Theoretical Investigation of Structures, Bonding and Electronic Properties for t

REN Hong-Jiang ZHU Gang LI Xiao-Jun HE Ya-Ping



Theoretical Investigation of Structures, Bonding and Electronic Properties for the Complexes of 6‑Mercaptopurine and Ag8Clusters①

REN Hong-Jiang②ZHU Gang LI Xiao-Jun HE Ya-Ping

(710065)

The Ag clusters have been investigated widely theoretically and experimentally. In particular, it has recently shown that the neutralAg8clusters embedded in an argon matrix have a strong fluorescence signal. As we can know, the metal clusters may have important effects on the structures and properties of biomolecules. More and more attention is paid to the interaction between nanomaterials and biomolecules. In this work, the B3LYP method in density functional theory was used on the complexes between the 6-mercaptopurine (6MP) and Ag8clusters combined with 6-311++G** as well as LANL2DZ base sets. The geometries of all the complexes were optimized with full degree of freedom and the structures, chemical bonds, orbital properties as well as Mulliken charges for ten possible complexes were analyzed based on the same theory level. In addition, the influence of temperature and pressure on the stabilities of the four complexes was further explored using standard statistical thermodynamic methods ranging from 50 to 500 K and at 100 kPa or 100 bar. The results show that the complex Ag8-6MP-7-5 can be the most stable one among the investigated complexes, in which the Ag(11) atom interacts with the S(10) atom forming the strong chemical bond. The Mulliken charges also show that the Ag–S chemical bond is formed and the related charge has transferred. Additionally, the temperature and pressure can significantly influence the stability of the four stable complexes.

6-mercaptopurine, Ag8cluster, density functional theory, bonding properties;

1 INTRODUCTION

Nowadays, the nanotechnology is swiftly disten- sible in some fields such as catalysts, synthesis, chemluminescence, separation and sensing[1, 2]. Based on the unique chemical and physical properties such as large surface area, sensitivity and activity, these nanomaterials and products are needed for some chemical reactions like catalysts, enhancers, energy acceptors, absorbing materials and CL resonance energy transfer platforms and so on[3, 4]. Silver nanoparticles (AgNPs) are prone to be composed from inexpensive forerunners, which have displayed many optical characteristics and possess excellent chemical stability, dramatic catalytic and electrocatalytic properties. It has also been found that silver nanoparticles can sufficiently improve the anticancer drug activity and intensely enhance the drug curative effect[5-9]. So far, most of the first- principles studies have been limited to the Agclusters with n≤13, and many candidate structures were considered[10, 11]. Huda.[10]investigated the electronic and geometric structures of neutral, cationic, and anionic Ag(n = 5～9) clusters using the second-order many-body perturbation theory (MP2). They found that the Ag8cluster is a magic- number cluster with the highest binding energy, a relative higher adiabatic and vertical ionization potential, and a lower electron affinity among all the clusters studied. Fernandez.[11]systematically studied the electronic properties and geometric structures of neutral, cationic and anionic metal cluster M(M = Cu, Ag or Au, n = 2～13). In par- ticular, it is recently shown that neutralAg8clusters embedded in an argon matrix have a strong fluore- scence signal[12].

6-Mercaptopurine (6MP) has been used for many years as one of the chemotherapy drugs, which is extensively utilized as an effective medicine in the treatment of a variety of diseases, including rheuma- tologic disorder, lymphoblastic leukaemia, inflame- matory bowel disease, and the prevention of rejec- tion following organ transplantation[13-22]. The adsorption of 6MP on nanomentals has received a great deal of attention because of the potential of linking the drug’s coordination to its chemothera- peutic activity[23-25].Vivoni.[24]have reported the interaction between 6MP and a silver electrode using surface-enhanced Raman spectroscopy (SERS) experimentally as well as Urey-Bradley force field and semiempirical calculations with the PM3 method, and it was concluded that 6MP attaches head-on through the N1 atom when the molecule is adsorbed onto a silver electrode surface. Chu.[25]have investigated self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) on a silver electrode in acid and alkaline media by a combination protocol of the SERS technique with Raman mapping. It was found that the adsorption mode of 6MP SAMs changed with the pH value of the environment. The coordination of 6MP to the metals has been studied to produce 6MP-metal complexes acting as slow release drugs of 6MP[26]and to exploit this reaction for heavy-metal determination[27]. Even though there are many reports on the coordination of 6MP to Ag nanoparticles forming AgNPs[10, 11, 23-27], they are only limited to the investigation of experimental Raman spectroscopy, nanomaterials scale and lower level theoretical prediction. And what are the structures and bonding properties after adsorption? How the temperature and pressure affect the adsorption of 6MP onto the Ag clusters? These problems have been unknown to nowadays. In this work, we provide the answers to these problems by carrying out the detailed density functional theory calculations using B3LYP method.

In this study, the stabilities of ten complexes between 6MP and Ag8clusters were investigated in detail. The structures, bonding properties, and orbital and Mulliken charges analyses were carried out to understand the interactions between 6MP and the Ag8clusters.

2 COMPUTATIONAL DETAILS

All theoretical calculations were performed using the Gaussian 09 program suite[28]. The geometries of all complexes were optimized with the DFT-B3LYP methodcombined with 6-311++G** basis sets for non-metal atoms (C, H, N, S) and an effective pseudo potential basis set LANL2DZ only for Ag atoms. Harmonic vibrational frequencies were analyzed to verify the stationary points at the same theoretical level. Zero-point vibrational energies (ZPVE) and thermal corrections were evaluated under standard state conditions (298.15 K and 100 kPa)frequency analysis. The thermochemistry energy parameters were calculated at the B3LYP functional level of theory. The binding energies between the Ag8clusters and 6MP molecule were denoted by Δb, which were given for each complex:

Δb= −[(Ag8-6MP-7-n) –(Ag8) –(6MP-7)]. (1)

Here is an example for 6MP-7, in which n represents the labeled number. The same calculated method is also used for another tautomer 6MP-9.

The standard statistical thermodynamic methods for the Gibbs free energies were utilized for different temperature from 50 to 500 K at 100 kPa and 100 bar. The bonding properties and bond strength variations were analyzed based on the NBO calcula- tions using the NBO package version 3.1, which were conducted at the DFT-B3LYP/6-311++G**//LANL2DZtheoretical level, implemented in the Gaussian 09 program. The analysis can provide a comprehensive insight for the complexes of 6MP on the Ag8clusters.

3 RESULTS AND DISCUSSION

3. 1 Structures and energies

Based on the reports of references 29 and 30, the 6MP molecule can exist in eight possible configura- tion tautomers, of which 6MP-7 and 6MP-9 can be predominant with the maximal quantity in the gas and aqueous phases. Other six tautomers can exist in a minimal quantity. Hence, in this work, only 6MP-7 and 6MP-9 are chosen for further consideration, and their geometries with atomic numbering scheme are shown in Fig. 1. It can be seen from Fig. 1 that when considering possible coordination atoms, three potential positions of N(3), N(7) and S(10) for 6MP-7 or N(3), N(9) and S(10) for 6MP-9 can be attacked by the mental Ag atom, mainly because of the lone pair electrons for the N and S atoms being extremely important for promoter action. Therefore, when considering the adsorption process, the orientation of adsorbed 6MP is critical in its action as a mediator of electron transfer. According to the reports by Huda et al.[10], the Ag8cluster is a magic-number cluster among the investigated clusters and it has two representative superiority structures. And two potential configurations were considered in this work based on the reported structures[10]. Six possible complexes geometries of 6MP-7 adsorbed on the Ag8clusters surface were found and depicted in Fig. 2. Four geometries of the complexes on 6MP-9 with the Ag8clusterswere also found and given in Fig. 3.The zero point virational energies, total energies, standard Gibbs free energies and enthalpies of all the complexes at 298.15 K at the B3LYP/6-311++G**//LANL2DZ level are obtained and listed inTable S1. The relative energy information is also shown in Table 1.

Fig. 1. Geometries and atomic number of two tautomers for 6-mercaptopurine (6MP)

Fig. 2. Six geometries of the formed complexes on 6MP-7 with the Ag8clusters (Bond lengths in Å

Fig. 3. Four geometries of the formed complexes on 6-MP-9 with the Ag8clusters (Bond lengths in Å) level for all the complexes

Table 1. Relative Standard Gibbs Free Energy (Gθ, kJ/mol) at 298.15 K Obtained at the B3LYP/6-311++G**//LANL2DZ

As can be seen from Figs. 2 and 3, the structures of Ag8cluster in complexes Ag8-6MP-7-1, Ag8-6MP-7-2, Ag8-6MP-7-3, Ag8-6MP-9-1 and Ag8-6MP-9-2 are dramatically different from those of Ag8-6MP-7-4, Ag8-6MP-7-5, Ag8-6MP-7-6, Ag8-6MP-9-3 and Ag8-6MP-9-4. As seen from Table 1, the configuration with the lowest Gibbs free energy is Ag8-6MP-7-5 and we take this energy as a zero so as to calculate the relative energies of other nine complexes. And the secondary one is Ag8-6MP-7-2 with a relative energy of 1.77 kJ/mol. The third and fourth ones are Ag8-6MP-7-3 and Ag8-6MP-9-2 with the relative energies to be 7.17 and 9.46 kJ/mol, respectively. The relative Gibbs free energies of other complexes Ag8-6MP-7-1, Ag8-6MP-7-4, Ag8-6MP-7-6, Ag8-6MP-9-1, Ag8-6MP-9-3 and Ag8-6MP-9-4 are 10.41, 11.73, 15.08, 27.69, 26.09 and 12.11 kJ/mol, respectively. These data imply that the complex Ag8-6MP-7-5 can be the most stable one of all configurations and Ag8-6MP-7-2 is also possibly the predominant one. Complexes Ag8-6MP-7-3 and Ag8-6MP-9-2 maybe exist in some quantity because of lower relative energies. Other complexes have slightly higher energies and the stabilities are slightly lower than that of the former four complexes.

In the complex Ag8-6MP-7-5, the Ag(11)–S(10) bond is formed with a distance of 2.692 Å which is slightly longer than the experimental value of 2.48 Å[18], implying the existence of classical chemical bond between the Ag(11) and S(10) atoms. The chemical bond S(10)–C(6) has been lengthened to 1.694 Å from 1.670 Å by 0.024 Å and the N(1)–H has also been lengthened from 1.013 to 1.025 Å by 0.012 Å. In Ag8-6MP-7-2, the chemical bond S(10)–C(6) is 1.688 Å, and in Ag8-6MP-7-1, Ag8-6MP-7-3, Ag8-6MP-7-4 and Ag8-6MP-7-6, the N(3)(or N(9))–Ag(10) chemical bonds are 2.449, 2.407, 2.645 and 2.419 Å, respectively, suggesting that the Ag–S or Ag–N chemical bonds are formed in the end. In these six complexes, the different positions can be attacked by Ag atom of the Ag8clusters. In Ag8-6MP-9 complexes,we try to find all the configurations but only four possible complexes were found. All the structures are shown in Fig. 2. In these four complexes, the Ag atom also formed chemical bonds with the N or S atoms, and it can seen that in Ag8-6MP-9-2 and Ag8-6MP-9-4, the N and S atoms simultaneously formed chemical bonds with one Ag atom. In Ag8-6MP-9-2, the N(7)–Ag(11) and S(10)–Ag(11) bond lengths are 2.578 and 2.936 Å, respectively. And in Ag8-6MP-9-4, the N(7)– Ag(11) and S(10)–Ag(11) bond lengths are 2.570 and 3.017Å, respectively. However, in Ag8-6MP-9-1 and Ag8-6MP-9-3, only the single N(3)–Ag(11) bond is formed with the distances of 2.438 and 2.460 Å, respectively. These data show that there are strong chemical bonds between the 6MP and Ag8clusters.

3. 2 Binding energies

The binding energies of all the complexes were also calculated and listed in Table 2, in which ΔErepresents the uncorrected interaction energies without basis set superposition error (BSSE) and ΔE(BSSE)is the corrected interaction energies inclu- ding BSSE corrections[31]. It can be seen from Table 2 that BSSE values are within the range of 5.27～6.95 kJ/mol, which occupied 10.46～17.06% of the uncorrected interaction energies, implying that the basis set effects must be considered in this work, and in the next discussions the corrected energies are employed. As seen from Table 2, the calculated binding energy of Ag8-6MP-9-2 is 50.63 kJ/mol and that of Ag8-6MP-7-2 is 48.70 kJ/mol, with their binding energy difference being 1.93 kJ/mol. Thecalculated binding energy of Ag8-6MP-7-5 is 46.61 kJ/mol and that of Ag8-6MP-9-4 is 44.02 kJ/mol. The binding energies of other complexes are more than about 28 kJ/mol. These data show that the interaction effects between the molecule 6MP and metal Ag8clusters are much larger and the corresponding chemical bonding may be formed. It can be also seen from Table 2 that the binding energies of those complexes having lower relative Gibbs free energies are much larger, such as complexes Ag8-6MP-7-2,Ag8-6MP-7-5andAg8-6MP-9-2. However, to our surprise, Ag8-6MP- 7-3 has lower binding energy than Ag8-6MP-9-2, which may be attributed to the different interaction strength. As we can see from Fig. 3, in Ag8- 6MP-9-2 there are two kinds of chemical bonds, N(7)–Ag(11) and S(10)–Ag(11), and this interaction effect must be much stronger than that of single bond interaction effect in Ag8-6MP-7-3 or Ag8-6MP-7-5, which can explain why the complex Ag8-6MP-9-2 has the greatest binding energy. For further insight into the bonding properties between the 6MP molecule and the Ag8cluster, the following orbital analyses are carried out for illustrating the corresponding problems.

Table 2. Binding Energy (ΔEb) and Corrected Binding Energy (ΔEb) including BSSE of All the Complexes (kJ/mol)

3. 3 Orbital analysis

The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distributions for the four relative stable complexes Ag8-6MP-7-2, Ag8-6MP-7-3, Ag8-6MP- 7-5 and Ag8-6MP-9-2 are shown in Fig. 4. As can be seen, the HOMO orbital electrons of Ag8-6MP-7-5 are mainly delocalized in the Ag8clusters and they are far from the 6MP molecule. No contributions could be seen from the purine ring atoms, indicating that the HOMO contribution does not mainly come from the purine ring. However, the LUMO electrons are delocalized in the purine ring and S atomdelocalizedbond, and these contributions are mainly due to covalent bond from the purine ring. The HOMO and LUMO distributions show that Ag8-6MP-7-5 has the largest energy gaps. The electronic clouds of the HOMO orbitals in Ag8-6MP-7-2, Ag8-6MP-7-3 and Ag8-6MP-9-2 complexes are not much larger thanthat of Ag8-6MP-7-5 due to the less overlapping field of Agelectrons, while the LUMO electronic clouds are attributed to the anti-bonding orbitals of purine ring, indicating that these complexes have smaller gaps. At the same time, the energy gaps of four complexes Ag8-6MP-7-2, Ag8-6MP-7-3, Ag8-6MP-7-5 and Ag8-6MP-9-2 were calculated to be 198.83, 159.50, 221.75 and 146.03 kJ/mol, implying that in these complexes, Ag8-6MP-7-5 has the highest energy gap. And hence, the stability for Ag8-6MP-7-5 is also the largest among the four complexes. This result is almost the same with the predicted stability from energy information. For further revealing the bonding properties, the NBO analysis obtained at the B3LYP/6-311++G**//LANL2DZ level of theory is listed in Table 3. As can be seen from Table 3, the second order perturbation interaction energy LP[S(10)] → LP*[Ag(1)] of Ag8-6MP-7-5 is 101.25 kJ/mol and the LP[S(10)] → LP*[Ag(1)] interaction of Ag8-6MP-7-2 is 92.76 kJ/mol, implying that there exist strong chemical bonds and the interaction energies of Ag8-6MP-7-2 and Ag8-6MP-9-2are57.66 and 50.38 kJ/mol, respectively. As we can know, the second order perturbation interaction energy can show the strength of formed chemical bonds.

Table 3. NBO Analysis at the B3LYP /6-311++G** Level of Theory

Fig. 4. Frontier molecular orbitals (HOMO and LUMO) of the considered four complexes Ag-6MP-7-5, Ag-6MP-7-2, Ag-6MP-7-3 and Ag-6MP-9-2

3. 4 Mulliken charges analysis

The Mulliken charges of the four stable com- plexes Ag8-6MP-7-2, Ag8-6MP-7-3, Ag8-6MP-7-5 and Ag8-6MP-9-2 adsorbed on Ag8clusters are listed in Table 4. It can be seen from Table 4 that the charges of atoms C(4), C(5), C(6), C(8), N(1), N(3) and S(10) in purine ring have changed dramatically. In Ag8-6MP-7-5, the charges of C(5) and C(6) atoms are 0.2029 and 0.2221 e, which are different from those of the isolated 6MP-7. This can mainly result from forming strong chemical bond between Ag(11) and S(10). The C(5) atom in the pyrimidine ring of Ag8-6MP-7-5 obtained much more electrons and the C(6) atom lost some electrons, and the S(10) atom obtained the electrons from –0.6295 to –0.6804e. In N(1) atom, the change is much greater by the value of 0.1381 e than other nitrogen atoms of both pyrimidine and imidazol rings. And the Ag(11) atom has changed from –0.0988 to –0.1154 e, showing the obvious electron transferring effect. In Ag8-6MP-7-2, the changes of C(5) and C(6) atoms are much greater with the –0.2561 and 0.2866 e, and the third one is the C(4) atom with a change of –0.2097 e. In four N atoms, only the N(1) has changed much by 0.1308 and other atoms almost keep the same. The Ag(11) atom becomes more negative from –0.1592 to –0.3163 e, implying that the electrons transfer greatly and the chemical bonding is formed. However, in Ag8-6MP-7-3, the electron C(8) atom changes the greatest with a transfer of –0.3219 e, which also results from the chemical bond forming between N(9) and Ag(11). And the N(9) atom lost the electrons and transferred to the C(8) atom. In this molecule, the most obvious change is for the C(5) atomtransferring electrons of 0.2719 e. The Ag(11) atom has changed from –0.1592 to 0.2732 e, showing that the chemical bond is also formed. And other atoms do not change greatly. In Ag8-6MP-9-2, the charges of all carbon atoms have changed dramatically, and especially for C(4) atom, it has changed from –0.4303 to 0.7516 e, followed by the C(2) atom with a change of –0.7192 e. The atoms C(5), C(6) and S(10) changed with –0.4831, 0.3691 and –0.2735 e, which deposes the formation of Ag–N chemical bond and the charge transfer.

Table 4. Mulliken Charges of Partial Atoms on the Considered Complexes Ag8-6MP-7-5, Ag8-6MP-7-2, Ag8-6MP-7-3 and Ag8-6MP-9-2

3. 5 Temperature and pressure effects

For exploring the temperature and pressureinfluence on the complexes nanomaterials, the different temperature and pressureare considered in this work. The temperature changed from 50 to 500 K and the pressure was given at 100 kPa and 100 bar.The Gibbs free energies calculated using the standard statistical thermodynamic method ranging from 50 to 500 K are employed in the next discussion. The relations between the Gibbs free energies and temperature for complexes Ag8- 6MP-7-5, Ag8-6MP-7-2, Ag8-6MP-7-3 and Ag8-6MP-9-2 at 100 kPa and 100 bar are illustrated in Fig. 5(a) and (b), respectively, in which the Gibbs free energies dramatically decrease with increasing the temperature from 50 to 500 K, implying that the temperature can affect their stabilities. In Fig. 5, the Gibbs free energy differences of the four complexes are very slight regardless of at 100 kPa or 100 bar, implying that the temperature can almost have the same effect on the stabilities of the four complexes. It can be seen from Fig. 5 the Gibbs free energy of Ag8-6MP-9-2 is the highest between 50 and 300 K either at 100 kPa or 100 bar. The stabilities of Ag8-6MP-7-5 and Ag8-6MP-7-2 are also the same within the investigated pressure and temperature ranges. The results also show that in different temperature these complexes have different stabi- lities, which can be attributed to the different chemical bonds formed. The Ag8-6MP-7-5 and Ag8-6MP-7-2 have obviously lower stabilities than other complexes.

Fig. 5. Gibbs free energies of the considered complexes Ag8-6MP-7-5, Ag8-6MP-7-2, Ag8-6MP-7-3 and Ag8-6MP-9-2 with the temperature increasing from 50 to 500 K at 100 KPa for (a) and 100 bar for (b)

4 CONCLUSION

Theoretical investigations have been performed using the density functional method B3LYP with 6-311++G**//LANL2DZ base set on the complexes between 6-mercaptopurine and Ag8cluster. And the complexes geometries were optimized with full freedom. The structure characters, bonding proper- ties and Mulliken charges of four stable complexes were analyzed in detail. Additionally, the influences of temperature and pressure on the stabilities of the four complexes were also explored using standard statistical thermodynamic methods. The result shows that the complex Ag8-6MP-7-5 can be the most stable among the investigated complexes, in which the Ag(11) atom interacts with the S(10) atom forming the strong chemical bond. The Mulliken charges also show that the Ag–S chemical bond is formed and has inspired the charge transfer. The temperature and pressure maybe significantly influence the stability of the four stable complexes.

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5 March 2018;

30 May 2018

①This project was supported by the National Natural Science Foundation of China (No. 21643014) and the Special Natural Science Foundation of Science and Technology Bureau of Xi’an City Government (No. 2016CXWL02 and SGH17H249)

. Ren Hong-Jiang. E-mail: hjren@xawl.edu.cn

10.14102/j.cnki.0254-5861.2011-1997