4 M compound 3a in Britton–Robinson buffer (20% MeOH); pH values a=2, b=4, c=7, d=9; scan
rate 100 mV s−1
.
Table 2
Square root of the scan rate−peak current and logarithm of peak current−logarithm of scan rate results of synthesized indole derivatives
Samples
Equation of ꢀ6−Ip
Correlation coefficient
Equation of log 6−log Ip
Correlation coefficient
3a
3b
3c
3d
3e
3f
y=0.368x−0.339
y=0.322x−0.476
y=0.431x−0.470
y=0.352x+0.165
y=0.305x+0.374
y=0.793x−1.180
y=1.218x−3.962
r=0.993
r=0.995
r=0.994
r=0.999
r=0.995
r=0.997
r=0.991
y=0.513x−0.504
y=0.510x−0.536
y=0.552x−0.530
y=0.483x−0.396
y=0.425x−0.308
y=0.608x−0.404
y=0.699x−0.530
r=0.995
r=0.997
r=0.998
r=0.999
r=0.994
r=0.997
r=0.999
3g

S. Su¨zen et al. / Il Farmaco 56 (2001) 835–840
839
Fig. 3. Effects of the pH on compound 3a peak potential.
Table 3
Calibration characteristics of synthesized indole derivatives
Sample
Medium
Linearity range (M) Equation
Correlation
Measured
coefficient
potential (V)
3a
3b
3c
3d
3e
3f
pH 2 Britton–Robinson buffer (%20 MeOH)
PH 5 Britton–Robinson buffer (%20 MeOH)
pH 3 Britton–Robinson buffer (%20 MeOH)
pH 10 Britton–Robinson buffer (%20 MeOH)
pH 2 Britton–Robinson buffer (%20 MeOH)
pH 5 Britton–Robinson buffer (%20 MeOH)
pH 10 Britton–Robinson buffer (%20 MeOH)
1×10−5–8×10−5
1×10−5–1×10−4
1×10−5–1×10−4
1×10−5–1×10−4
1×10−5–8×10−5
1×10−5–8×10−5
1×10−5–1×10−4
y=26333x−0.1246
r=0.998
r=0.998
r=0.997
r=0.998
r=0.996
r=0.987
r=0.998
0.87
0.73
0.83
0.51
0.87
0.73
0.51
y=19322x−0.1019
y=24811x−0.1119
y=21139x+0.0411
y=21054x−0.056
y=36842x+0.287
y=31968x+0.197
3g
logarithm of peak current versus the logarithm of the
scan rate gave a straight line with similar slopes and
correlation coefficients for all the indole derivatives.
Slopes of 0.50 and 1.0 are expected for ideal reactions
of solution and surface species, respectively [12].
buffer at different pH (Table 3). Different linear cali-
bration curves were obtained for indole derivatives. The
characteristics of the calibration plots are listed in
Table 3.
Fig. 4 shows a typical differential pulse voltam-
mogram obtained follow by analysis of 3a derivative.
This figure is given as example of DPV curve for all the
synthesized indole derivatives.
Voltammetric techniques are most suitable to investi-
gate the redox properties of a new drug, this can give
insights into its metabolic fate [13]. Some metabolites
can be differentiated from the parent drug since metab-
olization often proceeds through the addition or the
modification of a substituent, this will give rise to
additional waves or to a shift of the main wave. Such
situation may allow the simultaneous determination of
the initial compound and the metabolized drug. Cyclic
voltammetry has been used in studying the redox mech-
anism [14] that is related to antioxidant activity of the
indole derivatives synthesized in this study. The results
The peak potential shifted towards negative poten-
tials with an increase in pH. Similar peak potential–pH
curves were obtained for all the indole derivatives.
Thus, compound 3a is given in Fig. 3 as an example.
The quantitative evaluation is based on the depen-
dence of the peak current on indole derivatives concen-
trations. The application of the differential pulse
waveform (pulse amplitude: 50 mV) yielded voltam-
mograms in which the peak currents were greater than
those obtained by cyclic voltammetry. A pulse interval
of 0.25 s gave rise to the sharpest and symmetrical peak
shape. The optimum scan rate was found to be 20
mV s−1 for all samples. For analytical purposes, best
response (with regard to peak current sensitivity and
morphology) was obtained with Britton–Robinson

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S. Su¨zen et al. / Il Farmaco 56 (2001) 835–840
Acknowledgements
This work was supported by the Ankara University
Research and Development Grant 99-03-00-03.
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