An Attractive Route for CO Oxidation Process - American Chemical

Article pubs.acs.org/JPCC

DNA Base−Gold Nanocluster Complex as a Potential Catalyzing Agent: An Attractive Route for CO Oxidation Process Naresh K. Jena,†,‡ K. R. S. Chandrakumar,*,† and Swapan K. Ghosh*,†,‡ †

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai 400 085, India Department of Atomic Energy, Homi Bhabha National Institute, Mumbai, India

‡

ABSTRACT: The important contribution of DNA to modern day nanobiotechnology is sometimes ascribed to its potential role as a programmable building block for the assembly of nanomaterials. In this work, our density functional theory (DFT) based study explores the possibilities of using complexes of DNA bases (viz., adenine (A), thymine (T), guanine (G), and cytosine (C)) as well as DNA base pairs (such as A-T, C-G, and G4-quadruplex) with the gold cluster Au3 as a model catalyst system for oxidation of CO to CO2. The complexes are energetically stable, and our calculated binding energies of the gold cluster−DNA base complexes are found to match closely with the experimentally obtained heat of desorption of DNA bases on Au films. Our results also reveal that the oxidation of CO on these complexes takes place exclusively via an Eley−Rideal (E-R) type of mechanism whereas both E-R and Langmuir−Hinshelwood (L-H) mechanisms for this reaction are observed for a bare Au3 cluster. More interestingly, the CO oxidation reaction on the gold cluster (in the complexes) is found to be more facile, with a lesser activation barrier and higher heat of formation of the products, as compared to that of a pristine Au3 cluster. Our results suggest that DNA base− gold cluster complexes can be an attractive and efficient catalytic model system that can have wider ramifications. Since the nanoscale gold is already known as a potential catalyst for variety of chemical reactions, our approach will richly supplement to the expanding horizons of catalysis involving gold.

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electronics.18−20 Concepts like DNA origami have directed new paradigms for synthesizing nanoparticles of desired shapes and conformations with precision never achieved earlier.21,22 Schreiber et al.23 have further explored the DNA based origami to synthesize gold nanoparticles covering a wide range of shapes and sizes. For all these diverse applications, understanding the binding of DNA bases adenine (A), thymine (T), guanine (G), and cytosine (C) with gold is of central importance. Storhoff et al. have demonstrated that the binding and stabilities of deoxynucleosides to gold nanoparticles is highly sequence dependent.16 From combined Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), Whitman and co-workers have inferred that the adsorption affinity of DNA bases on Au-surface follows the trend as A > C ≥ G > T.17 Using temperature-programmed desorption studies, Demers et al. have quantitatively estimated the energetics of this fundamental interaction of DNA bases with gold.15 Hence, the very nature of interaction of DNA bases, nucleosides, or oligonucleotide sequences with Au forms the basis for all the versatile applications of the DNA−Au system. In sharp contrast to the attribute of bulk gold as the noblest of metals, an interesting variant, the nanoscale gold, has come

INTRODUCTION The larger aim of nanotechnology, apart from enriching of our scientific knowledge in terms of providing detailed insights into the structure, properties and phenomena at the nanoscale, is to render viable and sustainable technologies to the society for the betterment of our life. To this end, DNA-based nanobiotechnology has undergone an unprecedented advancement in recent years. The emergence of DNA as a “programmable assembler” for the bottom-up synthesis of materials has fascinated the material chemists.1 The possibilities of employing DNA as a template have opened up new avenues for the synthesis of nanomaterials pushing the cost and size limit imposed by the conventional top-down approaches like optical lithography. The interaction of DNA with metal nanoparticles finds diverse applications in the recent advancements of nanobiotechnology.2−11 Of particular interest is the DNA−gold interaction which forms the basis of several diagnostics applications involving DNA detection employing techniques like surface plasmon resonance spectroscopy (SPRS), surfaceenhanced Raman spectroscopy (SERS), electrochemical, colorimetric detection, etc.12−14 DNA molecule adsorbed on gold films, gold electrodes, or DNA coated with gold nanoparticles have been reported to have applications in the field of electronics, sensors, drug delivery, chips, imaging, etc.15−17 In some of the more recent developments of DNA nanotechnology, methods like metallization of DNA strands have emerged as promising scaffolds for the synthesis of 3D metallized objects which can find applications in nano© 2012 American Chemical Society

Received: May 14, 2012 Revised: July 12, 2012 Published: July 21, 2012 17063

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into the limelight by virtue of exhibiting pronounced catalytic activity particularly promoting the low-temperature CO oxidation which was recognized by Haruta et al. nearly two decades ago.24 More importantly, this reaction, apart from being a very important reaction dealing with the constituent of the automobile exhaust systems, where the oxidation of poisonous CO is imperative, serves as a prototype reaction for the larger spectrum of heterogeneous catalysis community.25 The nature of the support materials (mostly oxides) for the catalyst can also play a decisive role on the oxidation of CO. It has been demonstrated that charging effects due to presence of F-center-rich MgO surface greatly facilitates the CO oxidation on supported gold clusters.26 The related literature pertaining to noble metal supported catalysis has also witnessed an unprecedented expansion on the studies underpinning several aspects of nanoscale gold, including its varied structure and electronic properties and their applications in nanocatalysis, thus prompting some authors to call it as “nanocatalysis beyond the gold rush era”.27 Using small gold clusters as catalytic models, the essential aspects of the structural, energetic, and mechanistic details of many important chemical reactions, apart from CO oxidation, can be gleaned through with the help of computational methods.28 Kryachko et al. have theoretically demonstrated a unique feature of complexes between DNA bases and small gold clusters Au3 and Au4, namely a nonconventional N− H···Au hydrogen bonds.29 Interestingly, these clusters are strongly bound to DNA bases primarily by an N−Au or O−Au anchor bond along with the above-mentioned hydrogen bond.29 Very recently, Cao et al. have provided an experimental realization to the formation of above complexes through their combined photoelectron spectroscopy and density functional theory (DFT) calculations and have given estimates of the vertical detachment energies of nucleobase−Au− complexes, confirming simultaneously the presence of N−H···Au hydrogen bonds.30 Hence, on the basis of these considerations, we conceive the idea that the complexes of DNA bases with Au clusters can serve as ideal model nanostructures to act as catalytically active systems. Our objective for the current investigation is to employ density functional theory (DFT) based method and investigate the (1) binding ability of DNA base−Au3 cluster complex, (2) interaction of CO&O2 with gold cluster bound to the DNA bases, and (3) mechanism and energetics of CO oxidation reaction on these complexes. However, the larger perspective that we try to bring about from the present study is to critically assess the potentials of the DNA−gold system as a model catalyst for CO oxidation. Adhering strongly to the unique attributes of DNA as building block for nanoassembling and the growing interest in the DNA−gold system in recent years for varieties of other applications, we attempt to focus here an unexplored realm of possibility for these systems in the context of catalysis. Quintessentially, on comparison with the results obtained for a bare Au3 clusters, we are able to put forward an important observation that the DNA bases (individual bases, base pairs, and G4-quadruplex)−Au3 complexes facilitate the oxidation reaction by reducing the activation barrier. These observations can be taken as a first step toward designing promising heterogeneous catalyst systems with DNA as a basic building block or an active support.

Article

COMPUTATIONAL DETAILS

All the calculations have been carried out at the DFT level employing the TURBOMOLE program.31 For 60 core electrons of Au atom, we have used relativistic corrected effective core potential ecp-mwb as implemented in the above software. The 19 valence electrons of Au and all electrons of C and O atoms have been treated with def2-TZVP basis sets. The initial geometries of complexes of Au3 with DNA bases have been optimized by employing two different functionals, namely B3LYP and BP86. Apart from the gas phase calculations, we have also employed the continuum solvation model (COSMO) with water as the solvent (ε = 80) to investigate the effect of solvent on the binding affinities of gold cluster−nucleobase complexes. To study the oxidation of CO on the Au3−DNA base complexes, the initial state (IS), transition state (TS), and the final state (FS) have been optimized with BP86 functional. The transition state search has been carried out by choosing the distance between C and O1 (C of CO molecule and O1 of O2 molecule, described by O1−O2), dC−O1, as the reaction coordinate for the CO oxidation reaction. We have carried out constrained optimizations by varying dC−O1 as the reaction coordinate and generating the initial guess structures for the TS. The optimized TS is characterized with a single negative vibrational frequency as obtained from the Hessian calculation. The barrier height (ΔEa) for the CO oxidation reaction is calculated as the difference between the energies of TS and IS. The barrier height can be an important parameter in judging the catalytic efficiencies of gold cluster−DNA complexes. Hence, to further ascertain the reliability of DFT based methods in predicting the barrier heights for the reaction, we have carried out several single point energy calculations employing second-order Moller−Plesset perturbation theory with resolution of identity approximation (RI-MP2) method as implemented in TURBOMOLE package to estimate ΔEa. We have also computed the enthalpy of formation of the product (ΔHf) as the difference in the energies of the FS and IS.

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RESULTS AND DISCUSSION Binding of Au3 with DNA Bases. We begin our discussion by considering the complexes of individual DNA bases (A, T, G, and C) with the gold cluster Au3. For a particular DNA base, there can be several possible positions where the cluster Au3 can bind with the base via an anchor bond and a nonconventional hydrogen bond.29 We, however, have only considered the DNA base−Au3 complex which is having the highest binding energy for a given base−cluster complex. For instance, for A, Au3 bound via N3, H9 (anchor bond and hydrogen bond, respectively), as shown in Figure 1, corresponds to the complex having highest binding energy. Similarly, for T, G, and C, the most preferred binding sites for the cluster Au3 are O2, H1; N3, H9; and N3, H4, respectively (Figure 1). We have computed the binding energy of the cluster with the bases by employing B3LYP functional as well as BP86 functional, and the results are presented in Table 1. It is important to note that the former functional predicts the binding energy which is very close to experimentally obtained heat of desorption of DNA bases on Au films, in the range of 26.3−34.9 kcal mol−1.15 In our previous studies pertaining to CO oxidation on small gold clusters, we have shown that the performance of the functional BP86 is reasonably better as compared to some other functionals.32 Hence, in our subsequent discussion we present results obtained exclusively 17064

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Figure 1. The most stable complexes of DNA base−Au3 cluster with some important geometrical parameters.

Figure 2. Binding of cluster Au3 with DNA base pairs A-T, G-C, and G4-DNA.

by using this functional. The optimized geometries of the complexes along with some important geometrical parameters such as anchor bond length (dAu−N/dAu−O), H-bond length (dAu−H), and N/O−Au−Au bond angle have been presented in Figure 1. The relative affinities of gold cluster−DNA base complexes follow the order A ≥ C > G > T. This trend closely matches with the experimental results of A > C ≥ G > T obtained by Kimura-Suda et al.17 Apart from considering the interaction of the individual DNA bases with the gold cluster, to give more realism to our proposed models, we have also considered the cluster interaction with the nucleobase pairs A-T and G-C and the guanine quadruplex33 (G4 tetrad). In the double-stranded DNA, the complementary strands contain base pairs A-T and G-C (Figure 2), where the former is held by two hydrogen bonds and the latter by three. Similarly, in the G4 tetrad (Figure 2) four guanine units are held together through hydrogen bonds.33 The G4 tetrad is an important structure in the telemores, and any alteration in this chromosomic region can lead to processes such as aging or cancer.3435 Moreover, G4-DNA is also biotechnologically important for their

promising roles as biosensors or liquid crystals.3637 The binding energies of all these important variants of nucleobases with the cluster Au3 are summarized in Table 2, and the corresponding optimized geometries are presented in Figure 2. The basic structural features such as the N−Au or O−Au anchor bond and Au···H hydrogen bond in these complexes are essentially similar to that of the individual base−cluster complexes discussed earlier. It is interesting to mention that the most favored site of interaction of the cluster Au3 with individual bases as mentioned in the previous section is also retained for complexes Au3-A-T (Au3 cluster interacting with A side of A-T base pair, similar notation is used for other structures), Au3-TA, and Au3-G-C which are unaffected by the hydrogen bonds (Figure 2). For the case of Au3-C-G, the preferred site of interaction of Au3 is (O2, H1) as the (N3, H4) site (most preferred site in the case of isolated base−cluster complex) is involved in the intramolecular hydrogen bonding which is essential for the base pairing. If we compare the interaction energies of the complexes Au3-A-T, Au3-T-A, and Au3-G-C with their corresponding isolated base counterpart (Au3-A, Au3-T,

Table 1. Binding Energy of Individual DNA Bases with Au3, Activation Barrier (ΔEa), and Heat of Formation (ΔHf) of CO Oxidation Reaction on DNA Base−Au3 Complexes and Charge on the Cluster (qAu3)

a

cluster complex

Eb(BP86) (kcal mol−1)

Eb(B3LYP) (kcal mol−1)

Au3 Au3-A Au3-T Au3-G Au3-C

−37.74 −24.24 −34.35 −37.56

−32.16 −21.73 −28.75 −31.73

ΔEaa

ER

(kcal mol−1) 46.81 36.20 38.74 37.35 35.97

ΔHfa

ER

(kcal mol−1)

−39.76 −57.93 −51.27 −54.63 −54.61

qAu3 (e) 0.0 −0.110 −0.048 −0.122 −0.098

BP86 results. 17065

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Table 2. Binding Energy of DNA Base Pairs (A-T and G-C) and G4 Tetrad with Au3, Activation Barrier (ΔEa), and Heat of Formation (ΔHf) of CO Oxidation Reaction on These Complexes and Charge on the Cluster (qAu3) cluster complex

Eb(BP86) (kcal mol−1)

Eb(BP86/COSMO) (kcal mol−1)

Au3-A-T Au3-T-A Au3-G-C Au3-C-G Au3-G4

−37.58 −25.71 −37.61 −26.19 −38.24

−33.62 −19.65 −32.33 −20.88 −31.94

ΔEa

ER

(kcal mol−1) 37.29 39.25 37.31 38.90 35.74

and Au3-G, respectively, presented in Table 1), we find the binding energies to be almost comparable in both the cases. This demonstrates that the favorable interactions of the cluster−DNA base are preserved even by considering the base pairs. The interaction energy of the system Au3-C-G is −26.19 kcal mol−1, which is lesser than the corresponding individual base−cluster complex as the site of interaction is different in both the cases. Interestingly, the interaction energy for Au3-G4 system (−38.24 kcal mol−1) is the highest among all the systems. This sounds encouraging considering the fact that, like individual DNA bases, DNA base pairs including the G4DNA can also serve as an idealized model architecture involving Au clusters. To supplement our observations on the favorable cluster−nucleobase interactions discussed thus far based on the gas phase calculations, we have also made an attempt to investigate the effect of solvent on the overall stabilities of these complexes. The interaction energies obtained using the COSMO model with water as solvent have been presented in Table 2. The binding energy values are slightly lesser as compared to their gas phase counterparts. This observation is a well-known fact, as invoking of solvent models tends to reduce the charges on the system which can be associated with a decrease in the binding energies as compared to the gas phase results. The worth mentioning point, however, is that the overall trend of the binding energy remains unaffected, and thus the qualitative picture also remains the same. All these cluster− DNA base complexes which have been discussed in this section form the basis for our subsequent investigations of CO oxidation reactions on the gold clusters. CO Oxidation on Au3−DNA Base Complexes. In this section we discuss our results on the CO oxidation reaction taking place on the gold cluster−DNA base complexes. Gold nanoclusters are becoming overwhelmingly important on account of their importance in heterogeneous catalysis, a brief account of which has been discussed already in the introduction. The important reaction of CO combustion on supported gold proceeds through two different mechanisms, viz. Langmuir−Hinshelwood (L-H) and Eley−Rideal (E-R).38 In the L−H mechanism, both the reactants (CO and O2) are preadsorbed on the gold cluster in the IS. On the contrary, the E−R mechanism proceeds with the adsorption of only one of the reactant species in the IS. Before we start our discussion on DNA base−Au3 complex, we will first consider CO oxidation on a pristine Au3 cluster. On a bare Au3 cluster, the CO oxidation can take place via either of the two possible mechanisms (E-R and L-H). The optimized geometries of IS, TS, and FS for CO combustion on Au3 by the above two mechanisms have been presented in Figure 3. For the E-R mechanism, in the IS geometry only the CO molecule is adsorbed on the cluster, although it is important to note that in the IS geometry for E-R type of mechanism, oxygen can also be adsorbed on the bare cluster. Herein, we have, however, considered only CO adsorbed on the pristine cluster because in

ΔHf

ER

(kcal mol−1)

−58.25 −52.37 −56.07 −51.56 −57.41

qAu3 (e) −0.110 −0.059 −0.143 −0.060 −0.139

Figure 3. Oxidation of CO on an Au3 cluster via two possible mechanisms (L-H and E-R) showing the IS, TS, and the FS.

the case of Au3 bound to DNA bases we find that only CO and not O2 get adsorbed on the cluster. Hence, for the sake of comparison, we have only considered the case of CO adsorbed on the cluster rather than O2 in the IS geometry. It is also pertinent to mention that the binding of CO to small gold clusters is always favorable as compared to that of O2 which has been discussed previously in several studies.39 Likewise, for L-H type of CO oxidation on Au3, both CO and O2 are adsorbed on the cluster. We have calculated the barrier heights (ΔEa) for CO oxidation reaction as well as the heats of formation of the final product (ΔHf). The calculated ΔEa and ΔHf for L-H and E-R mechanisms are 42.43 and −39.61 kcal mol−1 and 46.81 and −39.76 kcal mol−1, respectively. It is interesting to note that on a pristine cluster Au3, CO oxidation can preferentially take place by a L-H mechanism which is having lesser ΔEa (42.43 kcal mol−1) as compared to E-R mechanism (46.81 kcal mol−1). Some previous reports of CO oxidation on Au8/ MgO(100) system also discuss the existence of two different types of reaction pathways based on temperature-programmed reaction (TPR) spectra and extensive ab initio calculations which support our findings that L-H mechanism can be a favorable channel for this reaction.38 In the FS as shown in Figure 3, the CO2 molecule is formed along with O atom adsorbed on the cluster. From the calculated values of ΔHf for the CO oxidation on Au3 cluster we infer that the reaction is highly exothermic with a net heat of formation of ∼−39 kcal mol−1. In furtherance of our discussion, we will now focus on the CO oxidation taking place on an Au3 cluster bound to the DNA bases. Before studying the CO oxidation process, we tried to study the adsorption of individual reactant species on different sites of the cluster. There are three possible binding sites for the individual CO or O2 molecule on the cluster, namely the Au atom involved in the anchor bond (AuA), Au atom involved in the H-bond (AuH), and the remaining Au atom flanked by AuA and AuH (central Au, AuC). We tried to optimize O2 adsorbed on the above three possible binding sites but found in all cases 17066

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different base−cluster complexes varies within a range of ∼3 kcal mol−1. For the T-Au3 system, the barrier is the highest as compared to its counterparts. This observation is explained on the basis of binding ability of bases with the cluster, where the T-Au3 system has the lowest binding energy as the Au−O anchor bond is relatively weaker than the stronger Au−N bond. Moreover, the weaker binding energy for the former complex also has minimum effect on the charging of the cluster (qAu3 from Table 1). The barrier heights obtained from RI-MP2 single point calculations performed on DFT optimized structures are found to be 59.74, 84.81, 73.61, and 48.20 kcal mol−1 for complexes A-Au3, T-Au3, G-Au3, and C-Au3 respectively. These values are larger than their corresponding DFT obtained results. It is also noted that for the pristine cluster Au3 the calculated barrier height at RI-MP2 level is 100.12 kcal mol−1, which is significantly higher than that of the DNA−gold complexes. In the case of MP2 method, the calculted barrier reduction is in the range of 20−40% compared to the pristine gold cluster while the barrier reduction predicted by the DFT method is within 20% . Nevertheless, it is pertinent to note that the qualitative trend remains same in both cases. More importantly, both the DFT and MP2 methods unequivocally predict the reduction of barrier height for the reaction on the gold cluster− DNA base complexes in comparison with that of the pristine cluster. The important observation that gold cluster bound DNA bases appreciably reduce the activation barrier for CO oxidation as compared to the bare cluster suggests that the cluster is activated by virtue of forming stable complexes with DNA bases. The heat of formation of the final product corresponds to a highly exothermic situation. It can clearly be noticed from Table 1 that the final states for the DNA base−Au3 system is more stabilized as compared to that for a bare Au3 cluster. This suggests that CO oxidation on gold cluster bound to a DNA base is more facile as compared to a pristine gas phase cluster. In an attempt to compare our results with the existing results for the classic case of Au/MgO system, we note that for the latter system the E-R mechanism predominates mostly in the low-temperature regime even at 90 K during the initial stages of dosing in the experiments.38 Our observation of similar kind of mechanism suggests that the complexes we have envisioned can be an active catalyst for CO oxidation at low temperatures, a wondrous attribute of nanoscale gold that has been brought to limelight by early works of Haruta.24 The results pertaining to CO oxidation process on the Au3DNA base pairs/G4-DNA complexes have been presented in Table 2. Correspondingly, the optimized geometries for IS, TS, and FS are shown in Figure 5. The activation barriers for the CO oxidation reaction on these complexes closely resemble that of isolated base-Au3 system presented in Table 1 (within a difference of 1−2 kcal mol−1). Similarly, the heats of formation of products are also comparable to their corresponding counterparts presented in Table 1. These important findings suggest that DNA base pairs/G4-DNA complexes with gold cluster can be as effective as the corresponding isolated nucleobases−gold cluster complexes in promoting the CO oxidation process. Hence, the DNA bases−gold cluster complexes can fulfill the essential features that an ideal catalyst system should possess to promote the CO oxidation reaction. The reduction in ΔEa for CO oxidation for DNA base−Au3 complex can be attributed to the charging of the cluster. We

the oxygen molecule to drift apart from cluster, suggesting its weak interaction with cluster−DNA base complexes. However, in the case of the CO molecule, we find AuC as the favorable binding site. This observation suggests that in the IS only one reactant (CO) is adsorbed on the cluster, and consequently the mechanism for CO oxidation is E-R type. The geometries corresponding to IS, TS, and FS for all the four DNA base−Au3 complexes have been presented in Figure 4. As a requirement

Figure 4. Oxidation of CO on individual DNA base−Au3 cluster complexes via E-R mechanism showing the IS, TS, and FS.

for the E-R type of mechanism, we find that in the IS the CO molecule is adsorbed on the gold cluster and the oxygen molecule approaches the CO molecule. The TS for the reaction involves a four-member O−C−O1−O2 structure adsorbed on the cluster. The FS of the reaction is characterized by an O atom bound to the Au3 cluster and CO2 being released. It is interesting to add that the atomic oxygen adsorbed on the gold cluster is also involved in H-bonding with one of the DNA− base H atoms. The energetics for the oxidation process has been summarized in Table 1. Most notably, we find that for all DNA base−Au3 complexes the barrier height is reduced as compared to that of the pristine cluster (E-R and L-H values). For instance, the barrier height for CO oxidation on Au3−C is the minimum (35.97 kcal mol−1) as compared to the corresponding value of 46.81 kcal mol−1 for the pristine cluster. This amounts to be ∼20% reduction in the activation energy for the reaction which is quite significant. For the different base−cluster complex, the trend in the ΔEa follows the order T > G > A > C. It is, however, noted that the barrier height for the CO oxidation reaction taking place on the four 17067

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which are commonly employed as substrates are (1) they provide strong adherence for the nanoparticles and (2) they modulate the electronic environment of active nanoparticles to facilitate catalysis. Moreover, added to these attributes the ease of manipulating the DNA structures to assemble nanomaterials of desired shape, size, functionality, or architecture can offer whole new range of possibilities. To this end, we assert that DNA base−gold complex can also be an ideal substitute for the well-explored oxide−gold system. The potential role of DNA as an ideal template for nanoassembling can be important in designing an active heterogeneous catalyst system.

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CONCLUSIONS

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AUTHOR INFORMATION

In summary, we have explored here, by employing DFT based methods, the possibilities of using individual DNA bases and DNA base pairs including G4 tetrad-Au3 cluster complexes as a catalytic model system for the CO oxidation reaction. The formation of such complexes has been theoretically demonstrated in the literature which has also met experimental realization, very recently.30 These complexes are energetically stable and our calculated interaction energies match closely with that of experimentally obtained desorption energies of DNA bases on Au film. We have demonstrated that on a pristine Au3 cluster the CO oxidation can take place by two different mechanisms such as E-R and L-H. On the contrary, on an Au3 cluster bound to individual DNA bases or base pairs or G4 tetrad, the mechanism is exclusively E-R. This mode of mechanism is well documented for CO oxidation on the Au/ MgO system at low temperatures. Thus, we suggest that the DNA base−Au system can also offer promises for being an active catalyst for low-temperature CO oxidation similar to the well-explored oxide/Au systems. Our estimation of the activation barrier for the oxidation reaction suggests that in all the DNA−gold cluster complexes the CO oxidation is more facile with lower activation energy as compared to the bare gold cluster. Moreover, this reaction also becomes more exothermic for the case of DNA base−cluster system in comparison to that of the pristine cluster. The charging of the cluster on account of complex formation can be of importance in deciding the catalytic activity of the cluster. Our results seem to be interesting in the light of some recent developments in DNA based nanotechnology pertaining to the DNA−nanoparticle complexes which can serve as an ideal bottom-up route to the assembling of nanomaterials with a better control over shape, size, and functionality. The role of DNA can be analogous to that of oxides or carbon nanomaterials in terms of promoting the reactions, which are exhaustively explored as substrates for heterogeneous catalysis. As evinced from our interpretations, apart from the several other applications of DNA−gold systems, these systems can also act as active catalytic centers for processes like CO oxidation. Our results are likely to foster further swathes of research interests in this direction.

Figure 5. Oxidation of CO on DNA base pairs/G4-tetrad-Au3 cluster complexes via E-R mechanism showing the IS, TS, and FS.

have computed the total charge transferred from the DNA base to the cluster from a natural population analysis which has also been presented in Tables 1 and 2. The net charge transferred to the gold cluster is always negative, suggesting the fact that the cluster becomes anionic when it forms complexes with DNA bases. It is already reported in the literature that anionic gold clusters are more active for CO oxidation as compared to their neutral counterparts.39 In our previous studies, we have demonstrated the importance of charging effects and modulation of local electronic environment around the reactive centers in the gold clusters for efficient CO oxidation.32,40 In this context, the role of DNA bases assumes significance from two counts: (1) they form stable complexes with gold cluster, and (2) they help in the charging of the cluster which in turn manifests in activation of the cluster for CO oxidation. The role of charging of Au clusters and their effect on CO oxidation has previously been stressed upon in the literature.26 Yoon et al. in their seminal work have demonstrated that charging of Au8 cluster on MgO surface can play a discerning role in CO oxidation.26 Two major advantages of oxides or surfaces of carbon nanomaterials like graphene or carbon nanotubes, etc.,

Corresponding Author

*E-mail: [email protected] (K.R.S.C.); [email protected] (S.K.G.). Notes

The authors declare no competing financial interest. 17068

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(34) Sen, D.; Gilbert, W. Nature 1988, 334, 364. (35) Gowan, S. M.; Harrison, J. R.; Patterson, L.; Valenti, M.; Read, M. A.; Neidle, S.; Kelland, L. R. Mol. Pharmacol. 2002, 61, 1154. (36) Alberti, P.; Mergny, J.-L. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 1569. (37) Bonazzi, S.; Capobianco, M.; De Morais, M. M.; Garbesi, A.; Gottarelli, G.; Mariani, P.; Ponzi Bossi, M. G.; Spada, G. P.; Tondelli, L. J. Am. Chem. Soc. 1991, 113, 5809. (38) Arenz, M.; Landman, U.; Heiz, U. ChemPhysChem 2006, 7, 1871. (39) Coquet, R.; Howard, K. L.; Willock, D. J. Chem. Soc. Rev. 2008, 37, 2046. (40) Jena, N. K.; Chandrakumar, K. R. S.; Ghosh, S. K. J. Phys. Chem. C 2009, 113, 17885.

ACKNOWLEDGMENTS N.K.J. gratefully acknowledges a senior research fellowship from Homi Bhabha National Institute (HBNI), Department of Atomic Energy, India. The authors also acknowledge supercomputing facility of BARC for providing high performance parallel computing facility. S.K.G acknowledges DST-India for Sir J. C Bose Fellowship and also support from the INDO-EU project MONAMI.

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dx.doi.org/10.1021/jp3046609 | J. Phys. Chem. C 2012, 116, 17063−17069