Molecular structure, homo-lumo analysis and vibrational spectroscopy of the cancer healing pro-drug temozolomide based on dft calculations

1.   Introduction

Temozolomide, an imidazotetrazine derivative, is an oral alkylating agent synthesized at Aston University in the United Kingdom in the early 1980s [1]–[3]. Its molecular formula is C₆H₅N₆O₂ [4]. The Food and Drug Administration approved it for the treatment of glioblastoma multiforme and recurrent anaplastic astrocytoma in the first-line treatment [5],[6]. All of these drugs work in a non-specific manner, affecting both malignant and non-cancerous cells [7],[8]. Cancer cells, on the other hand, divide more quickly than normal tissue, increasing vulnerability to these impacts [9]. Temozolomide, like other chemotherapy medicines, is highly costly, and drug prices might vary widely from place to place [3],[10]. The mechanism of action and current clinical applicability of temozolomide has been mostly limited to glioblastoma until now [11].

Recently, computational chemistry has really drawn a lot of attention among researchers and scientists as a means of solving real problems in chemical, medicinal, biotechnology, and material science [12],[13]. The main problem associated with temozolomide as a drug is its poor chemical stability in neutral and basic conditions [14]. So far, there has been no extensive theoretical research of the title compound in an anion state, and no similar studies comparing it in the presence of the solvent DMSO have been done. So, in this research paper, we optimized the temozolomide molecule at its minimum possible surface energy both in neutral and anion states and with the addition of DMSO solvent so that its frequency shifting, variation in charge distribution, and energy gap could be studied. It mainly focuses on the Mulliken charge distribution, MEP, HOMO LUMO energy gap, DOS spectrum, and vibrational spectral assignments with TED contribution in prominent peaks based on computational data compared with experimental work on previous work.

2.   Methodology

All the quantum chemical calculations were performed using the Gaussian09W program [15]. The DFT/B3LYP [16],[17] method and the 6-311G(d) basis set were used to generate the optimized geometrical parameters and vibrational frequencies of the molecule. Gauss View 5.0 [18] was used to perform molecular visualization and to analyze the findings. The geometry of the chosen molecule was first optimized on the potential energy surface with full relaxation at the B3LYP/6-311G (d) level and was re-optimized for subsequent calculations. The vibrational wavenumbers were then calculated using optimized structural parameters. The fact that all of the calculated wavenumbers are positive confirms the stability of optimized geometry. The DFT technique employing the same basis set has been proposed for computing the molecular electrostatic potential and electronic properties such as HOMO-LUMO energies, and DOS spectrum. According to Koopman's theorem, the energy gap is the difference between LUMO and HOMO energy [19]. The Gauss Sum program was used to observe the DOS spectrum [20]. The TED analysis was carried out using the VEDA4 program [21].

3.   Results and discussion

3.1.   Optimized molecular geometry

The minimum energy configuration of the temozolomide molecule is obtained by geometrical optimization of the molecule within the Gaussian 09W software. The molecular structure with its atomic numbering after optimization in a neutral state is presented in the Figure 1.

The potential surface scan with the dihedral angle C9–C10–C14–O2 was performed by rotating 10 degrees in the range of 0–360 degrees for neutral and anion states. Also, the potential surface scan was performed with the addition of Dimethyl Sulfoxide (DMSO) solvent, which is shown in Figure 2, and the minimum total surface energy obtained was tabulated in Table 1. The further calculations were done by re-optimizing the molecule at its respective dihedral angles.

3.2.   Molecular electrostatic potential analysis

The highest positive and negative regions, represented by blue and red colors on the MEP map, are favored sites for nucleophilic and electrophilic attacks, respectively [22]. It is very useful in the study of molecular structure and physicochemical characteristics since it uses color grading to represent positive, negative, and neutral electrostatic potential regions [23]. The temozolomide molecule has a minimum electrostatic potential primarily limited to oxygen, which is a more reactive region for an electrophilic attack. Similarly, the maximum electrostatic potential is mainly confined to hydrogen, nitrogen, and carbon, which are highly active regions for nucleophilic attack in a neutral state. However, in the anion state, almost every atom favors electrophilic attack.

3.3.   HOMO-LUMO analysis

The frontier molecular orbitals, namely the HOMO and the LUMO have a significant role in quantum chemistry. The HOMO-LUMO orbitals of the temozolomide molecule are calculated in the gas phase for neutral (singlet spin) and anion states (doublet spin) using the DFT method with the B3LYP/6-311G(d) basis set, as shown in Figures 4 and 5. The green and red colors represent the positive and negative phases, respectively. The energy values of HOMO (donor) and LUMO (acceptor) and their energy gaps reflect the molecule's chemical activity [24]. In general, the atom with more HOMO densities must have a higher capability to detach an electron, while the atom with higher occupied LUMO densities must have a stronger strength to gain an electron [25].

Table 2 lists all of the HOMO and LUMO energies, as well as their energy gaps, for different charge states of temozolomide molecule. The energy gap of the molecule in a neutral state (gas phase) is 4.40 eV and 4.30 eV when the DMSO solvent is present. Similarly, the calculated energy gaps for alpha and beta mode in anion state are found to be 3.11 eV and 4.37 eV, respectively. We have also calculated the HOMO-LUMO gap of the temozolomide molecule in water. It is found that the HOMO LUMO gaps of the title molecule in solvent water are 2.53 eV for alpha mode and 4.75 eV for beta mode respectively. The HOMO LUMO gap of the alpha mode of the title molecule is smaller in water in comparison to the molecule in DMSO. This suggests that the temozolomide molecule is more reactive in water than in DMSO. This result is consistent with previous similar work [26].

3.4.   Density of states

Since the nearby orbitals in the boundary region may have quasi-degenerate energy levels, using only HOMO and LUMO to describe the frontier orbitals may not be realistic [27]. Hence, the spectrum of density of states (DOS) for the temozolomide molecule was obtained by using the Gauss Sum 3.0 program with a full width at half maximum (FWHM) of 0.3 eV [20]. The DOS spectrum explains the contribution of electrons to the conduction and valence band. The spectrum in Figures 6 and 7 illustrates how many states are available at various energy levels.

The virtual orbital is also known as an acceptor orbital because it is not occupied. The filled orbital, on the other hand, is known as the donor orbital. The DOS has a positive value for bonding interaction, a negative value for anti-bonding interaction, and is zero for no bonding interaction [28]. The energy gaps observed in the DOS spectrum and HOMO LUMO are equivalent and in good agreement with each other for both α and β-state.

3.5.   Mulliken atomic charges

Mulliken atomic charge calculation is very useful in quantum chemistry since it has an influence on electronic structure, dipole moment, polarizability and various molecular properties [29]. The Mulliken atomic charges were determined in this study using the DFT/B3LYP/6-3116G+(d) basis set. The Mulliken charge values are displayed in the Figure 8. The C11 and C8 atoms have the highest positive and negative charges, respectively, both in neutral and in an anion state. Positive charges are found in all hydrogen atoms, while negative charges are found in oxygen atoms.

3.6.   Vibrational analysis

The title molecule is made up of 20 atoms and consists of 54 different vibrational modes. The intensive vibrational modes occurring between 1850 and 1400 cm⁻¹ were observed and analyzed with their TED assignment for the neutral and anion state of the compound. Also, the vibrational shifts were observed in addition of the DMSO solvent and isotope labeling of carbon atoms.

The most prominent peak for the title molecule in the selected range was observed at 1819 cm⁻¹ with TED contribution of 79% maximum in anion state. It is due to the stretching mode of C=0 which are usually observed in the region of 1850-1650 cm⁻¹ [30]. Usually, the ring C-C aromatic vibrations observed in the IR cover the spectral range from 1600 to 1400 cm⁻¹ [31]. In our study, the C-C stretching vibration bands with their TED assignments are calculated at 1456 (10), 1416 (12), 1458 (28), and 1411 (16) cm⁻¹ in the IR spectrum of neutral, anion, and in the presence of DMSO solvent for both states, respectively.

The stretching vibration of C-N is commonly found around 1400 cm⁻¹ [31]. The computed wavenumbers for C-N vibration are observed between 1700 cm⁻¹ and 1520 cm⁻¹ in neutral and anion states, as well as in the presence of solvent and isotope labeling of carbon atoms. The calculated value slightly differs since it is not scaled and resonance in the ring increases the force constant of the C-N bond [32].

When an atom is replaced by an isotope of larger mass, a downshift in the frequency can be observed [33]. The frequency of every mode of vibration between 1850 and 1400 cm⁻¹ has decreased slightly when the carbon atom was labeled with the C-13 isotope in neutral and anion states. In an average, the down shifting ranges from 20-80 cm⁻¹ which is remarkably noticeable in the Figures 9 and 10.

4.   Conclusions

The minimum stable energy for the potential surface scan was found in the anion state and in the presence of DMSO solvent. The nucleophilic and electrophilic reaction sites of the molecule are predicted by the MEP surface. The Mulliken atomic charges were examined and analyzed. The HOMO and LUMO energy gaps address every charge transfer interaction that occurs within the molecule and its bioactivity. The more value of the HOMO-LUMO gap in the neutral state of the temozolomide molecule reflects the higher kinetic stability and the lower chemical reactivity both in the gas phase and in solvents. The energy gaps observed in the DOS spectrum and HOMO-LUMO are equivalent and correspond well with each other. Also, the vibrational frequencies with their corresponding frequencies are analyzed both in the gas phase and in the presence of DMSO solvent. The calculated frequencies of all modes of vibration are present within the characteristic region of the spectrum, and the frequency shifting was observed significantly in different charge states and isotope labeling of carbon.