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Theoretical Study of the Formation of Inclusion Complex Between Cisplatin and Single Wall Carbon Nanotube Leonardo Souza, Camila A.S. Nogueira, Juliana Fedoce Lopes, Helio F. Dos Santos, and Wagner Batista De Almeida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01221 • Publication Date (Web): 26 Mar 2015 Downloaded from http://pubs.acs.org on March 29, 2015
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Theoretical Study of the formation of Inclusion Complex between Cisplatin and Single Wall Carbon Nanotube Leonardo A. De Souza1, Camila A.S. Nogueira1, Juliana F. Lopes2, Hélio F. Dos Santos3, Wagner B. De Almeida4,* 1
Laboratório de Química Computacional e Modelagem Molecular (LQC-MM), Departamento de
Química, ICEx, Universidade Federal de Minas Gerais (UFMG), Campus Universitário, Pampulha, Belo Horizonte, MG, 31270-901, Brazil. 2
Laboratório de Química Computacional (LaQC), Institutode Física e Química, Universidade Federal
de Itajubá (UNIFEI), Av.BPS, 1303 Pinheirinho, Itajubá, MG, 37500-903, Brazil. 3
Núcleo de Estudos em Química Computacional (NEQC) – Departamento de Química, ICE,
Universidade Federal de Juiz de Fora (UFJF), Campus Universitário, Martelos, Juiz de Fora, MG, 36036-330, Brazil. 4
Laboratório de Química Computacional (LQC), Departamento de Química Inorgânica, Instituto de
Química, Universidade Federal Fluminense, Campus do Valonguinho, Centro, Niterói, RJ, 24020-141, Brazil.
ABSTRACT In this work we report theoretical quantum chemical design and investigation of supramolecular structures formed by cisplatin and single wall carbon nanotube (SWCNT). Through Density Functional Theory (DFT) calculations, plausible modes of interaction between cisplatin and SWCNT zigzag (12,0) model were found: inclusion and adsorption complex forms. B3LYP/631G(d,p)/Lanl2DZ calculations of NMR chemical shifts for the free and interacting molecular structures showed very promising to assist the experimentalists to identify these structures. Our results strongly indicate that the cisplatin-nanotube system form a stable molecular complex that can be used as a new drug delivery device.
Keywords: Cisplatin, Carbon Nanotube, NMR, Drug Delivery System, DFT
*Corresponding author:
[email protected] 1 ACS Paragon Plus Environment
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1. INTRODUCTION In 1991, some years after the discovery of fullerenes, Iijima was able to synthesize a new allotrope of carbon with cylindrical structures, called carbon nanotubes (CNTs)1, 2. Thus, CNTs with several micrometers in length and nanometric diameters were produced suggesting different and broad potential applications. Regarding their physical properties, for example, the nanotubes can be classified as metallic or semiconducting according to their diameter and chirality3-5. CNTs, however, have some significant disadvantages major related to insolubility6,7, aqueous non-dispersion8 and toxicology, which are essential for practical application in medical and biological fields6, 9, 10. Lately, the integration between biomolecules and CNTs has shown that such nanostructures are strong candidates for various biomedical applications6, 11-15. Molecular modeling has advanced along to nanotechnology experimental studies. Quantum chemical calculations allow free design and detailed investigation of many systems projected for biological applications, especially drug delivery, frequently assisting the experimentalists in this area14, 15. Most of these works show that intermolecular forces such as van der Waals interactions and hydrogen bonds play an important role in the stability and drug delivery systems11, 13. Platinum drugs are still the most effective agents used in the cancer treatment. Cisplatin (cis-DDP) is a wide known chemotherapy agent or several cancers types16, 17. Its non-selectivity could lead to apoptosis not only cancer cells, but also by healthy cells causing various side effects to patient, which are limiting the drug administration10,
18
.In this sense, carbon nanotubes can serve as promising
materials in the selective transport of cis-DDP to the tumor reducing the undesirable side effects, as reported for in vitro and in vivo experimental studies19-21. Guven et al.21 performed the experimental preparation, characterization and in vitro testing of ultra-short SWCNT class with cisplatin encapsulated. The authors confirmed the formation of drug delivery system through sophisticated experimental techniques as high-resolution transmission electron microscopy and energy dispersive spectroscopy. The dialysis studies conducted in saline solution buffered with phosphate at 37°C showed the controlled release of the drug; in vitro tests was subsequently performed with two different cell lines of breast cancer, MCF-7 e MDA-MB-231. A high cytotoxicity compared with free cis-DDP in the culture medium after 24 hours when the complexes are used was observed. Li et al.22 studied multi-walled carbon nanotubes (MWCNT) as device capable of selectively trapping Pt(IV) complexes, releasing the active form of platinum(II) complexes through chemical reduction. The experimental results showed that platinum(IV) complex remained trapped inside the MWCNT by hydrophobic interactions and, achieved a dramatic inversion of hydrophobicity after chemical reduction facilitating the drug release. In vitro tests were performed to evaluate the drug release in the presence of deoxyguanosine monophosphate (GMP), adopted as a model to examine the DNA binding through the 2 ACS Paragon Plus Environment
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Pt-GMP adduct type formed. The authors revealed that in contrast to cisplatin, the platinum(IV) drug is selectively released after reduced in the presence of the reducing agent, showing a similar functional cytotoxicity of cisplatin. Our group has recently reported11 density functional theory (DFT) calculations to study inclusion complex formation constituted by the cisplatin encapsulated into carbon nanohorns (CNH). Our results indicate the cisplatin-CNH formation energy calculate at the M06-2x/6– 31G(d,p) level around −30 kcal mol−1, with the cisplatin placed close to the bottom of the CNH cone structure. Theoretical B3LYP/6–31G(d,p)/LanL2DZ 1H NMR and
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N NMR results showed a
variation ~20 ppm down field upon inclusion, which is expected to be an unambiguously assignment of the cis-DDP@CNH inclusion compound. In this work, new designed nanostructures for modulate cisplatin delivery are presented. DFT (Density Functional Theory) calculations were carried out aiming to determine if a CNT model and cisplatin molecule, can form a stable complex, which could be useful as drug delivery system.NMR chemical shifts calculations for all the structures were performed to identify the inclusion complex formation, providing relevant spectroscopic results that can aid further experimental studies on preparation and characterization of CNT-cisplatin associations.
2. COMPUTATIONAL DETAILS All calculations were performed with the Gaussian 09 package23 employing the DFT24 method with the B3LYP25 functional using the LanL2DZ26 effective core potential for platinum. Initially, the geometries of the one-end capped SWCNT zigzag (12,0)and cis-DDP molecules were optimized separately using the 3-21G basis set27 for C, H, N e Cl atoms. The optimized geometries were then used to calculate the potential energy curve (PEC), at the same theoretical level, simulating the inclusion throughout a fixed axis, with both frozen molecules. Cis-DDP was moved 20.0 Å along a fixed axis which passes through the center of mass of nanotube and the reference carbon atom situated in the nanotube cap (see Figure S1). The geometries corresponding to the minimum found on the PEC were fully optimized using the slightly improved 6-31G(d,p)/LanL2DZ basis set27. Furthermore, we expanded our study considering the geometry optimization of the inclusion complexes obtained using the same model of carbon nanotube with both opened ends. Harmonic frequency calculations were not attempted due to the size of the molecular system, since it is a quite demanding computational task. Finally, using the improved theoretical level, the gauge-independent atomic orbital (GIAO) method28 was used for calculation of 1H,
13
C and
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N magnetic shielding constants with chemical shifts (δ),
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obtained on a δ-scale relative to the TMS( C and 1H ) and NH4Cl (15N). This methodology was used successfully in other studies conducted by our group11, 29-32. 3 ACS Paragon Plus Environment
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3. RESULTS AND DISCUSSION
The one-side capped nanotube structure used to build the complexes contains six pentagons in its closed end, length and diameter equals to 20 and 9.5 Å, respectively. The model of CNT with its open end has 17 Å in length and it was idealized removing the carbon atoms that made up the tube cap. Tripisciano et al.12,
13
showed by atomic force microscopy that SWCNT and MWCNT with about
1000Å length were employed to encapsulate cisplatin. Thus, our theoretical models of SWCNT represent around 2% of the larger structures observed experimentally and have molecular formulas equals to C240H12 (Capped Carbon Nanotube-CCNT) and C216H24 (Opened Carbon Nanotube-OCNT). Cisplatin and CCNT geometries were first optimized separately. Cisplatin molecule was found to be square planar, as expected, with Pt─Cl and Pt─N bonds length in average 2.40 and 2.10 Å, respectively and the overall calculated structure is well compared with the X-ray one33. For the CCNT model, the average C─C bond was 1.42 Å for the tubular region and 1.43 and 1.45 Å for the bonds that form pentagons and hexagons in the closed end of the nanotube, respectively. From these structures, it was possible to calculate cis-DDP and CCNT interaction PEC which is shown in Figure 1, along with the minimum energy structures considered. The PEC is not a reaction coordinate profile, however, it shows a noticeable quite high energy barrier, 94.25 kcal mol-1, to cross the open rim of the tube. This is mainly due the rigid approach used that does not allow the tube breathing upon inclusion. B3LYP/6-31G(d,p)/LanL2DZ full optimized geometries complexes are depicted in Figures 2 and 3. It can be seen that the cisplatin is located near the open end (complex I), the middle (complex II) and the cap (complex III) of the nanotube. Other cis-DDP@CCNT (external adsorption, complex IV) was also optimized to compare the energy stability and NMR spectra. After the optimization, it was observed that the cisplatin is maintained close to its former axis (see Figure S1 – Supporting Information). In the complex II and III, the molecule is rotated 90° around an imaginary axis perpendicular to this former axis (compare Figure 1 with 2b-c). For complex IV, the dihedral angle ∠ Cl-Pt-C-C and distance calculated shows that the plane defined by guest molecule is displaced about 53° and 3.6 Å from the nanotube surface, respectively. The inclusion complexes models with OCNT were built removing 24 carbon atoms which formed the nanotube cap. The cis-DDP@OCNT complexes were named complex V, VI, VII and VIII as depicted on Figure 3. After the optimization, cisplatin remained relatively the same position found in the cis-DDP@CCNT complexes. For the complex VIII, the dihedral angle ∠ Cl-Pt-C-C and distance calculated shows that the plane defined by cis-DDP is also displaced about 73° and 3.7 Å from the nanotube surface, respectively. The average C─C bond for the OCNT was 1.42 Å for the tubular region and 1.41 Å for those on the hexagons in the opened ends of the nanotube. The geometry
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optimization calculations show that the cisplatin molecule has no direct influence on the average diameter of the CCNT and OCNT models in inclusion complexes studied compared to free models. The complex formation energies ( ∆E f ) are given in Table 1. Regarding the complexes formed with the CCNT, B3LYP/6-31G(d,p)/LanL2DZ energy of formation showed that the complex I is almost four times strongly bound than complex III, which is almost three times more stable than complex II. For the complexes formed with the opened nanotube a similar tendency is observed. For the two types of CNTs employed, the structure in which cisplatin is adsorbed outside of the tube (complexes I, IV, V and VIII) showed the most energetically favorable form using B3LYP functional (see Table 1). A local analysis of the charge distribution of these molecules (data not shown) suggest that the interaction between the monomers given through weak electrostatic interactions. In complexes I and V such interactions occur between hydrogen atoms which complete the carbon valence of tube and ligands groups of cis-DDP; in complex IV and VIII the interaction occurs between the cis-DDP molecule and the carbon atoms of tube closest to it. Recently11, for cisplatin and carbon nanohorns complexes, we showed that the M06-2x functional34, based on meta-GGA approximations and flexible functional parametrization, can provide better results of structure and molecular stability. These results reported motivated us to continue using the DFT M06-2x/6-31G(d,p)//B3LYP/6-31G(d,p) level for estimate complexes energies formation of these systems. M06-2x results show that systems stability are considerably enhanced as compared to the B3LYP/6–31G(d,p) values. Moreover, the complex containing cisplatin inside of the nanotubes become more energetically favorable than those with the molecule adsorbed outside of the tube. Regarding the complex formed with the CCNT, M06-2x/6-31G(d,p)//B3LYP/6-31G(d,p) energy of formation showed that the complex III is almost four times strongly bound than complex IV and this difference decreases 28% when compared to complex I. For the complexes formed with the opened nanotube, the inclusion complexes VI and VII energy of formation are practically the same, being almost twice strongly bound than external adsorption complex VIII. These could be more realistic result, since the M06-2x functional corrects the dispersion interactions which are expected to play a expressive role to stabilize these complexes. This effect also led to the formation of complexes in which cisplatin is located at the entrance of nanotube (complex I and V) energetically more favorable than those in which the molecule is adsorbed on the external surface of the tube (complex IV and VIII). This observation is valid for both calculations levels, but is more sensitive for B3LYP than for M06-2x functional. Figure 4 show B3LYP/6-31G(d,p)/LANL2DZ for free cisplatin and
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N NMR and 1H NMR (NH3 protons) spectra
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C NMR for SWCNT zigzag (12.0) models used for complexes design. The
calculated GIAO nuclear isotropic magnetic shielding tensors were scaled (1.062 and 0.83 factors respectively for 1H and
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N NMR chemical shifts) to aid the comparison with experimental NMR 5 ACS Paragon Plus Environment
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spectra. The calculated chemical shift values for the two different hydrogen atoms of free cisplatin, opposite to chlorine atom (Ha) and near the chlorine atom (Hb), are 4.0 and 3.8 ppm, respectively. The chemical shift value of 4.06 ppm found by Berners-Price et al.35 measured in a 5 mmol dm-3 solution containing cisplatin in 95% H2O-5% D2O with pH 4.72 was used as reference, showing a good agreement. Since, nanotubes used to build the complex contain only C─H and C─C bonds, 13C NMR chemical shifts are situated in a range from 50 to 150 ppm (see Figure 4b and 4c), as expected. In Figure 5 (left column), the
13
C NMR spectra are shown. If we compare Figure 4b with
Figures 5a and 5g, it can be seen that there is no significant difference between the spectra of complexes I and IV adsorption formed with respect to free nanotube spectrum. The electron density due to cis-DDP molecule externally adsorbed does not cause significant shielding effect of the carbon nuclei from the tube, and thus, the spectra of Figures 5a and 5g are similar to Figure 4b. But, when we evaluate Figures 5c and 5e some signs are observed with chemical shift values more positive in a range of 300 ppm (complex II) and 200 ppm (complex III) relative to 13C NMR shifts calculated for the free nanotube (Figure 4b). These signs relate to furthest carbon nuclei from cisplatin that is inside the nanotube in these complexes. The shielding effect due to electron density of cis-DDP is less intense in these carbon nuclei and thus, they are detected in the low-field regions. This effect is not only less intensified due to orientation of cisplatin, in addition to the fact that the molecule be close to the axis which passes through the center of mass of nanotube. If the cis-DDP has been adsorbed closer to nanotube wall (as observed in our study11 about inclusion complexes formed by cisplatin and carbon nanohorns), the carbon nuclei further away from this region could be identified in field even lower. If we imagine the cisplatin molecule used to form inclusion complexes inscribed in a sphere with a diameter of approximately 5.0 Å (this distance was calculated between the hydrogen (Ha) atom opposite chlorine in the molecule – see definition in Figure 4a) these variations observed in the spectra of Figures 5c and 5e may be even greater if one nanotube model with larger diameter than that utilized in our studies (d> 9.5 Å) is used. Experimentally, CNTs can be produced with 10-500 Å diameters1, 2, 36
, therefore, these variations probably can be observed in NMR experiment. Also, in Figure 5 (right column) are shown the spectra with 1H and 15N NMR chemical shifts
due to the formation of cis-DDP@CCNT complexes. When a comparison of 1H NMR spectra of the free cis-DDP (Figure 4a) is performed with the ones due to complexes formation with CCNT, most significant chemical shifts for the high-field region are observed for the hydrogen nuclei (NH3 group) of cisplatin in complex III. These shifts are ranging from 7.0 ppm for Hc to 9.5 ppm for Hb nucleus (see definition in Figure 4a). We can say that the protons of cis-DDP in the complex III are more strongly shielded by the induced field generated by the bound electrons movement of the large number of carbon atoms neighbors (tubular and cap region of nanotube), therefore, these protons tend absorb energy on greater intensity fields. On the other hand, this effect is less intense for the complexes I and 6 ACS Paragon Plus Environment
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II (-1.2< ∆δ
