Characterization of the reaction products, kinetics and mechanism of oxidation of the drug captopril by platinum(IV) complexes

Abstract

Captopril, the first pharmaceutical drug designed, synthesized, and used primarily for the treatment of hypertension and congestive heart failure, is an angiotensin-converting-enzyme inhibitor and is also an antioxidant. On the other hand, the kinetic and mechanistic aspects for the oxidation of captopril are not well understood. The oxidation of captopril by cisplatin prodrug and a model compound, cis-[Pt(NH₃)₂Cl₄] and trans-[PtCl₂(CN)₄]²⁻, was thus investigated in this work. A stopped-flow spectrometer was employed to follow the oxidation kinetics over a wide pH-range under the pseudo first-order conditions of [captopril] ≫ [Pt(IV)]. The oxidation by the Pt(IV) complexes displayed a second-order character, first-order each in [Pt(IV)] and in [captopril], whereas the metal-ion-catalyzed autooxidation made a very minor contribution to the overall kinetics in acidic media and was negligible in neutral media. Captopril was oxidized to form the captopril-disulfide as identified by ESI mass spectrometry under the conditions of the kinetic measurements. In the proposed reaction mechanism, the Pt(IV) complexes are reduced by the three protolytic captopril species in parallel as rate-determining steps, generating reactive species of chlorothiol and/or sulfenylchloride. The reactive species will be rapidly trapped, either directly or indirectly, by another molecule of captopril to form captopril-disulfide. The rate constants for the rate-determining steps have been derived, demonstrating that the fully deprotonated captopril is about 10⁵ to 10⁶ times more reactive than its corresponding thiol form toward the Pt(IV) complexes.

---

Introduction

High blood pressure is one of the major risk factors causing strokes and other cardiovascular diseases such as coronary heart disease and peripheral artery disease, while about 33% of deaths caused by strokes are attributed to untreated high blood pressure.¹ Angiotensin II, the peptide-based blood pressure regulating hormone, is a primary hypertensive compound, which is produced in vivo from Angiotensin I, catalyzed by angiotensin-converting enzyme (ACE), a metalloprotease possessing a zinc(II) ion at its active site.^1,2 Captopril, 1-(2S)-3-mercapto-2-methylpropionyl-L-proline, is an ACE inhibitor and is also the first pharmaceutical drug designed, synthesized, and used primarily for the treatment of hypertension and congestive heart failure.^1,2 Moreover, captopril has a low incidence of adverse side effects and is employed worldwide.^1–3 In addition, captopril is also an antioxidant and can reduce oxidative stress, which is often involved in the pathogenesis of arterial hypertension.^4–7 On the other hand, it has been shown that captopril has some properties to inhibit the growth of some types of tumors.^8–10

Thus, it is not surprising that many different analytical methods were proposed and developed for the determination of captopril in various samples due to the importance of this drug in the pharmaceutical industry; some of those methods were based on its redox chemistry, involving oxidants like Mn(VII), Fe(III), Ce(IV), bromine and iodate.^11–16 Although these redox reactions were successful in the establishment of analytical methods, the reaction rates, mechanisms and products were not studied.^11–16 As a matter of fact, few kinetic and mechanistic investigations on the oxidation of captopril have appeared, including the oxidants of oxygen, [Mo(CN)₈]³⁻ and [Mo(CN)₈]³⁻, nitrous acid/nitrite, bromate and bromine;^17–20 among them, only the nitrite reaction is related to oxidative stress.¹⁹ Furthermore, the proposed reaction mechanisms differentiate each other, relying on the oxidants.^17–20 Clearly, there is lack of a general trend to anticipate/predict some general kinetic and mechanistic characters on captopril oxidations.^17–20

We have undertaken an investigation on the oxidation of captopril by trans-dichloro-platinum(IV) complexes, namely trans-[PtCl₂(CN)₄]²⁻ and cis-[Pt(NH₃)₂Cl₄], which are platinum(IV) anticancer model compounds, and the cisplatin prodrug, respectively.²¹ In fact, the design and synthesis of new generation of platinum-based anticancer agents, including some Pt(IV) complexes, are still of current interest with the goal of overcoming the side effects of cisplatin and carboplatin.^22–26 In addition, trans-dichloro-platinum(IV) complexes have been found to be highly selective and efficient reagents for formation of intramolecular disulfide bonds in peptides and proteins.^27–32 In view of the biomedical- and biochemical-importance of the platinum(IV) compounds and the industrial importance of captopril, we have fully characterized the reaction products, kinetics and mechanism of the oxidation of captopril by these two Pt(IV) complexes. The structures of the Pt(IV) complexes and captopril are illustrated in Scheme 1. We herein report our results.

Scheme 1 Structures of the Pt(IV) complexes, captopril and captopril-disulfide.

Materials and methods

Materials

Platinum complexes cis-[Pt(NH₃)₂Cl₄] and K₂[Pt(CN)₄]·3H₂O, and captopril were purchased from Sigma-Aldrich (St. Louis, MO). K₂[PtCl₂(CN)₄] was synthesized by oxidation of K₂[Pt(CN)₄]·3H₂O with chlorine, as described previously.³³ The UV-vis spectrum of the aqueous solution of K₂[PtCl₂(CN)₄] agreed well with that reported earlier for trans-[PtCl₂(CN)₄]²⁻.³⁴ Sodium acetate, acetic acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium perchlorate, perchloric acid, sodium chloride and Na₂EDTA were all of analytical grade and obtained from either Fisher Scientific (in Beijing, China) or Alfa Aesar (in Tianjin, China), which were used to prepare buffer solutions. All the solutions were prepared by use of doubly distilled water.

Instruments

An Accumet Basic AB15 Plus pH meter, equipped with an Accumet AccutupH® combination pH electrode (Fisher Scientific, Pittsburgh, PA), was used to measure the pH values of buffer solutions. Standard buffers of pH 4.00, 7.00 and 10.00, also from Fisher Scientific, were used for calibration of the electrode just before pH measurements. UV-visible (UV-vis) spectra and time-resolved spectra were recorded on a TU-1900 spectrophotometer (Beijing Puxi, Inc., Beijing, China) using 1.00 cm quartz cells. Kinetic measurements were performed on an Applied Photophysics SX-20 stopped-flow spectrometer (Applied Photophysics Ltd., Leatherhead, U.K.). Both of the spectrometers were equipped with a thermostat (BG-chiller E10, Beijing Biotech Inc., Beijing). Mass spectra were run on an Agilent 1200/6310 ion trap mass spectrometer with electrospray ionization (ESI).

Solutions

Combinations of acetic acid–sodium acetate, NaH₂PO₄–Na₂HPO₄, and NaHCO₃–Na₂CO₃ were used to prepare buffer solutions. All the buffer solutions contained 0.10 M of NaCl and 2.0 mM of EDTA; sodium perchlorate was used to adjust the ion strength (μ) of buffers to 1.0 M. The addition of NaCl was to suppress the hydrolysis of the platinum(IV) complexes. EDTA in the buffers was to eliminate the catalytic effects in the autooxidation of captopril caused by trace amounts of metal ions such as Cu(II) and Fe(III).^35–37 Stock solutions of 1.0 mM cis-[Pt(NH₃)₂Cl₄] and of trans-[PtCl₂(CN)₄]²⁻ were prepared by dissolving weighed solid samples, respectively, in solutions containing 0.90 M of NaClO₄, 0.09 M of NaCl and 10 mM of HCl. Stock solutions of 20.0 mM captopril were prepared with buffer solutions and used only for a few hours.

Kinetic measurements

Time-resolved spectra were recorded for the reaction of trans-[PtCl₂(CN)₄]²⁻ with captopril in 10 mM HClO₄ and of cis-[Pt(NH₃)₂Cl₄] with captopril in a buffer solution of pH 4.41, cf. Fig. 1 and 2. For kinetic measurements, solutions of cis-[Pt(NH₃)₂Cl₄] or trans-[PtCl₂(CN)₄]²⁻, and captopril were prepared by adding an appropriate amount of each stock solution to 10 mL of specific buffer solution, respectively. These solutions were flushed with nitrogen for at least 10 min, followed by loading these solutions into the stopped-flow machine; moreover, those solutions were used only for a couple of hours. Reactions were followed under pseudo-first-order conditions with captopril in at least 10-fold excess over the Pt(IV) complexes. Kinetic traces were followed at 255 nm in the case of trans-[PtCl₂(CN)₄]²⁻ and at 240 nm (pH < 7.4) and 280 nm (pH > 7.4) in the case of cis-[Pt(NH₃)₂Cl₄].

Fig. 1 Time-resolved spectra for reaction between trans-[PtCl₂(CN)₄]²⁻ and captopril under the reaction conditions: [Pt(IV)] = 0.08 mM, [captopril] = 1.00 mM, [H⁺] = 0.010 M, [Cl⁻] = 0.10 M, μ = 1.0 M and 25.0 °C. The first spectrum was obtained ca. 10 s after reaction; the time between scans was 35 s.

Fig. 2 Time-resolved spectra for reaction between cis-[Pt(NH₃)₂Cl₄] and captopril under the reaction conditions: [Pt(IV)] = 0.08 mM, [captopril] = 1.00 mM, [Cl⁻] = 0.10 M, pH = 4.41, μ = 1.0 M and 25.0 °C. Spectra corresponding to the reaction times were: 30, 79, 128, 226, 324, 422, 520, 618, 716, 814, and 912 s. Inset: enlarged scale between 240 nm and 320 nm. The arrows in the figure indicate the direction of absorbance changes.

Products analysis

ESI-mass spectra were run for product analysis. The reaction mixtures of 1.0 mM trans-[PtCl₂(CN)₄]²⁻/cis-[Pt(NH₃)₂Cl₄] with 8.0 mM captopril in 20.0 mM HCl solution were used for recording the ESI-mass spectra. In addition, mass spectra were also recorded for 1.0 mM captopril in dilute HCl, which were utilized as a reference for mass analysis.

Results and discussion

Time-resolved spectra

Time-resolved spectra are displayed in Fig. 1 for the reaction between trans-[PtCl₂(CN)₄]²⁻ and captopril in a 10 mM HCl solution. The absorption around 255 nm band increased drastically, which can be assigned to the formation of [Pt(CN)₄]²⁻.³⁴ Two isosbestic points, observed in the time-resolved spectra at 242.6 nm and 285.8 nm, are a good indication that reduction of trans-[PtCl₂(CN)₄]²⁻ to [Pt(CN)₄]²⁻ is a simple process while the absorption change caused from the captopril oxidation makes a trivial contribution to the overall spectra.

For the reaction between cis-[Pt(NH₃)₂Cl₄] and captopril, the time-resolved spectra are shown in Fig. 2. The absorbances at two bands, one band around 228 nm and the other around 266 nm (a weak band, see the insert of Fig. 2), all decreased as the reaction proceeded. Absorbance readings from Fig. 2 at 228 nm and 266 nm as a function of time are illustrated in Fig. 3 (data points). The absorbance–time curves appeared to be exponential and were thus fitted by eqn (1), where A_t, A₀, and A_∞ stands for absorbance at time t, zero and infinity, respectively. The resulted fittings are good and also shown in Fig. 3.

A_t = (A₀ − A_∞)exp(−k_obsdt) + A_∞ (1)

Moreover, the value of k_obsd obtained at 228 nm is in good agreement with that acquired at 266 nm within experimental error. The single exponential fittings, together with the good agreement of the k_obsd values suggest that the absorbance decrease in Fig. 2 corresponds to a single process, most likely the reduction of cis-[Pt(NH₃)₂Cl₄] (see more discussions below).

Fig. 3 Kinetic traces at 228 nm and 266 nm for the reaction of cis-[Pt(NH₃)₂Cl₄] with captopril; the data points were from the absorbance readings in Fig. 2. The solid curves were obtained by fitting eqn (1) to the experimental data by use of a nonlinear least-squares method.

Kinetic data and rate law

Under the pseudo first-order conditions ([captopril] ≥ 10 [Pt(IV)]), kinetic traces in all the reaction media used in the present work could be simulated very well by single exponentials for both Pt(IV) complexes, indicating that the reduction is first-order in [Pt(IV)] and is described in eqn (2):

d[Pt(CN)₄²⁻]/dt = k_obsd[PtCl₂(CN)₄²⁻] (2a)

or

−d[Pt(NH₃)₂Cl₄]/dt = k_obsd[Pt(NH₃)₂Cl₄] (2b)

Pseudo first-order rate constants, k_obsd, derived from the simulations, are reported as the average values from 5–7 runs. Standard deviations are usually much less than 5%. Values of k_obsd as functions of [captopril] and pH at 25.0 °C and μ = 1.0 M are summarized in Tables S1 and S2 in the ESI.† By use of the data in Tables S1 and S2,† plots of k_obsd versus [captopril] according to eqn (3) are linear and shown in Fig. 4 for the reaction of trans-[PtCl₂(CN)₄]²⁻ and in Fig. 5 for the reaction of cis-[Pt(NH₃)₂Cl₄], respectively.

k_obsd = k_c + k′[captopril] (3)

Fig. 4 Pseudo first-order rate constants, k_obsd, as a function of [captopril] for oxidation of captopril by trans-[PtCl₂(CN)₄]²⁻ at 25.0 °C and μ = 1.0 M using the data of Table S1 in the ESI.†

Fig. 5 k_obsd as a function of [captopril] for oxidation of captopril by cis-[Pt(NH₃)₂Cl₄] at 25.0 °C and μ = 1.0 M using the data of Table S2 in the ESI.†

From the linear plots, values of k′ from the slopes (summarized in Table 1) and of k_c from intercepts (listed in Tables S1† and S2†) were derived, where k′ denotes the pH-dependent observed second-order rate constants.

Table 1 Observed second-order rate constants k′ for the oxidation of captopril as a function of pH at 25.0 °C and μ = 1.0 M

Pt(IV) complex	pH	k′/M^−1 s^−1
trans-[PtCl2(CN)4]^2−	3.22	57.3 ± 0.6
	3.69	207 ± 2
	4.28	737 ± 7
	4.82	(2.65 ± 0.04) × 10^3
	5.32	(6.39 ± 0.09) × 10^3
	5.35	(8.19 ± 0.10) × 10^3
	5.97	(3.78 ± 0.08) × 10^4
	6.85	(1.88 ± 0.03) × 10^5
	7.25	(4.42 ± 0.09) × 10^5
	7.35	(3.34 ± 0.09) × 10^5
	7.85	(1.26 ± 0.03) × 10^6
cis-[Pt(NH3)2Cl4]	3.69	1.4 ± 0.1
	4.21	5.7 ± 0.2
	4.82	19.9 ± 0.2
	5.35	64 ± 2
	6.09	277 ± 5
	6.85	(1.23 ± 0.02) × 10^3
	7.22	(2.76 ± 0.06) × 10^3
	7.35	(2.99 ± 0.06) × 10^3
	7.85	(6.25 ± 0.15) × 10^3
	8.47	(6.40 ± 0.20) × 10^4
	8.96	(1.05 ± 0.04) × 10^5
	9.46	(2.26 ± 0.08) × 10^5

---

By examining the values of the intercepts k_c, they are negligible in the neutral buffer solutions such as pH 6.85 and 7.25 for trans-[PtCl₂(CN)₄]²⁻ and pH 7.22 and 7.35 for cis-[Pt(NH₃)₂Cl₄]. Meanwhile in acidic media, the intercepts are generally very small and could not be obtained with certainty (see the large errors associated with the values). The very small intercepts k_c in acidic media may stem from the autooxidation of the thiol group catalyzed by metal ions (especially by Cu²⁺),^35–38 although 2 mM EDTA was used to suppress the catalytic effects in our experiments; the suppression seemed incomplete.³⁸ However, in the neutral pH buffers as mentioned above, the oxidation rates by the Pt(IV) complexes are so rapid and dominant that the catalytic reaction is not observable. Nevertheless, the contribution from the autooxidation to the overall reaction is small even in the acidic media and the small intercepts k_c, derived with large errors, cannot be rationalized. We will thus focus on the observed second-order rate constants k′.

Product analysis

ESI mass spectra (both positive and negative modes of ionization) recorded for the reaction mixtures of 8 mM captopril with 1.0 mM trans-[PtCl₂(CN)₄]²⁻ and of 8 mM captopril with 1.0 mM cis-[Pt(NH₃)₂Cl₄] are displayed in Fig. S1 and S2,† respectively, in the ESI. The peak assignments, given also in the Figures, clearly indicated that the major peaks orginated only from the excess captopril and from the oxidized product captopril-disulfide (comparatively, platinum complexes displayed much lower sensitivities, see Fig. S3 in the ESI†). Moreover, no further oxidation products of captopril such as captopril-sulfenic acid, captopril-sulfinic acid, and/or captopril-sulfonic acid were found on the mass spectra.^3,20 Thus, it can be concluded that under the conditions used, which were similar to those employed in our kinetic measurements, captopril was oxidized to form captopril-disulfide predominantly by both Pt(IV) complexes (see the structure of captopril-disulfide in Scheme 1).

Previously, it was found that trans-dichloro-platinum(IV) were the selective and efficient reagents for formation of intramolecular disulfide bonds in peptides, as well as in dithiol-containing molecules.^21,27–31 Our present work demonstrates that formation of intermolecular disulfide bond for thiol-containing molecules by trans-dichloro-platinum(IV) is also favored under the conditions used.

Mechanistic elucidation

From eqn (3) and the related discussions on the small contributions from the autooxidation of captopril in acidic media, we know that the oxidation of captopril by the Pt(IV) complexes is of an overall second-order kinetic nature, with a first-order in [captopril]. This implies that one captopril molecule is involved in the rate-determining step(s). The observed second-order rate constants k′ increase drastically (cf. Table 1) with the increase of pH, rendering the thiolate species of captopril much more reactive than its thiol forms. On other hand, the formation of captopril-disulfide as the oxidation product requires another captopril molecule participating in the overall reaction process; the participation of the second captopril molecule is thus anticipated to be after the rate-determining step(s). The present reactions are generally fast, which is in contrast with the previous observations that substitution reactions on Pt(IV) complexes are rather slow (substitution inertia).³⁹ Hence, the possibility of substitution reactions on the Pt(IV) complexes (such as substitution of coordinated chloride) prior to the electron transfer step can be ruled out. The time-resolved spectra in Fig. 1 with two clear isosbestic points strongly substantiate this reasoning.

By taking all the above considerations into account, a reaction mechanism is proposed and delineated in Scheme 2, encompassing parallel reactions between the Pt(IV) complexes and the three protolytic species of captopril as rate-determining steps depending on the solution pH. Reduction of trans-dihalido-Pt(IV) complexes by thiol-compounds taking place via a halide-bridged activated complex has been well-described previously.^40–42 This can be briefly described by the attack of the sulfur atom of captopril on one of the axially-coordinated chlorides in the Pt(IV) complexes, resulting in the partial formation of a Cl–S bond and partial breaking of Cl–Pt–Cl bonds:

Scheme 2 The suggested reaction mechanism.

By the end of the attack, the Cl–Pt–Cl bonds will be totally broken and the Cl–S bond formation is complete, giving rise to reactive species of chlorothiol and/or sulfenylchloride (1–3) in Reactions (4)–(6) in Scheme 2. This is equivalent to a Cl⁺ transfer from the Pt(IV) center to the attacking sulfur, and concurrently, the Pt(IV) center is reduced to its Pt(II) counterpart by the loss of the two axially-coordinated chlorides. Subsequently, reactive species (1–3) will be rapidly trapped by the excess captopril to generate captopril-disulfide, see Reaction (7) in Scheme 2. On the other hand, a small portion of the reaction species will probably be hydrolyzed to captopril-sulfenic acid (Reaction (8) in Scheme 2), which also will be trapped rapidly by the excess of captopril to form captopril-disulfide,⁴³ cf. Reaction (9) in Scheme 2 with the reactive species 3 as an example.

Evaluation of rate constants

According to Scheme 2, a theoretic rate law expressed in eqn (10) can be derived for the oxidation of captopril by trans-[PtCl₂(CN)₄]²⁻ and cis-[Pt(NH₃)₂Cl₄], where a_H denotes to the proton activity, corresponding to the pH measurements.

It is obvious that eqn (10) corresponds to the second-order term of eqn (3). Therefore:

Protolysis constants pK_a1 = 3.52 and pK_a2 = 10.0 at 25.0 °C for captopril were reported earlier.¹⁹ Eqn (11) was employed to fit the k′ – pH data listed in Table 1, using weighted nonlinear least-squares routine with k₁, k₂ and k₃ as adjustable parameters and with K_a1 and K_a2 values fixed. Briefly, the values of k′ and their associated errors in Table 1 were used. In the beginning, some initial values were given to k₁–k₃ to try the simulations in order to find a reasonable convergence. When a convergence was obtained, the initial values were then varied to see if the same convergence could be achieved. In such a way, a common and robust convergence was finally found, of course offering a unique set of values for k₁, k₂ and k₃.

The fittings gave negative k₁ values for both Pt(IV) complexes; thus k₁ was indeterminate in the pH range studied. If the k₁ term in eqn (11) was assumed to make a negligible contribution to the overall reaction rate, eqn (11) could be simplified to eqn (12).

The simulations of eqn (12) to the experimental data in Table 1 afforded nice fittings as shown in Fig. 6 and 7, respectively, for trans-[PtCl₂(CN)₄]²⁻ and cis-[Pt(NH₃)₂Cl₄]. Moreover, the values of k₂ and k₃ were evaluated from the fittings, which are summarized in Table 2. The ratios of k₃/k₂ (1.8 × 10⁶ for trans-[PtCl₂(CN)₄]²⁻ and 3.8 × 10⁵ for cis-[Pt(NH₃)₂Cl₄]) are huge, highlighting that the thiolate form of captopril is much more reactive than its thiol forms. These ratios can also account for the observation that k′ increases several orders of magnitude when the solution pH is increased from 3 to about 10 in Table 1. The reaction mechanism delineated in Scheme 2 can explain all the experimental observations and is thus convincing. On the other hand, it is totally different from those derived for the captopril oxidations so far by other oxidants.^17–20

Fig. 6 Second-order rate constants k′ as a function of pH at 25.0 °C and μ = 1.0 M for oxidation of captopril by trans-[PtCl₂(CN)₄]²⁻ (data points). The solid curve was obtained by fitting eqn (12) the experimental data by use of a weighted nonlinear least-squares method.

Fig. 7 Second-order rate constants k′ as a function of pH at 25.0 °C and μ = 1.0 M for oxidation of captopril by cis-[Pt(NH₃)₂Cl₄] (data points). The solid curve was obtained by fitting eqn (12) the experimental data by use of a weighted nonlinear least-squares method.

Table 2 Values of rate constants of the rate-determining steps derived from curve-fittings at 25.0 °C and μ = 1.0 M

Pt(IV) complex	km	Value/M^−1 s^−1
a Could not be derived from the kinetic data collected.
trans-[PtCl2(CN)4]^2−	k1	—^a
	k2	152 ± 8
	k3	(2.8 ± 0.1) × 10^8
cis-[Pt(NH3)2Cl4]	k1	—^a
	k2	5.3 ± 0.3
	k3	(1.5 ± 0.1) × 10^6

---

Captopril was shown to have the property of scavenging reactive oxygen species (ROS) including superoxide anion, hydroxyl radical, hypochlorous acid/hypochlorite, which can cause oxidative stress.^4,44,45 Of those ROS, hypochlorous acid/hypochlorite showed some common mechanistic characters as trans-dichloro-platinum(IV) complexes toward thiol-containing compounds.²¹ In other words, platinum(IV) anticancer prodrugs can also cause oxidative stress presumably similar to hypochlorous acid/hypochlorite. In this regard, captopril might be used to relieve the oxidative stress induced by Pt(IV) anticancer prodrugs.

Conclusions

The oxidation of captopril by trans-[PtCl₂(CN)₄]²⁻ and cis-[Pt(NH₃)₂Cl₄] was carefully studied over a wide pH range, enabling us to establish the rate law. The oxidation product was characterized as captopril-disulfide by ESI mass spectrometry. An overall mechanistic picture was delineated for the oxidation process, accounting convincingly for all the experimental observations. The thiolate species of captopril is 10⁵–10⁶ times more reactive than its thiol forms toward the platinum(IV) complexes. Captopril might be used to relieve the oxidative stress caused by Pt(IV) anticancer prodrugs.

Acknowledgements

We thank the financial support to this work by grants from the Natural Science Foundation of Hebei University (2012ZD01) and from the Youth Foundation of Hebei Educational Committee (Q2012058, Z2012041).

References

1. W. C. Hou, H. J. Chen and Y. H. Lin, J. Agric. Food Chem., 2003, 51, 1706–1709.
2. B. J. Bhuyan and G. Mugesh, Org. Biomol. Chem., 2011, 9, 1356–1365.
3. W. M. M. Mahmoud and K. Kummerer, Chemosphere, 2012, 88, 1170–1177.
4. O. I. Aruoma, D. Akanmu, R. Cecchin and B. Halliwell, Chem.-Biol. Interact., 1991, 77, 303–314.
5. G. X. Li and Z. Q. Liu, Eur. J. Med. Chem., 2009, 44, 4841–4847.
6. J. Miguel-Carrasco, S. Zambrano, A. J. Blanca, A. Mate and C. M. Vazquez, J. Inflammation, 2010, 7, 21–30.
7. J. Miguel-Carrasco, M. T. Monserrat, A. Mate and C. M. Vazquez, Cardiovasc. Pharmacol., 2010, 632, 65–72.
8. S. l. Hii, D. L. Nicol, D. C. Gotley, L. C. Thompson, M. K. Green and J. R. Jonsson, Br. J. Cancer, 1998, 77, 880–883.
9. T. Sjoberg, L. A. G. Rodriguez and M. Lindblad, Clin. Gastroenterol. Hepatol., 2007, 5, 1160–1166.
10. S. Attoub, A. M. Gaben, S. Al-Salam, M. A. H. Soltan, A. John, M. G. Nicholls, J. Mester and G. Petroianu, Ann. N. Y. Acad. Sci., 2008, 1138, 65–72.
11. J. A. M. Pulgerin, L. F. G. Bermejo and P. F. Lopez, Anal. Chim. Acta, 2005, 546, 60–67.
12. N. Rahman, N. Anwar, M. Kashif and N. Hoda, Acta Pharm., 2006, 56, 347–357.
13. W. T. Suarez, A. A. Madi, L. C. S. de Figueiredo-Filho and O. Fatibello-Filho, J. Braz. Chem. Soc., 2007, 18, 1215–1219.
14. J. A. V. Prior, J. L. M. Santos and J. L. F. C. Lima, Anal. Chim. Acta, 2007, 600, 183–187.
15. A. M. El-Didamony, J. Chin. Chem. Soc., 2009, 56, 755–762.
16. A. M. El-Didamony and E. A. H. Erfan, Spectrochim. Acta, Part A, 2010, 75, 1138–1145.
17. T. Y. Lee and R. E. Notari, Pharm. Res., 1987, 4, 98–103.
18. R. N. Zahdeh, R. A. Zaru and H. A. Hodali, Polyhedron, 2007, 26, 3069–3073.
19. A. Sexto and E. Iglesias, Org. Biomol. Chem., 2011, 9, 7207–7216.
20. T. Tran, W. Mbiya, R. Adigun, P. Ndungu, B. Martineigh and R. H. Simoyi, J. Phys. Chem. A, 2013, 117, 2704–2717.
21. S. Huo, S. Shen, D. Liu and T. Shi, J. Phys. Chem. B, 2012, 116, 6522–6528.
22. N. J. Wheate, S. Walker, G. E. Craig and R. Oun, Dalton Trans., 2010, 39, 8113–8127.
23. Z. Wexselblatt and D. Gibson, J. Inorg. Biochem., 2012, 117, 220–229.
24. A. V. Klein and T. W. Hambley, Chem. Rev., 2009, 109, 4911–4920.
25. G. M. Rakic, S. Grguric-Sipka, G. N. Kaluderovic, M. Bette, L. Filipovic, S. Arandelovic, S. Radulovic and Z. Lj. Tesic, Eur. J. Med. Chem., 2012, 55, 214–219.
26. H. P. Varbanov, M. A. Jakupec, A. Roller, F. Jensen, M. Glanski and B. K. Keppler, J. Med. Chem., 2013, 56, 330–344.
27. T. Shi and D. L. Rabenstein, J. Org. Chem., 1999, 64, 4590–4595.
28. T. Shi and D. L. Rabenstein, J. Am. Chem. Soc., 2000, 122, 6809–6915.
29. T. Shi and D. L. Rabenstein, Tetrahedron Lett., 2001, 42, 7203–7206.
30. T. Shi and D. L. Rabenstein, Bioorg. Med. Chem. Lett., 2002, 12, 2237–2240.
31. T. Shi, S. M. Spain and D. L. Rabenstein, Angew. Chem., Int. Ed., 2006, 45, 1780–1783.
32. M. Narayan, E. Welker, C. Wanjalla, C. Xu and H. A. Scheraga, Biochemistry, 2003, 42, 10783–10789.
33. T. Shi and L. I. Elding, Inorg. Chim. Acta, 1998, 282, 55–60.
34. L. Drougge and L. I. Elding, Inorg. Chim. Acta, 1986, 121, 175–183.
35. J. E. Taylor, J. F. Yan and J.-l. Wang, J. Am. Chem. Soc., 1966, 88, 1663–1667.
36. A. V. Kachur, C. J. Koch and J. E. Biaglow, Free Radical Res., 1998, 28, 259–269.
37. L. Ehrenberg, M. Harms-Ringdahl, I. Fedorcsak and F. Granath, Acta Chem. Scand., 1989, 43, 177–187.
38. N. Bhattarai and D. M. Stanbury, Inorg. Chem., 2012, 51, 13303–13311.
39. W. R. Mason, Coord. Chem. Rev., 1972, 7, 241–255.
40. T. Shi, J. Berglund and L. I. Elding, Inorg. Chem., 1996, 35, 3498–3503.
41. T. Shi, J. Berglund and L. I. Elding, J. Chem. Soc., Dalton Trans., 1997, 2073–2077.
42. K. Lemma, T. Shi and L. I. Elding, Inorg. Chem., 2000, 39, 1728–1734.
43. L. Turell, R. Radi and B. Alvarez, Free Radical Biol. Med., 2013, 65, 244–253.
44. D. Bagchi, R. Prasad and D. K. Das, Biochem. Biophys. Res. Commun., 1989, 158, 52–57.
45. M. Bartosz, J. Kedziora and G. Bartosz, Free Radical Biol. Med., 1997, 23, 729–735.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45020a

---