Kinetic and polarographic study on atropine N-oxide: its obtaining and polarographic reduction

Article abstract of DOI:10.1007/s11696-021-01638-3

The work presents the obtaining of atropine N-oxide using various peroxyacids (organic monoperoxyacid, diperoxyacids and inorganic peroxyacids). The kinetics of atropine oxidation with various oxidants, for example Oxone, m-chloroperoxybenzoic acid, diperoxysebasic acid and diperoxyazelaic acid, was studied. The optimal conditions for obtaining of atropine N-oxide (oxidation duration, pH) are given in the work. It was established that the best oxidant was potassium peroxymonosulfate, since 100% yield of atropine N-oxide was achieved within 15?min. In this work, we showed that the oxidation reaction of atropine to N-oxide was a second-order reaction. The rate constants of these reactions were established. The electrochemical behavior of atropine N-oxide obtained using potassium peroxymonosulfate and m-chloroperoxybenzoic acid on a mercury dropping electrode was investigated. Atropine N-oxide was reduced forming two peaks. Each reduction peak involved 1 electron and 1 proton.

Full text of DOI:10.1007/s11696-021-01638-3

Chemical Papers
https://doi.org/10.1007/s11696-021-01638-3
ORIGINAL PAPER
Kinetic and polarographic study on atropine N‑oxide: its obtaining
and polarographic reduction
Liliya Dubenska1 · Olha Dushna1 · Mykola Blazheyevskiy2 · Solomiya Pysarevska3 · Iryna Klymiuk1
Received: 6 February 2021 / Accepted: 1 April 2021
© Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
The work presents the obtaining of atropine N-oxide using various peroxyacids (organic monoperoxyacid, diperoxyacids and
inorganic peroxyacids). The kinetics of atropine oxidation with various oxidants, for example Oxone, m-chloroperoxybenzoic
acid, diperoxysebasic acid and diperoxyazelaic acid, was studied. The optimal conditions for obtaining of atropine N-oxide
(oxidation duration, pH) are given in the work. It was established that the best oxidant was potassium peroxymonosulfate,
since 100% yield of atropine N-oxide was achieved within 15 min. In this work, we showed that the oxidation reaction of
atropine to N-oxide was a second-order reaction. The rate constants of these reactions were established. The electrochemical
behavior of atropine N-oxide obtained using potassium peroxymonosulfate and m-chloroperoxybenzoic acid on a mercury
dropping electrode was investigated. Atropine N-oxide was reduced forming two peaks. Each reduction peak involved 1
electron and 1 proton.
Keywords Atropine · N-oxide · Peroxyacid · Kinetics · Electrochemistry
Introduction
of the circular muscle of the iris; closing the angle of the
anterior chamber of the eye and obstruction of the outfow
of the intraocular fuid, which increases the intraocular
pressure; accommodation paralysis due to relaxation of the
cervical muscle and reduction of lens curvature (impaired
close vision); suppresses the reaction of the pupils to light
(Ainsworth 2014).
Atropine (Atr) is an alkaloid naturally occurring in a num-
ber of Solanaceae plants like Atropa belladonna, Datura and
Mandragora (Brust 2004). Atropine inhibits the response of
many organs and tissues to impulses from parasympathetic
nerve fbers, as well as their sensitivity to acetylcholine
introduced from outside (Rodrigo 2002). This alkaloid is
also used to treat spasms in the stomach, intestines, bladder
or other organs. Atr reduces the secretion of all glands of
the digestive tract, as well as salivary causing dryness in
the mouth. In addition, Atr inhibits sweating, but in high
doses, it can increase body temperature (Mofatt 1986). It
also causes several eye efects: mydriasis due to relaxation
Due to such a wide range of action, Atr is used to treat
bradycardia (Rang 2007), decrease saliva production during
surgery, peptic ulcer and duodenal gastritis, acute pancreati-
tis, gallstone and urolithiasis. In gynecology, it is used in the
treatment of persistent spasm of the cervix and gastanium of
the uterus, when is the threat of its rupture. In neurology, Atr
is used to reduce salivation in bulbar syndrome, sometimes
in urinary incontinence (“Atropine” 2015). Previously, it was
used in reanimation in case of cardiac arrest, but in 2010,
due to lack of evidence of its efectiveness, it was forbidden
for such use (Field 2010). In ophthalmology, Atr is used
during eyeglass selection, for examination of the fundus,
keratitis, and to slow the progression of myopia in children
(Lindsley 2011). Atr is also used as an antidote agent when
poisoning with anticholinesterase drugs or organophospho-
rus compounds. In pharmacology, it is known in the form of
salt namely atropine sulfate. Atropine sulfate is most com-
monly used in the form of injectable solutions and eye drops.
* Solomiya Pysarevska
solomiya.pysarevska@lnu.edu.ua
1
Department of Analytical Chemistry, Ivan Franko National
University of Lviv, Kyryla i Mephodia Str. 8, Lviv 79005,
Ukraine
2
Department of Physical and Colloid Chemistry, National
Pharmaceutical University, Valentynivska Str. 4,
Кharkiv 61168, Ukraine
3
Department of Life Safety, Ivan Franko National University
of Lviv, Doroshenko Str. 41, Lviv 79000, Ukraine
Vol.:(0123456789)
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Chemical Papers
At the same time, these efects also make Atr to be poison,
and there are known cases of such use of it (Brown 2019).
According to chemical structure, Atr is an alkaloid of
a group of tropan derivatives and is similar in structure to
cocaine (Fig. 1) (Ainsworth 2014). Various methods were
used for quantitative determination of Atr such as non-
aqueous (Brown 2019, Turkevich 2003) and redox titration
(Dubey 2004), extraction spectrophotometry, high-per-
formance liquid chromatography (HPLC) (Rbeida 2005),
polarography (Blazheyevskiy 2012), potentiometry using
ion-selective electrodes (Bibire 2003), chromatomospec-
trometry (Chen 2006) and chemiluminescence (Sun 2006).
European Pharmacopeia recommends using HPLC with
UV-detection at 257 nm for quantitative determination of
Atr sulfate in eye drops and solution for injection (European
pharmacopeia 2007).
of physiologically active substances due to similarity of
these reactions to those occurring in metabolism processes.
The nature of products and the rate of metabolism process
can diferentiate under various factors such as medium and
nature of reagents. It is widely accepted that electron trans-
fer processes occurring on electrodes and in living cells are
similar to some extent. Therefore, voltammetric methods are
appropriate to be used for investigation of redox processes,
stability, ability to adsorb as well as kinetics of hydrolysis of
biologically active substances (Wang 1988, Blazheyevskiy
2017). For the analytical chemistry specialists, the mecha-
nism of the chemical process is interesting in terms of the
interpretation of the analytical signal, in order to optimize
the conditions of determination. Thus, the aim of this study
was the investigation of kinetics of atropine oxidation, as
well as the mechanism of reduction of corresponding oxida-
tion products on the dropping mercury electrode.
Oxidation reactions of drugs are very important for
understanding the processes of metabolism, as well as the
degradation of such substances in the environment. Several
scientifc groups reported oxidation of Atr by various oxi-
dants (Fawzy 2016, Meti 2014, Vaz 2016). Strong oxidants
like permanganate ion in the medium of perchloric and
sulfuric acid solutions oxidized Atr to tropine and phenyl-
malonic acid (Fawzy 2016).
Experimental
Materials
Chemicals
It is known that the main metabolites of Atr are nor-atro-
pine (24%), atropine N-oxide (equatorial isomer) (15%),
tropine (2%) and tropic acid (3%) (Van Der Meer 1986). In
the work (Van Der Meer 1986), m-chloroperoxybenzoic acid
(mCPBA) was used to obtain the N-oxide of Atr. Tertiary
nitrogen of Atr is easily oxidized by peroxyacids resulting
in formation of N-oxide (Mandava 1968). There are some
other works (Craig 1970, Lu 1987, Bai 1995, Cai 2012, Bel-
ler 2014) devoted to the study of oxidation of alkaloids to
N-oxide, as well as oxidation of Atr (Blazheyevskiy 2012).
Earlier we have reported the polarographic determination
of alkaloids and amide type anesthetics, that was based on
the obtaining of polarographically active derivatives, namely
N-oxides, and we have proposed mechanism of N-oxide
reduction (Plotycya 2016, Dubenska 2020, Plotycya 2019).
The experts in medical chemistry are highly interested in
mechanism of chemical and electrochemical redox reactions
Atropine sulfate. The substance of atropine sulfate mono-
hydrate (CAS 5908–99-6) with content of active substance
99% was purchased from SIGMA-ALDRICH. Standard
solution of Atr with concentration of 0.5·10–2 mol L–1 was
prepared by dissolving the exact amount in distilled water in
volumetric fask. Working solutions of Atr were prepared by
diluting the standard solution. Aqueous solution of Atr was
stable when stored in the refrigerator for two weeks.
The oxidizing agents utilized in the work were triple
potassium salt of Caro acid – Oxone, m-chloroperoxyben-
zoic acid, diperoxysebasic acid (DPSA) and diperoxyazelaic
acid (DPAA).
Oxidizing agents. “Extra pure” Oxone was purchased
from Acros Organics. The active ingredient of Oxone is
potassium peroxymonosulfate (KPMS), KHSO5 (CAS
10058–23-8), commonly known as potassium monopersul-
fate, that is present as a component of a triple salt with the
formula 2KHSO5·KHSO4·K2SO4 potassium hydrogen per-
oxymonosulfate sulfate, (CAS 70693–62-8). This reagent is
well soluble in water and its solutions are quite stable during
exploitation and storage (Blazheyevskiy 2017, Belej 1986,
Steele 1982). Stock solution of 1·10–2 mol L–1 Oxone was
prepared by dissolving the exact amount in distilled water
in a 100 mL volumetric fask.
m-Chloroperoxybenzoic acid (CAS 937–14-4) was pur-
chased from Acros Organics. The content of active sub-
stance according to quality certifcate was 70.2%; and also
it contained 10.9% of 3-chlorobenzoic acid and 18.4% of
Fig. 1 Structure formula of Atr
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Chemical Papers
crystallization water. 2.0·10–3 mol L−1 solution of mCPBA
was prepared by dissolving the exact amount in 20 mL of
ethanol in 100 mL volumetric fask. The volume of solution
was adjusted to the mark at 20 ºC with distilled water and
mixed thoroughly. This solution was used for polarographic
studies. For kinetic investigation, higher concentrations of
mCPBA were used. For this purpose, 0.35 g of mCPBA was
suspended in 100 mL of double-distilled water at tempera-
ture 30 ºC. The obtained suspension was fltered and then
aqueous solution was standardized by iodometric titration.
The concentration of this solution was 1·10–2 mol L–1.
Diperoxy acids (DPAs) (diperoxysebasic acid, diperoxya-
zelaic acid) were synthesized using the known procedure
described by Swern (Parker 1957, Blazheyevskiy 2014).
The corresponding carboxylic acid was dissolved under
stirring in concentrated sulfuric acid (95–98%) in an open
beaker. The obtained solution was cooled in ice water bath
to 10–15 °C. 50–65% H2O2 was slowly added dropwise
during 15 min under constant stirring. White precipitate
formed in all cases. The white precipitate was fltered out
and washed with cold half saturated ammonium sulfate solu-
tion (4·10 mL). The obtained product was dried at room
temperature. The content of main substance according to
iodometric titration was 96–98%.
electrode were used. The accuracy of potential measure-
ment is 1 mV. The uncertainty of current measurement is
0.1%. The employed DME had the following characteris-
tics: m = 5.94·10–4 g·s−1; τk = 10 s in 0.2 mol L–1 NH4Cl
with open circuit.
The study of electrochemical process was conducted
using cyclic voltammetry (VA) and the linear sweep
VA. This method can be considered as a quick screening
method, which is characterized by a low limit of determi-
nation and a quick response.
The pH of the solutions was measured potentiometri-
cally using MV 870 DIGITAL-pH- MESSGERÄT pH-
meter with silver chloride reference electrode.
Procedure of kinetic studies
All kinetic measurements were conducted in aqueous solu-
◦
tion under second-order conditions at 293 0.2 K. The
reaction of Atr with DPAs was carried out in oxygen-free
bufer solution by observing the decrease in diperoxy acids
concentration using iodometric titration under conditions
of [DPAs] > [Atr].
20.0 mL of Atr standard solution, 50 mL of bufer solu-
tion and 20.0 mL of mCPBA standard solution or 20.0 mL
of KPMS or 10.0 mL of DPA were successively intro-
duced in 100 mL volumetric fask. The countdown was
started (the chronometer was turned on) and the mixture
was adjusted to the mark with the bufer solution. After
1 min, 5 min, 10 min, 20 min and 30 min, 10.00 mL of the
reaction mixture was sequentially collected and added to
150 mL conical fasks containing 1.0 mL of 1 mol L–1 sul-
furic acid and 1.0 mL of 5% KI solution for titration. The
oxidation of Atr by peroxyacid stopped in acidic medium.
The oxidant residue interacted with I–. After 1 min, the
released iodine was titrated with sodium thiosulfate solu-
tion until the yellow color of the triiodide disappeared.
Signifcant spontaneous decomposition of peroxyacid
during the experiment was not observed. It was established
in a blank experiment of iodometric titration of peroxyacid
solutions.
Stock solutions of DPAs (2.0·10–2 mol L−1) were prepared
by dissolving pure compound in ethanol. Intermediate solu-
tions were prepared from stock solution using a mixture of
ethanol/water (50:50, v/v). Working solutions were prepared
from Intermediate solutions using the same solvent. The
fnal ethanol content of all calibrators and quality control
samples was less than 5%.
The Britton-Robinson bufer was used to provide the
required pH. 20.2 g of Na2B4O7·10H2O, 28.7 mL of glacial
acetic acid CH3COOH and 17.6 mL of H3PO4 orthophos-
phoric acid were dissolved in the volumetric fask. The
necessary pH was obtained by adding 2.5 mol L–1 sodium
hydroxide (monitored with pH meter) and distilled water up
to 1 L of solution.
Also, there were used sulfuric, hydrochloric acid, 5%
solution of KI, and sodium thiosulfate solution (standardized
iodometrically with a concentration of 0.01–0.02 mol L–1).
All these reagents were “pure for analysis” (qualifcations
acquired in Ukraine).
The reaction was followed by estimating the unreacted
DPAs as a function of time using the iodometric method.
The liberated iodine was titrated against standard sodium
thiosulfate solution. From the titer values, plots of
Purifed argon was used to remove dissolved oxygen.
(
)
(
)
Ct
Ct
Apparatus
log
or for DPA as log
vs. time
CAtr− C −C
(
CAtr−2 C −C
(
)
)
0
t
0
t
were constructed and the slope of such plots gave the sec-
ond-order rate constants, kobs (L mol−1 min−1). C0is the
initial molar concentration of oxidant (for time “0” min),
mol L−1; CAtr—concentration of Atr, mol L−1; Ct—current
concentration of oxidant (for time t, min), mol L−1. The
results were reproducible within 5% error.
Voltammetric measurements were carried out on digital
device MTech OVA-410 [MTech Lab) and temperature-
controlled three-electrode cell, volume 10 mL. An indi-
cator dropping mercury electrode (DME), a saturated
calomel reference electrode and platinum wire auxiliary
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Chemical Papers
The procedure of polarographic measurements
products on reaction rate. The obtained experimental data
of reaction kinetic studies for KPMS are shown in Fig. 2a.
The dependence of the apparent second-order rate con-
stants on pH has form of curve with maximum, corre-
sponding to the value of pKa of alkaloid salt form indicat-
ing the ionic nature of the reaction (Fig. 2c). Similar data
were obtained in the kinetic study of Atr oxidation by
other peroxyacids, in particular, diperoxysebasic acid
(DPSA) (Fig. 2b, d).
4 mL of BR bufer solution with certain pH for oxidation (for
instance 10.2 for KPMS), the needed amount of Atr stand-
ard solution to achieve certain concentration, and 1.0 mL of
1.0·10–2 mol L−1 oxidant were successively introduced into
25 mL volumetric fask. Obtained mixture was maintained
during the optimal oxidation time (for example, for KPMS
15 min), then the pH 5.0 (with pH meter) was adjusted by
hydrochloric acid solution, and the obtained solution was
diluted to the mark with distilled water. The obtained solu-
tions were introduced into the electrochemical cell, and
deoxygenated with argon for 10 min. Polarogramms were
recorded in the range of potentials from –0.5 to –1.8 V. The
current was measured at the peak of polarogramm relatively
background line.
Based on the hypothesis that the transitional state of
reaction involves a non-dissociated form of acid (H2An),
its monoanion (HAn–) and base form of alkaloid (tertiary
amine, Atr), the kinetic equation, establishing a connection
between the apparent rate constant kobs and the second-
order rate constant, K was obtained:
−
kobs = K ⋅ ꢀAtr ⋅ (ꢀ
+ ꢀHAn
)
H2An
(1)
where αAtr is the molar fraction of alkaloid base,
Results and discussion
(ꢀH An + ꢀHAn− )—sum of molar fractions of protonated form
2
of DPA and its monoanion (HAn–) particles.
Kinetic studies of Atr N‑oxidation
Obtained plots agreed well with the experimental
results. kobs was a linear function of the product of molar
The comparative study of the kinetic laws of the reaction
of atropine oxidation with peroxymonosulfate and higher
diperoxycarboxylic acids and mCPBA was conducted. A
fractions ꢀAtr ⋅ (ꢀH An + ꢀHAn− ) in the pH range studied
2
(Fig. 3a), confrming the adequacy of the proposed above
equation to experimentally obtained dependence.
The molar fractions that correspond to acid–base forms
at diferent concentrations of H+ ions were calculated
using the known equations:
(
)
Ct
plot of log
vs. time for DPAs remained linear
CAtr− C −C
(
)
0
t
up to 100% of conversion, demonstrating the second-order
kinetic reaction and that there was no efect of the formed
Fig. 2 Semi-logarithmic plots
(
)
t
Ct
of log
vs. time for
CAtr− C −C
0
(
)
Atr N-oxidation by KPMS a
and for DPSA b at diferent pH.
The dependence of the apparent
second-order rate constant
(Kapp) on pH for the reaction of
Atr with KPMS c and with
DPSA d. CAtr =1.0·10–3 mol
L−1, CDPSA =1.78·10–3 mol L–1,
C
KPMS =2.0·10–3 mol L–1
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Chemical Papers
Fig. 3 A plot of apparent
second-order rate constants for
Atr N-oxidation with DPSA vs.
(
)
−
ꢀAtr ⋅ ꢀH A + ꢀ
a and
(
)
HA
2
1
−
2−
versus ꢀAtr
⋅
ꢀHSO + ∕ ꢀSO
2
5
5
b with KPMS. CAtr =1.0·10–
3 mol L−1, CDPSA =1.78·10–
3 mol L–1, CKPMS =2.0·10–3 mol
L–1, 293 °K
Ka
Ka + [H+]
αAtr
=
;
(2)
(3)
[H+]2
α
=
;
H2An
+
[H ]2 + KH An ⋅ [H+] + KH An ⋅ KHAn
−
2
2
K
H An ⋅ [H+]
2
−
αHAn
=
(4)
[H ]2 + KH An ⋅ [H+] + KH An ⋅ KHAn
+
−
2
2
where Ka is the acid dissociation constant of alkaloid salt
form; KH An—dissociation constant of diperoxy acid proto-
2
Fig. 4 A plot of Atr N-oxide yield vs. time of Atr N-oxidation
by mCPBA (pH 8.2), DPSA (pH 9.2) and KPMS (pH 10.5).
CAtr =1.0·10–3 mol L–1; 293 °K
nated form; KHAn–—acid dissociation constant of acid salt
of diperoxy acid (monoanion).
The apparent second-order rate constant (kobs) for Atr
N-oxidation with KPMS was linear function vs. product of
(
)
2−
reduction of Atr N-oxide. Anodic peaks were not registered
in the polarogramms indicating the irreversibility of the
electrochemical process. Atr did not undergo any changes
at potentials from 0.0 to –1.8 V. Thus, this method only
allows detection of Atr N-oxide, which is Atr metabolite.
No changes in the anodic region of polarogramms were
observed when experimental conditions varied. Thus,
cathodic linear sweep voltammetry was used in further
studies.
1
−
5
molar fractions ꢀAtr ⋅ ꢀHSO + ∕ ꢀSO
in the pH range
2
5
studied (Fig. 3b), confrming mainly the participation of
monoanion of Caro acid in the reaction.
Therefore, using kinetic method, it was shown that inter-
action of diperoxy- or monoperoxy acids (KPMS and DPA)
with Atr in aqueous medium occurred quantitatively due to
nucleophilic substitution mechanism at β-peroxide oxygen
atom in peroxy acid with the formation of corresponding
N-oxides. The reaction kinetics agreed with the laws of spe-
cifc acid–base catalysis.
Earlier we showed that the better medium for maintaining
pH was BR bufer, since the nature of background electrolyte
remained the same in wide pH range (Plotycya 2018).
Aqueous solution of Atr N-oxide was sufciently stable
during three hours. The stable shape of polarogramms, cur-
rent and potential values confrmed stability of these solu-
tions. Tenfold excess of ascorbic acid or Na2SO3 had no
efect on N-oxide polarogramms. First of all, these reducers
were studied since they were typical components of drugs,
in particular Na2SO3 was used as stabilizer for injection
solutions.
The full conversion of Atr to N-oxide occurred most rap-
idly during oxidation with KPMS (Fig. 4). Oxidation reac-
tion of Atr with DPA and especially with mCPBA was much
slower. The results of kinetic studies are given in Table 1.
Electrochemical reduction of Atr N‑oxide
Figure 5 displays the polarogramms of Atr N-oxide obtained
using KPMS (1) and mCPBA (2) as oxidants at conditions
stated in Table 1. The presented polarogramms for diferent
oxidants have the similar shape confrming the similarity of
oxidation products obtained using diferent oxidants. Two
peaks at about –1.05 V and –1.36 V were caused by the
The shape of polarogramm, peak current and potential
depended on pH (Fig. 6). In particular, in all cases, reduc-
tion peak potentials of atropine N-oxide shifted toward nega-
tive region with increase in pH. This fact indicated that the
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Chemical Papers
Table 1 The results of kinetic
Oxidizing agent
KPMS
Optimal pH of
oxidation
Optimal duration of
oxidation, min
K (L·mol−1·min−1
)
References
studies
10.2–10.5
15
0.193
This work
and (Blaz-
heyevskiy
2012)
DPAA
DPSA
9.5
9.0–9.2
20–30
35–40
0.09
0.077
This work
This work
and (Blaz-
heyevskiy
1999)
mCPBA
8.0–8.2
>60
0.03
This work
stage of proton transfer was involved in the electrochemical
process.
The dependences of –E, V on pH are linear (Table 2).
The value of b parameter in these equations can be used to
estimate the ratio of the number of protons to the number
of electrons involved in the electrochemical reaction zH+/n:
dE
2.3RT ⋅ zH+
=
,
dpH
ꢀnF
where α is the charge transfer coefcient, that is 0.5 for fully
irreversible processes.
Values of peak potential (Ep) and half peak potential (Ep/2)
can be used to evaluate the number of electrons involved in
the electrochemical reaction according to Eq. (3) (Gosser
1999):
Fig. 5 Polarogramms of Atr N-oxide obtained with KPMS (1) and
mCPBA (2), as well as in Britton Robinson (BR) bufer (3). Condi-
tions: CKPMS =CmCPBA =1.0·10–4 mol·L–1, CAtr =4.0·10–5 mol·L–1, pH
5.0; ν=0.5 V/s
Fig. 6 The dependence of cur-
rent a and potential b for the
frst and the second reduction
peaks of Atr N-oxide obtained
with KPMS on pHpol at BR
bufer background. Condi-
tions: Cbufer =0.4 mol··L–1,
CKPMS =1.0·10–4 mol·L–1,
CAtr =4.0·10–5 mol·L–1, pH
oxs with KPMS 10.2, ν=0.5 V/s,
toxs KPMS =15 min
Table 2 The equations of linear
dependence of – E, V on pH on
a BR bufer
Oxidant
Peak
pH range
Equation
– E=(0.56 0.02)+(0.091 0.004) pH
Correlation
zH+/n
0.79
0.99
0.73
0.86
coefcient, R
KPMS
P1
P2
P1
P2
4.5–7.1
4.5–7.1
4.2–7.2
4.2–7.2
0.98973
0.95975
0.89801
0.98134
– E=(0.76 0.07)+( 0.114 0.012) pH
– E=(0.68 0.09)+( 0.084 0.016) pH
– E=(0.88 0.05)+( 0.099 0.008) pH
mCPBA
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Chemical Papers
(for the second peak). The slopes were 0.8 and 0.9 for the
frst and the second peaks, respectively, when mCPBA was
used as oxidant.
ꢀn = −47.7∕(Ep − Ep/2)(mV),
(5)
where α is the charge transfer coefcient, n—the number
of electron transferred in a stage of electrode process,
According to Eq. (5), n is calculated using polarogramms
of atropine N-oxide obtained at various conditions: pH from
4 to 7 and diferent concentrations of N-oxide. Totally, 15
polarogramms of atropine N-oxide obtained with KPMS and
7 polarogramms obtained with mCPBA were processed. Cal-
culated n varied in the range from 1.1 to 1.3 for the frst peak
and in the range 0.8–0.9 for the second peak.
All data obtained by various methods showed that each
peak corresponded to transfer of one electron. Thus, accord-
ing to data in Table 2, zH+ is also equal to 1 for each peak.
Such mechanism was reported for N-oxide of nefopam
alkaloid (Dubenska 2020). In the frst stage of reduction
of N-oxide, a hydroxyimino derivative is formed, which is
reduced to the original tertiary amine in the second stage.
The second stage of reduction occurred at very negative
potentials. The reduction of N-oxide in two stages is also
characteristic for other alkaloids (Dubenska 2020). How-
ever, N-oxides of local anesthetics are reduced in one single-
electrone stage (Plotycya 2016, Plotycya 2019).
The important step for elucidation of the mechanism of
electrochemical process is the investigation of the efect
of scan rate on polarographic characteristics (current and
potential). Polarogramms were obtained at scan rates var-
ied from 0.3 to 4.0 V/s (Fig. 7). Increase in scan rate led to
increase in peak height and shifting of peak potential toward
cathodic region indicating the irreversibility of reduction
process.
Peak currents of polarogramms are linearly dependent on
concentration of Atr N-oxide. For the frst peak: –I=(0,09
0,01)+((2,1 0,3)·104) C, R=0,99,957 and for the second
peak: –I=(0,18 0,01)+((3,5 0,5)·104) C, R=0,99,962.
This dependence can be used for the quantitative determina-
tion of Atr N-oxide. LOD and LOQ for Atr N-oxide were
0.9·10–6 and 2.7·10–6 mol·L−1, respectively, using single
sweep voltammetry with scan rate 0.5 V/s as quantitative
method of determination. Since reduction current of Atr
N-oxide is adsorption-controlled, it can promote the adsorp-
tion accumulation of N-oxide on stationary mercury drop
or amalgam electrode with mercury meniscus for analytical
The dependences of log Ip on log ν were linear with the
slopes varied from 0.55 to 0.73 (for both peaks at various
conditions). This indicated slight adsorption of N-oxide on
electrode surface.
The number of electrons can also be determined from the
dependence–Ep = f(ln ν), where the slope of this depend-
ence is equal to RT/αnF (Fig. 7b). The slope for N-oxide
obtained with KPMS was 1.2 (for the frst peak) and 1.1.
Fig. 7 Polarogramms a and
dependence of –E vs. ln ν b
and log Ip vs. log ν c for the
frst and the second peaks using
KPMS as oxidant. Condi-
tions: Cbufer =0.4 mol·L–1,
CKPMS =1.0·10–4 mol·L–1,
CAtr =4.0·10–5 mol·L–1, pHox
10.2; ν=0.5 V/s tox =15 min
1 3

Chemical Papers
Hydroxylamines and Sulfur Analogues, and Hydrazines. Dieter
Enders. Georg Thieme Verlag KG. 40b
determination in real objects. Thus, LOD and LOQ can be
lowered. Analytical characteristics can also be improved
when using diferential-pulse or square-wave voltammetry.
According to our previous studies (Dubenska L.2020,
Plotycya 2016), a simple and fast procedure of Atr oxida-
tion to N-oxide using KPMS can be utilized as a method of
derivatization for obtaining of a new analytical form.
Bibire N, Tântaru G, Dorneanu V, Stan M, Apostu M (2003) Mem-
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Conclusions
The kinetics of oxidation reaction of atropine in aqueous
medium with peroxymonosulfate in comparison with higher
diperoxycarboxylic acids and m-chloperbenzoic acid has
been studied. The interaction of diperoxy- or monoperoxy
acids with atropine in aqueous medium occurred quan-
titatively due to nucleophilic substitution mechanism at
β-peroxide oxygen atom in peroxy acid with the formation of
corresponding N-oxides. The reaction kinetics agreed with
the laws of specifc acid–base catalysis. The rate constants
of the second-order reactions were determined for the frst
time in this work.
Blazheyevskiy M, Ya Anatska, Kissel O (2012) Voltammetric determi-
nation of atropine in the form of N-oxide obtained with potassium
hydrogen peroxomonosulfate. Ukr med almans. 15(1):26–29
Brown K, Menemy M, Palmer M, Baker M, Robinson D, Allan P, Den-
nany L (2019) Utilization of an electrochemiluminescence sen-
sor for atropine determination in complex matrices. Anal Chem
91:12369–12376. https://doi.org/10.1021/acs.analchem.9b02905
Brust J (2004) Neurological aspects of substance abuse, 2nd edn. Else-
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Cai I, Sha M, Guo C, Pan M (2012) Synthesis of tertiary amine
N-oxides. Asian J Chem 24(9):3781–3784
Chen H, Chen Y, Du P, Han F, Wang H, Zhang H (2006) Sensitive
and specifc liquidchromatographic-tandem mass spectrometric
assay for Atropine and its eleven metabolites in rat urine. J Pharm
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2005.06.027
For the frst time, we have investigated the mechanism
of electrochemical reduction of atropine N-oxide, which is
a metabolite of atropine. N-oxide is reduced at relatively
negative potentials in two stages. Each stage of reduction is
a single-electron process.
Craig J, Purushothaman K (1970) Improved preparation of tertiary
amine N-oxides. J Org Chem 35(5):1721–1722. https://doi.org/
10.1021/jo00830a121
Atropine N-oxide could be a perspective analytical form.
Therefore, the work can be adapted for the determination
of atropine or its N-oxide in medicines and plant materials.
Dubenska L, Dushna O, Plyska M, Blazheyevskiy M (2020) Method of
polarographic determination of platyphylline in a form of N-oxide
and its validation in solution for injection. Methods Objects Chem
Anal 15(2):83–92. https://doi.org/10.17721/moca.2020.83-92
Dubenska L, Dushna O, Pysarevska S, Blazheyevskiy M (2020a) A
new approach for voltammetric determination of nefopam and
its metabolite. Electroanalysis 32(3):626–634. https://doi.org/10.
1002/elan.201900595
Acknowledgements This work was fnanced by the Ministry of Educa-
tion and Science of Ukraine (Grant number 0116U001541).
Declaration
Dubey B, Renu U, Tiwari A, Shukla I (2004) Assay of some ophthal-
mic drugs with ammonium metavanadate reagent. J Indian Chem
Soc 81(6):511–514
Conflict of interest On behalf of all authors, the corresponding author
states that there is no confict of interest.
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