Oxidative Degradation of Atropine Drug by Permanganate Ion in Perchloric and Sulfuric Acid Solutions: A Comparative Kinetic Study

The kinetics of oxidations of atropine (ATR) by permanganate ion in both perchloric and sulfuric acid solutions was studied using spectrophotometric technique at a constant ionic strength of 1.2 moldm -3 and at 25°C. In both acids, the reactions showed a first order dependence with respect to [permanganate], whereas the orders with respect to [ATR] and [H + ] were found to be less than unity. The effect of acid concentration suggests that the reactions were acid-catalyzed. Variation of either ionic strength or dielectric constant of the medium had no effect significantly on the oxidation rates. The reactions mechanism adequately describing the kinetic results was proposed. In both acids, the main oxidation products of atropine were identified by spectral and chemical analyses as tropine and phenylmalonic acid. Under comparable experimental conditions, the oxidation rate of atropine in sulfuric acid was approximately three times higher than that in perchloric acid. The reactions constants involved in the different steps of the reactions mechanism have been evaluated. With admiration to the rate-limiting step of these reactions, the activation parameters have been evaluated and discussed.


Introduction
Alkaloids are naturally occurring compounds that act on a diversity of metabolic systems in humans and other animals. They are produced by various organisms such as bacteria, fungi, plants, and animals. Alkaloids have significant pharmacological activities as antimalarial, antiasthma, anticancer, cholinomimetic, antibacterial, psychotropic and stimulant activities [1][2][3]. Among the most famous of the alkaloids is tropine alkaloid or atropine (ATR) which is an anticholinergic drugcontaining two cyclic structures (alicyclic nitrogen-containing alcohol tropine and aromatic tropic acid) joined by an ester linkage [4]. This structure allows for its rapid absorption through the blood-brain barrier. Atropine is structurally similar to cocaine as illustrated below.
Atropine occurs naturally in plants in the nightshade family including deadly nightshade, Jimson weed and mandrake [5]. It is a secondary metabolite of such plants and serves as a drug with a wide variety of effects. Atropine is considered as a core medicine in the World Health Organization (WHO), which is a list of minimum medical needs for a main health care system. It is a competitive antagonist for the muscarinic acetylcholine receptor. Furthermore, it is the most essential drug in the treatment of nerve agent poisoning. Its degradation by microorganisms has been reported by several groups [6] and in the initial stage, the hydrolysis of the ester linkage to give two separate cyclic components takes place.
Oxidation reactions are very important in organic synthesis. Among the important oxidizing agents, permanganate ion is widely used in the oxidations of many organic compounds in neutral, alkaline and acidic media [7][8][9][10][11][12][13][14][15][16][17]. The mechanism of oxidation reactions by permanganate ion is governed by pH of the medium [18]. During oxidation by permanganate, it is evident that the Mn (VII) in permanganate is reduced to various oxidation states in acid, alkaline and neutral media. In acid media, permanganate ion (MnO 4 -) can exist in several different forms, HMnO 4 , H 2 MnO 4 + , HMnO 3 , and Mn 2 O 7 depending on the nature of the reductant. The oxidant has been assigned with an inner-sphere and an outer-sphere mechanism pathways in their redox reactions [19,20]. In general, reduction of permanganate ion in acid medium goes to either Mn IV or Mn II , where the reduction potential of the Mn VII /Mn IV couple is 1.695 V and that of the Mn VII /Mn II couple is 1.51 V [21].
Although some reports on the oxidation of atropine by some oxidants have been published [22][23][24][25][26], there is a lack of literature on the kinetics of oxidation of this drug by permanganate ion in acid solutions. This observation prompted us to investigate the title reactions. The objectives of the present study are to check the reactivity of atropine drug towards permanganate ion in different acid solutions, and to propose the oxidations mechanism of such drug to understand its interaction with metal ions and its mode of action in biological systems.

Materials
The chemicals employed in the present work were of reagent grade and their solutions were prepared by dissolving the samples in bidistilled water. The stock solution of atropine was prepared by dissolving the sample, atropine sulfate monohydrate (C 17 H 23 NO 3 ) 2 .H 2 SO 4 .H 2 O (Aldrich), in bidistilled water. Potassium permanganate freshly solution was prepared and standardized as reported earlier [27]. Sodium perchlorate and acetic acid were used to attain the required ionic strength and dielectric constant of the reactions medium, respectively.

Kinetic Measurements
The kinetic measurements have been followed under pseudo-first order conditions where atropine drug was existed in a large excess over that of permanganate. Initiation of the reactions were done by mixing the formerly thermostatted solutions of permanganate and atropine that also contained the required amounts of the acid and NaClO 4 . The courses of the reactions were followed by monitoring the decay in the absorbance of permanganate as a function of timeat its absorption maximum (λ = 525 nm), whereas the other constituents of the reaction mixtures did not absorb considerably at this wavelength. The absorption measurements were done in a temperature-controlled Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer.
First order plots of ln(absorbance) versus time were recorded to be straight lines up to at least 75% of the reactions completion. The observed first order rate constants (k obs ) were calculated using non-linear least-squares fitting to the first order dependence of the absorbance versus time plots. The rate constants were the main values of at least three kinetic measurements. The rate constants were reproducible to within 3-4%.

Stoichiometry and Product Identification
Reaction mixtures containing various amounts of permanganate ion and atropine at constant [H + ], ionic strength, and temperature were allowed to react for 24 h in closed vessels for completion of the oxidation reactions. The unconsumed [permanganate] was determined spectrophotometrically at 525 nm. The results indicated that four moles of permanganate are consumed by five mole of atropine drug to yield the oxidation products as shown in the following equation, The above stoichiometric equation agrees with the product characterization. Tropine and phenylmalonic acid as the main reaction products of atropine were identified by spectral analysis as described elsewhere [25,[28][29][30]. Tropine was also identified by its hydrazone derivative [28].

Spectral Changes
The spectral scans during the oxidation of atropine by permanganate ion in perchloric and sulfuric acid solutions are shown in Fig. 1

Order of Reactions
The orders of the reactions with respect to the reactants were determined from the slopes of the log k obs versus log(conc.) plots by varying the concentrations of atropine and acids, in turn, while keeping other conditions constant.
Permanganate ion oxidant was diverse in the concentration range of (1.0 -10.0) x 10 -4 mol dm −3 while the rest of the reactant concentrations were kept constant. Both pH and temperature were also reserved constant. It has been found that, plots of ln(absorbance) versus time were linear up to about 75% of the reactions completion. Furthermore, the increase in [MnO 4 -] did not change the oxidation rates as listed in Table 1. These results indicate that the order of the reactions with respect to the oxidant is confirmedto be one.
The observed first order rate constants were measured at various concentrations of atropine keeping others constant. Plots of k obs versus [ATR] were found to be linear with positive intercepts on k obs axes in both perchloric and sulfuric acid solutions as shown in Fig. 2 confirming the less than unit order dependences with respect to atropine concentration.
The influence of both perchloric and sulfuric acid on the rates was investigated by varying the hydrogen ion concentration in the range of 0.2 -1.2 mol dm -3 , keeping all other reactants concentrations constant. The rate constants increased with increasing acid concentration (Table 1) and the oxidation rate in sulfuric acid was approximately three times higher than that in perchloric acid. Plots of log k obs versus log [H + ] were also linear with slopes of 0.81 and 0.79 in perchloric and sulfuric acid solutions, respectively, Fig. 3, suggesting that the orders of reactions with respect to [acid] were less than unity.

Effect of Ionic Strength and Dielectric Constant
The ionic strength effect has been investigated by varying the concentration of NaClO 4 in the reactions media at constant concentrations of permanganate, atropine and acid. It was found that variation in ionic strength did not affect the rates as observed from the data listed in Table 1.
The effect dielectric constant (D) was also studied by varying the acetic acid -water content in the reaction mixture with all other conditions being kept constant. The data clearly revealed that the rate constants did not significantly affected by the decrease in dielectric constant of the solvent mixture; i.e. increase in acetic acid content.

Effect of Temperature
The rates of the reactions were carried out at five different temperatures between 283 and 308 K at constant concentrations of the reactants and other conditions being constant. The results indicate that the rate constants increased with rise in temperature. The activation parameters of the second order rate constant (k 2 ) are calculated using Eyring and Arrhenius plots and are listed in Table 2.

Polymerization Study
To check the existence of free radicals in the reactions under investigations, the reactions mixtures were mixed with identified quantities of acrylonitrile monomer and kept for 4 hours under nitrogen. On dilution with methanol, white precipitates were formed, indicating the participation of free radicals in the oxidation reactions. The blank experiments which were carried out with either permanganate or atropine with acrylonitrile did not induce polymerization under the same experimental conditions.

Discussion
The enhancement of the reaction rate with increasing acid concentration and the chemistry of potassium permanganate [31] suggest the formation of a more powerful oxidant, namely permanganic acid, by the equilibrium: whereK 1 is the protonation constant of permanganate ion (K 1 = 2.99 x 10 -3 dm 3 mol -1 at 25°C) [32]. The protonation of permanganate ion shifts the Mn VII /Mn VI couple to a more positive value (+1.3 V), which makes HMnO 4 a stronger oxidizing agent than MnO 4 - [32]. Many investigators [9][10][11][12][13][14][15][16][17] have suggested that, most of the permanganate oxidation reactions proceed through intermediate complex formation between the oxidant and substrate. In the present work, the kinetic evidences for such complexes were establishedby the linearity of the plots between 1/k obs and 1/[ATR], Fig. 4, in favor of possible formation of a transient complex flanked by oxidant and substrate comparable with the well-known Michaelis-Menten mechanism [33] for enzyme-substrate reactions. The observed insignificant effect of either ionic strength or dielectric constant of the medium on the reactions rates implies association of two neutral molecule [34,35], i.e. between atropine and acid permanganate.
In view of the above arguments, the following reactions mechanism, illustrated in Scheme 1, can be suggested. The mechanism involves attack of the powerful oxidant, acid permanganate, on the atropine drug leading to the formation of a complex (C) in a prior equilibrium step. The cleavage of such complex leads to the formation of manganite (VI) and a free radical intermediate derived from atropine drug. The latter is attacked by manganite (VI) to yield the corresponding aldehyde as an intermediate product. Owing to the proposed mechanistic Scheme 1, the oxidation rate can be expressed by the following rate law: The relationship between the oxidation rate and the oxidant, substrate and hydrogen ion concentrations is deduced (See Appendix A) to give the following equation:

Rate
(2) Under pseudo-first order condition in the presence of a large excess of substrate over that of , the rate-law can be expressed by Eq. (3), Comparing Eqs. (2) and (3) and rearrangement, the following equations are obtained:  Table 3. The obtained values of K 1 are in a good agreement with those reported in the literature [13,32].   The activation parameters listed in Table 2 may be interpreted as follows. The obtained large negative values of ∆S ≠ suggest compactness of the formed complexes and such complexes are more ordered than the reactants due to loss of degrees of freedom [36]. Also, the obtained values of ∆S ≠ are within the range of radical reactions. The positive values of both ∆H ≠ and ∆G ≠ confirm endothermic formation of the intermediate complexes and their non-spontaneities, respectively.

Conclusions
The kinetics of oxidations of atropine by permanganate ion in both perchloric and sulfuric acid solutions has been studied. The main oxidation products of atropine were identified in both cases as the as tropine and phenylmalonic acid. Under comparable experimental conditions, the oxidation rate in sulfuric acid was approximately three times higher than that in perchloric acid. The reactions constants involved in the different steps of the oxidations mechanism along with the activation parameters have been evaluated and discussed.