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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Operando Electron Paramagnetic Resonance for Elucidating the Electron Transfer Mechanism of Coenzymes Mian A. Ali, Ayaz Hassan, Graziela Cristina Sedenho, Renato V. Gonçalves, Daniel Rodrigues Cardoso, and Frank Nelson Crespilho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01160 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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The Journal of Physical Chemistry

Operando electron paramagnetic resonance for elucidating the electron transfer mechanism of coenzymes Mian A. Ali†, Ayaz Hassan†, Graziela C. Sedenho†, Renato V. Gonçalves‡, Daniel R. Cardoso†, Frank N. Crespilho†* †São Carlos Institute of Chemistry, University of São Paulo, 13560-970 São Paulo, Brazil ‡ São Carlos Institute of Physics, University of São Paulo, 13560-970 São Paulo, Brazil Keywords: Electron transfer, electron paramagnetic resonance, electrocatalysis, coenzymes, NADH ABSTRACT: One of the most important challenges in chemistry with direct implication in biochemistry is probing the mechanism of electron transfer originating from biological molecules. On the basis of protein film voltammetry, mediated electron transfer and molecular adsorption followed by heterogeneous catalysis result in similar responses for steady-state currents; both processes increase the faradaic current at a low overpotential. This is typical of NAD-dependent alcohol dehydrogenase (ADH), an oxidoreductase enzyme that uses the interconversion of NAD+/NADH coenzyme to catalyze the oxidation of alcohol to aldehyde. We propose a setup based on operando electron paramagnetic resonance (EPR) spectroscopy to investigate the NADH/NAD+ redox reaction and introduce how to probe free electrons on a carbon electrode surface and correlate them with the electrocatalytic mechanism. Since knowledge of the g-factor may provide information about the electronic structure of the paramagnetic center at the carbon surface, it was found that the concentration of unpaired free electrons respond to both applied overpotential and NADH oxidation, enabling measurement of the in situ dynamics of the electron transfer reaction. A new correlation for the spin concentration reveals an increasing number of free unpaired electrons with increasing applied overpotential and NADH oxidation, which corroborates the controversial hypothesis that quinone groups act as electrocatalysts and not as redox mediators towards the oxidation of NADH to NAD+. Furthermore, operando EPR provides useful information in probing the electron transfer dynamics on a carbon surface and may be extended to other chemical systems involving electron transfer reactions.

INTRODUCTION Electron transfer involving biological molecules is a significant phenomenon in both biochemistry and technology. However, important questions remain to be answered, such as how to determine the dominant reaction pathways for efficient proton and electron motion from their initial positions to their final positions in biomolecules. In particular, bioelectrocatalysis using coenzymes usually follows complex reactions pathways and is therefore difficult to explain. For instance, dehydrogenase enzymes have pivotal roles in metabolism and oxidize substrates by transferring hydrogen to electron acceptors. NAD+ (an oxidized form of nicotinamide adenine dinucleotide) is one of most important coenzymes essential to the growth and survival of organisms and for technological applications in organic molecule oxidation for producing fuel and energy. One typical example is NAD-dependent alcohol dehydrogenase (ADH), an oxidoreductase enzyme that uses the interconversion of the NAD+/NADH redox couple1–3 to catalyze the oxidation of alcohol to aldehyde. The coenzyme binds to the enzyme by oxidizing the alcoholic moieties in it. The dissociation of the enzyme–NADH complex is the rate determining step according to a recently reported mechanism.4 Currently, voltammetry is used to measure the kinetics and dynamics of NAD+/NADH interconversion, where carbon electrodes functionalized with quinone groups are usually used to study this reaction. The terms “electrocatalysis” and

“catalytic electro-oxidation” have been frequently used to explain the NADH oxidation5–9 mechanism on a quinonemodified electrodes, mainly owing to the shape of the cyclic voltammogram. The “electrocatalytic” oxidation of NADH has been typically reported for ortho-quinones, anthraquinones (see the scheme in Figure 1), and their derivatives. In contrast, several authors have reported that quinones act as redox mediators.10–15 Even though the quinone–electrolyte interface plays an important role as an electron donor/acceptor, there is no consensus about the mechanism of electron transfer. As far as the experimental approach is concerned, there is an intrinsic problem in solving the reaction mechanism: both the mediated electron transfer mechanism and electrocatalysis can result in similar responses in steady-state electrochemistry because both increase the faradaic current at a low overpotential. However, if there is a catalytic effect on a reaction taking place at the surface of a quinone-modified electrode due to the electrode itself, an increase in the electrochemical reaction rate is expected, and the electronic properties of the electrode surface should change. However, to the best of our knowledge, there are no studies in the literature in so far. In addition, the NADH electrocatalysis mechanism involving direct electron transfer has never been reported in terms of the spectroscopic data of the electrode. Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that has been used to investigate in situ electrochemical phenomena16–22 or electrochemical systems under operando conditions.23,24 This is because EPR is a highly

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specific technique capable of detecting the formation and disappearance unpaired electrons or radicals involved in redox processes.

Figure 1. (a) Illustration of immobilized ADH (PDB 4W6Z) on a carbon electrode highlighting the electro-oxidation of the coenzyme NADH to NAD+ involving different quinone groups on the carbon electrode surface. (b) Chemical structure of the coenzyme NAD+. (c) General reaction pathway of NADH electrooxidation involving quinones as frequently reported in the literature.5,8,25

In this context, we propose a new experimental approach that may contribute to the understanding of the NADH oxidation mechanism (and eventually to other redox coenzymes) on a quinone-modified carbon electrode. This approach is based on operando electron paramagnetic resonance (EPR) spectroscopy of modified carbon electrodes under NADH electro-oxidation, where the quantity of unpaired electron spins can be measured with an applied overpotential, thus revealing the role of the quinone functional groups on the electro-oxidation of NADH. The versatility of operando EPR is discussed to clarify mechanistic ambiguities and in terms of broader application to other electrocatalytic systems. In this context, we used operando EPR measurements, because EPR is a powerful technique used to investigate in situ electrochemical reactions or electrochemical systems under operando conditions. This is because EPR is a highly specific technique capable of detecting the formation and disappearance unpaired electrons or radicals involved in redox processes. RESULTS AND DISCUSSION Carbon electrodes functionalized with quinones. There are several ways to promote the functionalization of a carbon electrode surface. We have selected two different methodologies to modify the surfaces of carbon electrodes with quinone groups towards the electro-oxidation of NADH to NAD+. The first one consists of covalent functionalization through electrochemical grafting to introduce anthraquinone functionalities to flexible carbon fiber (FCF) electrodes, named FCF-AQ here. This methodology has been consolidated from several studies described elsewhere.26–28 The second methodology consists of the modification of FCF with quinone groups by using a one-pot reaction with KMnO4 under acidic conditions.3,29,30 This methodology has been proven to be very useful for engineering more robust and stable electrodes for applications in enzyme bioelectrochemistry, particularly for the high electrochemical performance of dehydrogenase-based electrodes. In this case, one-pot synthesis improves the

electrode surface by subjecting it to oxidation through a direct chemical reaction, avoiding the complicated separation and purification of the intermediates involved, which sufficiently reduces the time and the use of chemicals. An anode FCF electrode modified by this procedure is named FCF-O. The proposed reaction pathways26–30 of both of the functionalized FCF-O and FCF-AQ electrodes are shown in Figure 2.

Figure 2. (a) Possible reaction pathway of FCF oxidation by KMnO4 under acidic conditions. (b) Electrochemical grafting of AQ groups to FCF in a HCl solution (0.5 mol L-1) containing 1aminoanthraquinone (1.0 mmol L-1) and NaNO2 (4.0 mmol L-1) in an Ar atmosphere.

In order to obtain FCF-AQ electrodes, the protocol initially involves the in situ generation of an anthraquinone diazonium cation in an aqueous solution containing 1aminoanthraquinone, NaNO2, and HCl.26–28 Then, cyclic voltammograms obtained during this process are shown in Figure S1a in the Supporting Information, where the formation of a reduction wave at 0.10 V is indicative of the attachment of anthraquinonyl groups to the surface of FCF. The decrease in the reduction wave current density and the formation of welldefined redox peaks in the subsequent cycles are consistent with the surface functionalization. This electrode was further characterized by cyclic voltammetry, and the results are compared with pristine FCF, as shown in Figures S1b and S1c. The FCF-AQ anode showed a very well-defined electroactivity for both acid and alkaline electrolytes. The surface density of anthraquinone groups obtained from the charge of the anodic peak was found to be 2.8 × 10-9 mol cm-2, which is in accordance with the literature.26,28 In a similar way, the surface density of o-quinones present on FCF-O was found to be 1.3 × 10-9 mol cm-2. The electrodes were further characterized electrochemically using the K4[Fe(CN)6]/K3[Fe(CN)6] redox probe (see Figure S2). Pristine FCF, FCF-O, and FCF-AQ anodes were further characterized by scanning electron microscopy (SEM), micro Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Figure 3a displays an SEM image of pristine FCF, which exhibits uniform surface with parallel lines and shallow grooves. This morphology is characteristic of FCF obtained by the carbonization process of polyacrylonitrile (PAN) filaments through a wet spinning procedure.21 Figure 3b shows an SEM image of an FCF-O electrode, which shows an increase in the depths of the grooves and the formation of defects on the surface. An SEM image of an FCF-AQ electrode (Figure 3c) shows a rougher surface with wider and deeper stretches. On the basis of the SEM images, it can be inferred that the surfaces of both of the FCF-O and FCFAQ electrodes are strongly affected by the functionalization processes. As observed from the SEM results, the presence of defects on the electrode surfaces after the functionalization processes is clear. In this context, micro-Raman


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540 537 534 531 528 525 Binding energy (eV)

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Figure 3. SEM images of (a) FCF, (b) FCF-O, and (c) FCF-AQ. Raman spectra of (d) FCF, FCF-O, and FCF-AQ. All spectra were fitted using a Lorentzian curve with a linear baseline and normalized according to the G-band intensities. XPS spectra of the (e) C 1s and (f) O 1s regions for FCF, FCF-O, and FCF-AQ. Plots of the concentration versus the functional groups of FCF, FCF-O, and FCF-AQ in the (g) C 1s region and (h) O 1s regions of the XPS spectra in (e) and (f).

spectroscopy was conducted in order to characterize these defects. Figure 3d shows the first-order region of the Raman spectra of pristine FCF, FCF-O, and FCF-AQ electrodes. Each spectrum consists of two main characteristic bands: D and G bands centered at about 1365 and 1603 cm-1, respectively. The G band is associated with the presence of sp2 carbon networks in the electrodes, while the D -band is a defect-induced Raman feature related to the presence of amorphous or disordered carbon forms.31 The line decomposition of the raw spectra (see Figure S3) shows the presence of a weak shoulder, the A band at 1555cm-1. This band is assigned to amorphous forms of carbon32 and is commonly observed for carbon fibers.33,34 The ratio of the intensities of the D and G bands (ID/IG)31 was used to characterize the number of defects present in carbon fibers promoted by surface modification methodologies. The calculated values of ID/IG were 0.98, 1.27, and 1.03 for FCF, FCF-O, and FCF-AQ respectively (see Table S1). Since the values of ID/IG increase with increasing disorder,31 the results indicate that the introduction of quinone functionalities by both methodologies increases the number of defects in the carbon fiber structure. In addition, it can be concluded that the chemical treatment with KMnO4 promotes a larger number of defects on the FCF-O surface than functionalization through electrochemical grafting. The defects are intrinsically related to the presence of oxygenated groups, as we shall show in following. In order to obtaining chemical information on the surface of pristine FCF, FCF-O, and FCF-AQ electrodes, X-ray photoelectron spectra were recorded. Figure 3e shows the deconvoluted C 1s core-level XPS spectra for pristine FCF, FCF-O, and FCF-AQ electrodes. The C 1s spectra for FCF-O electrode was fitted with five peaks using pseudo-Voigt function, while four peaks were observed for FCF-AQ. For all electrodes, peak I is attributed to sp2 carbon (C–C aromatic) observed at 284.7 eV for pristine FCF, 283.9 eV for FCF-O, and 284.4 eV for FCF-AQ. The presence of sp3-bonded carbon

atoms (C–H, C–N) is indicated by peak II located at 284.5 eV for FCF-O and 285.1 eV for FCF-AQ. Peak III belongs to phenolic, alcohol, or ether groups and is observed at 286.1 eV for pristine FCF, 285.1 eV for FCF-O, and 286.1 for FCF-AQ. Carbonyl or quinone groups are associated with peak IV and are observed at 286.0 eV for FCF-O and 288.2 eV for FCFAQ.3,35,36 The surface concentrations of different functional groups, as calculated on the basis of the peaks observed in the XPS spectra, are shown in Figure 3g, where a decrease in the concentration of sp2 carbon and an increase in the concentration of sp3 carbon (C–H, C–N) for the FCF-O and FCF-AQ electrodes, respectively, clearly confirm the surface functionalization of these electrodes. On the other hand, the increases in the carbonyl peak at 286.0 eV for FCF-O and 288.2 eV for FCF-AQ clearly confirm the formation of quinone molecules on their surfaces. Figure 3f shows the deconvoluted XPS spectra in the O 1s region for pristine FCF, FCF-O, and FCF-AQ electrodes, where three peaks were observed for pristine FCF and the FCF-O and FCF-AQ electrodes. Here, peak I is related to carbonyl or quinone and is observed at 531 eV for pristine FCF, 531.6 eV for FCF-O, and 531.4 eV for FCF-AQ.3,35 Peak II corresponds to carbon in phenolic, alcoholic, or ether groups, which is observed at 532.0, 532.6, and 532.7 eV for pristine FCF, FCFO, and FCF-AQ electrodes, respectively.3,35 Additional peaks were observed at 533.0, 533.6, and 533.7 eV for pristine FCF, FCF-O, and FCF-AQ electrodes, respectively, which correspond to carboxylic acid groups. The deconvoluted XPS spectra in the O 1s region corroborate the spectra in the C 1s region, exhibiting an increase in surface oxidation for both electrodes. Figure 3h shows a decrease in the concentration of C–O groups and an increase in the concentration of C=O groups on the FCF-O and FCF-AQ electrodes compared to pristine FCF. Electro-oxidation of NADH. The electro-oxidation of NADH was evaluated for both FCF-O and FCF-AQ electrodes. First,



cyclic voltammetry was performed in a phosphate buffer solution (0.1 mol L-1, pH 7.5) in the absence of NADH, as shown in Figures 4a and 4b (Figures 4c and 4d show magnified regions of Figure 4a and 4b, respectively). Well-resolved redox behavior is observed for both electrodes, which is consistent with surface modification with quinones. No redox process was observed for pristine FCF (see Figure S4). The values of Epa and Epc are -0.05 and -0.13 V for FCF-O and -0.33 and -0.84 V for FCF-AQ, respectively. In order to obtain the heterogeneous electron transfer (HET) constant for both electrodes, the Butler– Volmer equation was used since the quinone reaction is controlled by charge transfer at the electrode and

Figure 4. Cyclic voltammograms obtained in a phosphate buffer solution (0.1 mol L-1, pH 7.5) for (a) FCF-O and (b) FCF-AQ. (c) and (d) Magnified regions of FCF-O (a) and FCF-AQ (b) at different scan rates of 5–1000 mV s-1 (FCF-O) and 5–500 mV s-1 (FCF-AQ). Cyclic voltammograms obtained in a phosphate buffer solution (0.1 mol L-1, pH 7.5) containing NADH (1.0 mmol L-1) for (e) pristine FCF (●) and FCF-O (●) and (f) pristine FCF (●) and FCF-AQ (●) at a scan rate of 5 mV s-1. All experiments were performed in Ar atmosphere at T = 25 oC. not by mass transfer. By using the Butler–Volmer model, (see Figure S5), the HET constants at the zero overpotential condition (k0) were estimated to be 6.4 and 11.94 s-1 for FCF-O and FCF-AQ, respectively, and these values are consistent with the literature.29 The electrochemical behavior toward NADH oxidation was then evaluated in the presence of 1.0 mmol L-1 NADH, as shown in Figures 4e and 4f. Unlike pristine FCF, NADH results in drastic changes in the cyclic voltammograms, where the anodic peak current increases and the cathode peak disappears. The wave shape indicates the typical electro-oxidation of NADH on the surfaces of both electrodes. The value of Epa is 0.62 V for FCF-O and 0.72 V for FCF-AQ in the presence of NADH in solution. The FCF-AQ electrode shows a higher activity than FCF-O, as confirmed by the peak current density of 0.24 mA cm-2, which is three times higher than 0.08 mA cm-2 observed for the FCF-O electrode. As expected, the presence of quinones on the electrode surface enhanced the faradaic current. In following, the nature of this behavior is described in terms of operando EPR. Operando EPR and NADH electrocatalysis. Operando EPR experiments were carried out using an X-band EPR spectrometer coupled with a potentiostat/galvanostat (see Figure S6) for the electrochemical regeneration of NAD+ from NADH catalyzed by quinone modified carbon-based electrode. The electro-oxidation of

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NADH is extremely necessary to regenerate an NAD-dependent enzyme. Previous studies showed in-situ electrochemical EPR for investigation of charge transfer across the liquid/liquid interface16 and to probe electrical double layer capacitance,17 as well as to study the electrochemistry of polyaniline films,18,21 carbon nanotubes and fullerene,20 enzyme,22 the formation of mossy lithium and dendrite on lithium anodes and redox mechanisms in lithium-ion batteries.23,24 Here, we propose observe how a redox process taking place on the surface of carbon electrodes affect the concentration of unpaired electrons.

An operando EPR electrochemical cell was designed for use with aqueous electrolyte. This cell consists of a capillary system with three electrodes comprising pristine FCF, FCF-O, or FCFAQ as the working electrode (WE), platinum as the counter electrode, and saturated Ag/AgCl as the reference electrode, as schematically shown in Figure 5a. Using this experimental setup, the X-band EPR spectra are recorded simultaneously with chronoamperometric measurements after 50 s, when the steady-state (SS) current is obtained. Figure 5b shows the Xband EPR spectra of pristine FCF (black), FCF-O (red), and FCF-AQ (blue) electrodes in a phosphate buffer (0.1 mol L-1, pH 7.5) in the absence of NADH at 0.60 V. Increase in the EPR signal intensities are observed for the FCF-O and FCF-AQ electrodes compared to pristine FCF, suggesting that the presence of quinone functionalities causes an increase in the number of unpaired electrons on the electrode surface. Interestingly, in the presence of NADH (Figure 5c) the concentration of unpaired electrons increases for all electrodes. However, for FCF-O and FCF-AQ, the EPR signal is significantly higher compared to that for pristine FCF. For FCFO, the concentration of unpaired electrons (spin concentration, SC) increased 44% (ΔSC) in the presence of NADH, while ΔSC was 80% for FCF-AQ (Figure 5d). This value indicates that FCF-AQ presents a superior activity for the oxidation of NADH. The spin ratio between FCF-O and FCF-AQ ( 𝑆𝑅FCF ― O FCF ― AQ) provides useful quantitative information about the electro-oxidation process. 𝑆𝑅FCF ― O FCF ― AQ can be determined as follows: Δ𝑆𝐶FCF ― O

𝑆𝑅FCF ― O FCF ― AQ = Δ𝑆𝐶FCF ― AQ

(1)

𝑆𝑅FCF ― O FCF ― AQ, 0.75 which indicates a higher unpaired electron concentration for FCF-AQ under the operando condition (in the presence of NADH under 0.60 V). This value agrees with the ratio of the surface concentration of quinone groups for FCF-O (1.3 × 10-9 mol cm-2) and FCF-AQ (2.8 × 10-9 mol cm-2), which is 0.46. This concordance corroborates the hypothesis that quinone interacts with NADH molecules during the electro-oxidation reaction. Figure 5e shows a plot of the relative spin concentration versus the potential for FCF-O in a phosphate buffer in the absence of NADH (shown by the red line) and in the presence of NADH (shown by the black line). The increase in the spin concentration with the applied potential suggests that the kinetics of NADH electro-oxidation have Butler–Volmer behavior, as in the polarization curve. This result also corroborates that the limiting step in the reaction is NADH oxidation since the observed values of k0 were 6.4 and 11.94 s-1 for FCF-O and FCF-AQ, respectively. Since an unpaired electron is associated with the semiquinone radical, the EPR results suggest a new mechanism for electron transfer, as shown in Figure 6. The proposed mechanism is based on clear surface. This is a typical characteristic of electrocatalytic behavior.


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Figure 5. (a) Operando EPR cell design for applications in electrochemistry. X-band EPR spectra of pristine FCF (●), FCF-O (●), and FCFAQ (●) in a phosphate buffer solution (0.1 mol L-1, pH 7.5) in (b) the absence of NADH and (c) the presence of NADH (1.0 mmol L-1) at 0.60 V. (d) Spin concentration obtained with FCF-O (●) and FCF-AQ (●) in a phosphate buffer solution (0.1 mol L-1, pH 7.5) in the absence of NADH and in the presence of NADH (1.0 mmol L-1). (e) Plot of the relative spin concentration versus the applied potential for FCF-O in the absence of NADH (●) and in the presence of NADH (1.0 mmol L-1) (●). All measurements were carried out at 273 K.

A two-step mechanism could be assumed to explain the electrooxidation of NADH by the quinones on the surface of the electrode. It is possible that there is proton-coupled electron transfer (PCET) taking place at the surface. However, quinones are also known to undergo single electron reduction to a semiquinone.

Figure 6. Mechanism of the formation of unpaired electrons on the surface of a quinone-modified carbon electrode. Semiquinone is a free radical. This scheme shows an anion radical as an intermediate between the fully reduced and fully oxidized states catalyzed by NADH. The NADH intramolecular reactions are omitted in this model.

EPR measurements were carried out with an applied potential (ss current), and two steps can be considered for electron transfer during NADH oxidation: 1) the proton coupled electron transfer from NADH to quinone and 2) the reduction of hydroquinone. Semiquinone is a free radical, that can be assumed as a possible intermediate electron donor to the electrode surface, promoting an increasing of free unpaired electrons. The scheme in Figure 6 shows an anion radical as an intermediate between the fully reduced and fully oxidized states catalyzed by NADH. In the past, this reaction was proposed in terms of abortive side reactions caused by the initial reaction between catechol (from NADH) and o-quinone, “presumably caused by an intermediate semiquinone”.8 For the first time, we show a new correlation for the spin content that clearly reveals an increasing number of free unpaired electrons with increasing applied overpotential and NADH oxidation, which corroborates the fact that quinone groups on carbon surfaces act as electrocatalysts towards the oxidation of NADH to NAD+. CONCLUSION Operando EPR spectroscopy proved to be very useful for investigating the NADH/NAD+ redox reaction. We introduce how to probe the number of free electrons on a carbon electrode surface and correlate it with the electrocatalytic mechanism. The correlation with the spin concentration reveals an increasing number of free unpaired electrons with increasing applied overpotential and NADH oxidation, which corroborates the controversial hypothesis that quinone groups act as electrocatalysts towards the oxidation of NADH to NAD+. Furthermore, operando EPR provides useful information by probing the electron transfer dynamics on a carbon surface, and



we anticipate that this approach can be extended to other chemical dynamics systems involving electron transfer.

ASSOCIATED CONTENT

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Supporting Information. Supporting information is available free of charge on the ACS publication website. Supplementary figures (Figures S1–S7) and supplementary tables (Table S1). (13)

AUTHOR INFORMATION Corresponding Author * [email protected]

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Author Contributions These authors contributed equally to this manuscript.

Notes

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The authors declare no competing financial interest. (16)

ACKNOWLEDGMENT

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The authors would like to thank the Third World Academy of Sciences (TWAS) and gratefully acknowledge the financial support provided by the Sao Paulo Research Foundation (FAPESP) for the ongoing projects of F. N. C. (Grant Nos. 2015/16672-3 and 2013/14262-7), A. H. (Grant No. 2016/25806-6), and G. C. S. (Grant No. 2015/22973-6). F. N. C. would also like to thank Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) for financial support (Project No. 478525/2013-3).

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