Mechanistic Study for the Facile Oxidation of Trimethoprim on a

ARTICLE pubs.acs.org/JPCC

Mechanistic Study for the Facile Oxidation of Trimethoprim on a Manganese Porphyrin Incorporated Glassy Carbon Electrode Leena Rajith,† A. K. Jissy,†,‡ Krishnapillai Girish Kumar,*,† and Ayan Datta*,†,‡ † ‡

Department of Applied Chemistry, Cochin University of Science and Technology, Kochi-682022, Kerala, India School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Thiruvananthapuram-695016, Kerala, India

bS Supporting Information ABSTRACT: The electrocatalysis of [5,10,15,20-tetrakis(4methoxyphenyl)porphyrinato]Mn(III)chloride (TMOPPMn(III)Cl) toward the oxidation of trimethoprim was investigated by electrochemical and computational methods. Voltammetric analysis demonstrates the ability of TMOPPMn(III)Cl to act as an electrocatalyst for the oxidation of trimethoprim. DFT calculations were performed at the B3LYP level to detect the site of oxidation as well as to survey the role of manganese porphyrin in the facile oxidation of trimethoprim. Quantum chemical calculations affirm that the deprotonation of NH2 attached to C4 is more energetically favorable as compared to deprotonation of the C2 amino group. Also, the deprotonation of trimethoprim in the TMOPPMn(III)Cl-catalyzed reaction results in the formation of a more stable anion as compared to the uncatalyzed reaction. Atoms in molecule (AIM) analysis and natural bond orbital (NBO) analysis also substantiate the utility of TMOPPMn(III)Cl in trimethoprim oxidation by confirming the coordination of trimethoprim to manganese porphyrin on removal of proton.

’ INTRODUCTION Natural tetrapyrrolic macrocycles, in the form of metal complexes, play a vital role in certain biological processes, such as those concerned with respiration, drug detoxification, photosynthesis, and others. Porphyrin and chlorin derivatives are among the most important tetrapyrrolic macrocycles.1
3 The design of new materials with electrocatalytic properties constitutes a subject of interest for chemical, analytical, and photochemical applications. Metalloporphyrins and metallophthalocyanins are remarkable precursors in supramolecular chemistry. The rapid development of this chemistry led to assemblies possessing various architectures and different photo, electro, and catalytic properties.4 Axial coordination of a metalloporphyrin provides only one or two binding sites for other molecules. Additional binding centers can result in the increased stability of supramolecular assemblies and can be created by incorporating coordinating substituents at the periphery of a porphyrinic cycle. Porphyrins possess the ability to bind a large variety of substrates depending on the porphyrin structure. Many different supramolecular architectures based on porphyrin
substrate complexes are used for molecular recognition purposes, as receptors and sensors. Utilization of porphyrins in catalysis is also based on their ability to bind substrates.5
8 Several experimental and computational works have been performed on oxidative catalysis by metalloporphyrin, especially Mn(III) and Fe(III).9
23 DFT computational approaches have afforded an important new tool for assessing the roles of axial ligation, spin state r 2011 American Chemical Society

crossings, and charge distribution in the metalloporphyrincatalyzed reaction.22,23 Electrochemical methods are very sensitive and selective. These techniques are used in the analysis of ions and electroactive compounds, require small sample volumes, and can be used for in natura determination of a great variety of samples. Modification of electrodes with suitable biocompatible materials enables the electrochemistry of the redox biological compounds to proceed without hindrance, which results in increased selectivity and sensitivity of analytical determination.24
31 Metalloporphyrins can act as a sensing material and show excellent electrocatalytic activity and stability. This opens a new avenue for fabricating electrochemical devices. Metalloporphyrins supported on electrodes have proven to be potential candidates as suitable modification materials.27
33 Trimethoprim, chemically known as 5-(3,4,5-trimethoxybenzyl)pyrimidine-2,4-diamine, belongs to the class of chemotherapeutic agents known as dihydrofolate reductase inhibitors. It is a synthetic antibiotic used in prophylaxis treatment and urinary tract infections for HIV-affected patients, which interferes with the production of tetrahydrofolic acid from dihydrofolic acid. Bacteria are unable to take up folic acid from the environment, i.e., the infection host, and are thus dependent on their own de Received: August 20, 2011 Revised: September 16, 2011 Published: September 27, 2011 21858

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Scheme 1. Oxidation of Trimethoprim

Figure 1. Differential pulse voltammogram of trimethoprim at (a) bare GCE and (b) TMOPPMn(III)Cl/GCE.29

Figure 2. Variation of anodic peak potential of 10
3 mol/L of trimethoprim with pH.

novo synthesis. Inhibition of the enzyme starves the bacteria of nucleotides necessary for DNA replication, causing in certain circumstances cell lethality due to thymine-less death. This drug was developed by George H. Hitchings and collaborators, who shared the Nobel Prize for Physiology in 1988 for the discovery of antifolates. The focus of this article is the computational investigation of how manganese porphyrin assists in the oxidation of trimethoprim.

’ COMPUTATIONAL METHODS Quantum chemical calculations have been performed using the Gaussian 03 suite of programs.34 Calculations have been carried out only for the species involved in the reaction, omitting the electrode. Barth et al.35 have shown by ab initio calculations and scanning tunneling microscopy (STM) observations of tetrapyridyl and Fe(II)-tetrafluoride-porphyrin molecules on the Ag(111) surface that the molecular structure remains largely unaffected by the adsorption on the silver surface. Many studies have been carried out in which density functional theory (DFT) has been used as a tool to investigate mechanisms of reactions occurring on surfaces, without considering the electrode or the surface on which these reactions occur.36 D’Souza et al. also showed that the HOMO
LUMO energy gaps obtained for porphyrin systems by DFT calculations follow the trends of the electrochemical results. Their studies revealed good

agreement between the experimental results and computational predictions.37
39 However, it is important to note that these in vacuo calculations provide a qualitative idea about the oxidation mechanism, and a quantitative estimation will require an explicit consideration for the presence of the electrode and solvation effects. All the geometries were fully optimized by density functional theory (DFT) using the Becke, three-parameter, Lee
Yang
Parr (B3LYP) hybrid functional theory.40,41 The 6-31+G(d) basis set42,43 was used to optimize all the structures involved in the uncatalyzed reaction of trimethoprim. Because quantum mechanical studies on the oxidation in the real system containing four bulky substituents on the porphyrin catalyst are prohibitively expensive, we have used a chemically relevant model for studying the mechanism of this reaction. For the calculations, the porphyrins investigated in the experiments have been simplified to the corresponding basic porphyrins; that is, the peripheral 4-methoxyphenyl groups have been replaced by hydrogen atoms. Thus, [Mn(III) porphyrin]Cl is chosen as a model for TMOPPMn(III)Cl in the present investigation. The major interactions in our present study of the oxidation of trimethoprim occur between the dyz orbitals of manganese and the pz orbital of the nitrogen (N8) atom of trimethoprim. Our model can effectively explain such interactions.36 For the optimization of the geometries involved in the [Mn(III) porphyrin]Cl-catalyzed reaction, the split basis set method with 6-31+G(d) for the main group elements and compact effective potentials split valence set (CEP-31G)44,45 for manganese were utilized. These geometries were also optimized using the Los Alamos 19 electron shape consistent relativistic ECP (LANL2DZ) basis set46,47 to carry out natural bond orbital (NBO) and atoms-in-molecules (AIM) analysis. All minimum energy structures have been characterized through frequency calculations to ensure that there are no saddle points (imaginary frequencies). The AIM approach relies on an analysis of the topological properties of the charge density, F(r), and its quality depends on the computational level chosen. Both the gradient and the Laplacian of the charge density, rF and r2F, respectively, can be analyzed and provide complementary information on bonds. The critical points of rF give information about the existence of bonds, while the sign of r2F at that point reflects the kind of interaction. Nuclei attract the charge density so that maxima of rF are found there. A bond corresponds to a saddle point (the bond critical point), where rF becomes zero, a maximum only in one plane of space, and is found joining two trajectories of maximum r along the space, toward the nuclei.48 NBO analysis is based on a method for optimally transforming a given wave function into localized form. In NBO analysis, the input atomic orbital basis set is transformed via natural atomic orbitals (NAOs) and natural hybrid orbitals (NHOs) into natural bond orbitals (NBOs). The NBOs obtained in this fashion correspond to the widely used Lewis picture, in which two-center bonds and lone pairs are localized.49 21859

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Scheme 2. Mechanism of Oxidation of Trimethoprim

’ RESULTS AND DISCUSSION Voltammetric behavior of trimethoprim has been investigated at the bare glassy carbon electrode (GCE) and at TMOPPMn(III)Cl/GCE. Details of the experiment and NMR characterization of the porphyrin
Mn(III) complex, TMOPPMn(III)Cl, have been reported previously.29 Figure 1 shows the differential pulse voltammograms of 10
3 mol/L trimethoprim in phosphate buffer solution at bare GCE (curve a) and TMOPPMn(III)Cl/ GCE (curve b). At the bare GCE, trimethoprim yields a very low oxidation peak at 1.160 V. Under the same conditions, a welldefined oxidation peak appears at 1.088 V on TMOPPMn(III)Cl/

GCE. Obviously, the peak potential shifts toward a more negative potential compared to that of a bare GCE. The negative shift of oxidation potential may be attributed to the electrocatalytic activity of TMOPPMn(III)Cl. The electrochemical studies of 10
3 mol/L carried out in the pH range 3
10 show that the peak current is highest at pH 5. However, the anodic potential decreases with an increase in pH value as seen from Figure 2. The number of electrons involved in the electrochemical reaction of trimethoprim was calculated to be 3.8 (close to 4). On the basis of this experimental finding, the proposed possible mechanism is shown in Scheme 1. 21860

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Figure 3. Deprotonation of trimethoprim.

The voltammetric behavior of trimethoprim investigated at the [5,10,15,20-tetrakis(4-methoxyphenyl)porphyrinato]Mn(III)chloride-modified glassy carbon electrode (TMOPPMn(III)Cl/GCE) showed that TMOPPMn(III)Cl acts as an electrocatalyst for the oxidation of trimethoprim.29 There are two amino groups, one at position 4 of the pyrimidine ring and the other at position 2 of the stable pyrimidine system.50 Deprotonation of the NH2 group attached to either C4 or C2 of trimethoprim can occur, and thus both of these possibilities were investigated computationally. The internal energy change (ΔE), enthalpy change (ΔH), and free energy change (ΔG) of the deprotonation of NH2 attached to C4 are
35.9,
30.8, and
31.4 kcal/mol, respectively, while those attached to C2 are
26.4,
27.3, and
28.3 kcal/mol, respectively. Clearly, deprotonation of the former is more facile than that of the latter. Thus, for further calculations, we have considered the removal of a proton from the C4 amino group. The detailed mechanism50
53 is outlined in Scheme 2. The first step involved in the conversion of trimethoprim (I) to nitroso derivative (IX) is the deprotonation of the NH2 group at the fourth position to give the trimethoprim anion (II). The formation of (II) from (I) is an exothermic process (ΔH =
30.8 kcal/mol). The distance between C4 and N8 in (I) and (II) is 1.38 and 1.32 Å, respectively, as shown in Figure 3. [Mn(III) porphyrin]Cl 3 3 3 trimethoprim was optimized with 0, 1, and 2 spin states to analyze their relative stabilities. The geometry with spin state 2 (multiplicity = 5) was found to have the lowest energy and hence is the most stable one. Hence, all the further studies were carried out using [Mn(III) porphyrin]Cl 3 3 3 trimethoprim with spin state 2. As for the uncatalyzed reaction, the deprotonation of NH2 of trimethoprim attached to C4 and C2 for the [Mn(III) porphyrin]Cl-catalyzed reaction was also investigated. ΔE, ΔH, and ΔG corresponding to the deprotonation of NH2 attached to C4 are found to be
48.3,
48.4, and
48.4 kcal/mol, respectively, while those attached to C2 are
43.4,
45.0, and
42.2, respectively. The values of thermodynamic properties suggest that deprotonation at the fourth position is more favorable than that at the second position. Compared to the uncatalyzed reaction, the coordination of [Mn(III)

Figure 4. Deprotonation in [Mn(III) porphyrin]Cl 3 3 3 trimethoprim.

porphyrin]Cl with the trimethoprim anion increases the exothermicity of the deprotonation. This suggests that [Mn(III) porphyrin]Cl 3 3 3 trimethoprim (A) gets further stabilized by deprotonation. It is interesting to note that the distance between N8 and Mn is 3.64 Å in (A), whereas in the [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion (B) it is 2.23 Å as shown in Figure 4. The trimethoprim anion gets stabilized by the interaction of Mn(III) of the metalloporphyrin with N of the NH2 group at the fourth position of trimethoprim which carries the reaction forward. HOMO-1 for (A) and (B) shown in Figure 5 reveals that HOMO-1 of (B) is interacting, whereas that of (A) is not. Thus, on deprotonation, the trimethoprim anion coordinates to [Mn(III) porphyrin]Cl, thus facilitating further oxidation. It is possible that other steps further down the reaction mechanism might also be playing an important role. Nevertheless, the highly exothermic process of binding of the anion to the Mn(III) complex suggests that the step I f II will be a dominating path in the reaction. On the basis of the topology of electron density, which can be obtained from either quantum mechanical computations or accurate experiments, the AIM theory provides rigorous 21861

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Table 1. NBO Analysis of [Mn(III) porphyrin]Cl 3 3 3 trimethoprim and[Mn(III) porphyrin]Cl 3 3 3 trimethoprim Anion at the B3LYP/6-31+G(d),CEP-31G Level of Theorya complex Anion

Neutral

donor orbital

hybrid 0.26 0.18

acceptor orbital

hybrid 4.66

E(2) (kcal/mol)

ej
ei (au)

Fij (au)

LP* Mn

sp

RY* N8

sp

3.19

1.08

0.169

LP* Mn BD C4-N8

sp3.73d0.04


BD* N8-H8 LP* Mn


sp3.73d0.04

1.07 2.28

0.17 1.1

0.052 0.066

BD N8-H8




LP* Mn

sp3.73d0.04

3.57

0.89

0.074

CR N8

s

LP* Mn

sp3.73d0.04

2.45

14.43

0.25

LP N8

p

LP* Mn

sp3.73d0.04

22.21

0.67

0.158

LP N8

p

RY* Mn

sp0.32d0.58

2.9

3.79

0.144

LP N8

p

BD* Mn-N8

sp1.7

3.73

0.3

0.045

RY* N8

sp67.74

2.36

0.48

0.112

0.14 0.87

0.002 0.03

d

0.04

LP* Mn

sp

LP* Mn BD N8-H8

sp0.04


BD* N8-H8 LP* Mn


sp21.89

0.28 0.59

CR N8

s

LP* Mn

sp21.89

0.06

LP N8

p

LP* Mn

sp21.89

1.36

0.46

0.032

LP N8

p

RY* Mn

sp0.1d1.22

0.05

4.31

0.019

14.4

0.038

“BD” for 2-center bond, “CR” for 1-center core pair, “LP” for 1-center valence lone pair, “RY*” for 1-center Rydberg, and “BD*” for 2-center antibond. The unstarred and starred labels correspond to Lewis and non-Lewis NBOs, respectively. a

Figure 5. HOMO-1 of (A) [Mn(III) porphyrin]Cl 3 3 3 trimethoprim and (B) [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion.

definitions for atoms in molecules/crystals and the bonds which link the atoms and thus transforms the qualitative concepts into a quantitative description. The AIM theory allows for the characterization of the nature of chemical interactions based on experimental and/or theoretical electron densities rather than being limited to the classical connectivity of atoms. According to Bader’s notation, the critical points are characterized by the rank and signature of the Hessian matrix r2F(r) of the electron density. The rank is the number of nonzero eigenvalues of F(r), and the signature is the number of positive eigenvalues minus the number of negative ones. In general, theoretical points of electron density for energetically stable molecules or crystals are all of rank three, and there are just four possible signature

values, i.e., four kinds of critical points: (3,
3) or local (nucleus or non-nucleus) maxima, (3,
1) or bond critical points (BCP), (3,1) or ring critical points (RCP), and (3,3) or cage critical points (CCP). The observable topologic parameters are obtained, which are the electronic density (F) and Laplacian field (r2F), which describe the molecular stability through the identification of charge density centers within the chemical bond, and so, the internuclear pathways are classified as shared (covalent) or closed-shell (noncovalent) when the electronic density is concentrated or depleted, respectively. The plot of the gradient of the charge density F in [Mn(III) porphyrin]Cl 3 3 3 trimethoprim and [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion is depicted in Figure 6(A) and (B), respectively. The most striking feature of the 21862

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Fij for the LPN f LP*Mn for [Mn(III) porphyrin]Cl 3 3 3 trimethoprim are 1.36 kcal/mol, 0.46 au, and 0.032 au, and for (B) [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion the corresponding values are 22.21 kcal/mol, 0.67 au, and 0.158 au, respectively. These values show that much higher stabilization is provided by the electron delocalization between manganese and N8 (trimethoprim) on removal of the proton. The results of second-order perturbation theory analysis of the Fock matrix are collected in Table 1. The molecular orbital coefficients represent the contribution of a particular orbital on an atom to the molecular orbital formed. The orbital coefficients, in the case of the HOMO orbital, for N8 and Mn are 0.026 and 0.007, respectively, in [Mn(III) porphyrin]Cl 3 3 3 trimethoprim, whereas for [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion, they are 0.39 and 0.13, respectively. The Wiberg bond index matrix in the NBO analysis also points out a bond order of 0.02 and 0.45 between N8 and Mn in (A) and (B), respectively. This is further evidence for the fact that trimethoprim coordinates with the Mn of the [Mn(III) porphyrin]Cl only when it is converted to its anionic form on deprotonation.

Figure 6. Plots of the gradient of charge density in (A) [Mn(III) porphyrin]Cl 3 3 3 trimethoprim and (B) [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion. Please note that the BCP is represented by a red dot between the atoms.

plots is the absence of critical points between N8 and Mn in [Mn(III) porphyrin]Cl 3 3 3 trimethoprim [Figure 6(A)], whereas in [Mn(III) porphyrin]Cl 3 3 3 trimethoprim anion, there exists a (3,
1) bond critical point between these atoms [Figure 6(B)], with charge density, rF = 0.0814, and Laplacian of charge density, r2F =
0.0786. The value of charge density indicates that the accumulation of charge between the metal atom and nitrogen on the formation of the anionic species is not very substantial, as electron density gets depleted for these intermolecular interactions. For closed-shell interactions, where charge is depleted (measured by r2(r) BCP) along the BP, the Laplacian is positive. However, in AIM 2000, the negative of the Laplacian is calculated. Therefore, the negative sign for r2(r) observed for our complex represents noncovalent interaction between manganese and the anion of trimethoprim which stabilize the complex with respect to the neutral species.54,55 Natural bond orbital analysis stresses the role of intermolecular orbital interaction in the complex, particularly charge transfer. This is carried out by considering all possible interactions between filled donor and empty acceptor NBOs and estimating their energetic importance by second-order perturbation theory. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with electron delocalization between the donor and acceptor is estimated as Eð2Þ ¼ qi

’ CONCLUSIONS We have shown by electrochemical experiments and computational methods that TMOPPMn(III)Cl acts as an electrocatalyst for the oxidation of trimethoprim. From the voltammetric oxidation of trimethoprim investigated at the bare and TMOPPMn(III)Cl-modified GCE, it is inferred that the anodic potential required for the oxidation of trimethoprim gets lowered on the TMOPPMn(III)Cl/GCE compared to that on bare GCE. Quantum mechanical calculations explain the role of manganese porphyrin in the easy oxidation of trimethoprim. The calculations show that the oxidation of the amino group attached to C4 of trimethoprim is easier than that attached to C2. In comparison to the uncatalyzed, the catalyzed deprotonation, which is the preliminary step in the oxidative mechanism, is energetically more favorable. AIM analysis shows the existence of a bond critical point between manganese and the nitrogen of the trimethoprim anion. Natural bond orbital analysis further supports the interaction between trimethoprim anion and manganese porphyrin. A bond order of 0.45 is calculated between N8 of trimethoprim anion and manganese of metalloporphyrin. It is thus deduced that trimethoprim anion interacts with the manganese of the metalloporphyrin. We thus attribute the facile oxidation of trimethoprim in the presence of TMOPPMn(III)Cl to the interaction between the metal ion and trimethoprim on deprotonation, which assists the transfer of electrons in the first step of the reaction. Thus, the computational calculations shed light on the role of manganese porphyrin in the easy oxidation of trimethoprim. ’ ASSOCIATED CONTENT

bS

Supporting Information. Cartesian coordinates for ground state structures, their energies in Hartrees, harmonic frequencies, and complete ref 34. This material is available free of charge via the Internet at http://pubs.acs.org.

F2ði, jÞ ej
ei

where qi is the orbital occupancy; ei, ej are diagonal elements; and Fij is the off-diagonal NBO Fock matrix element. E(2), ej
ei, and

’ ACKNOWLEDGMENT The authors thank DST
Fast Track scheme, CSIR, and University Grants Commission (UGC) for partial funding. 21863

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