DFT and TD-DFT Study of Favipiravir Tautomerism as RNA Polymerase Inhibitors : COVID-19

Favipiravir is an antiviral medication currently being trialled as a COVID-19 treatment. To help accelerate these efforts, we have performed a research for tautomers formations of favipiravir as possible RNA polymerase enzyme inhibitors and mitigating the virus ability. This study provides important electronic and optical properties of tautomers determined by density functional theory (DFT) and time-dependent density function theory (TD-DFT) calculations in gas phase and in water. A series of favipiravir derivatives was designed, and study the effect of the HOMO-LUMO energy gap on the efficacy of inhibitors. It has been determined that H-atom positions change and substituting fluorine (F) by hydroxyl (OH) group of tautomers affects the energy gap and dipole moment values. Among all compounds, the results have shown that Fb4 form with OH is most potent inhibitory activity in both gas phase and water. These investigations indicated that these tautomers could be potentially developed into drugs, but further investigations are still required to examine the cytotoxicity and consequent side reactions.


Introduction
Since the December 2019, novel coronavirus disease (COVID-19) was first identified amid outbreak of respiratory illness cases in Wuhan city, Hubei province China and made several serious problems to the global health [1][2][3][4]. Without any available approved antiviral, considerable efforts have been done to examine available drugs to rapid detection of a way for pharmacotherapy [1,3].
Early exploratory studies show that inhibitors of enzymatic activity may be effective in treating COVID-19 [5,6]. Favipiravir is a broad antiviral drug (such as Arenavirus, Bunyavirus, Favivirus, Alphavirus, Norovirus, and Ebolavirus) belongs to the pyrazine carboxamide family, developed by FUJIFILM of Japan with activity against influenza viruses [7,8]. Recently has shown promising results in the treatment of COVID-19 patients in the clinical trials [9], as it is thought to inhibit the RNA-dependent RNA polymerase enzyme, and mitigating the virus ability to spread from one cell to another [10][11][12].
While the use of favipiravir drug, uncertainty remains about its safety and effectiveness [12], therefore, tautomers have been used in the synthesis some favipiravir compounds. The transfer of the protons and electrons from one place to another in an intramolecular changes the electronic structure and, hence, variety the properties, a reaction which involves proton transfer is called a tautomerism [13][14][15]. As a result, the tautomeric forms differ in shape, and double bonds pattern. Biological activity is one of the properties that are heavily affected by the proton transfer, the vital importance of knowing the tautomers states has been underlined by many authors [7]. The energy gap previously described can be used of drugs to investigate structure activity, and molecular  [16]. The energy gap of the inhibitors will also be influential in determining drug cytotoxicity and side effects in the body [17].
In this study, it is possible change the molecular properties of drugs and makes them possible candidates by changes in the band gap energy, which specifies the molecular reactivity of the drug. We applied density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to tautomers formations of favipiravir based on the H-atom movement, to shed light on geometric, electronic and optical properties like the energy gap (Eg), dipole moments (DM), electronic spectrum, highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) to candidate the best.

Computational Methodology
The computational model of favipiravir consists of 5 carbon, 3 nitrogen , 2 oxygen, 1 fluorine atoms with 4 hydrogen atoms in ends [18,19]. Structures of available tautomers of favipiravir have been analyzed by performing density functional theory (DFT) investigations at the level B3LYP functional and the 6-32G basis set as implemented in the Gaussian 09W software [20,21]. The values including energy levels of (E HOMO and E LUMO ), the energy gap ( = − ) [22], total energy (E total ), and dipole moments (DM) have been obtained for the optimized structures. The UV-Vis spectra, maximum absorption wavelengths (λ max ) and oscillator strengths (f) of the compounds were investigated with the time-dependent DFT (TD-DFT). We also computed these parameters for tautomers, by substituting fluorine (F) by hydroxyl (OH) group ( Figure 1) for the hope of better understanding and possible application for COVID-19 Patients. All the calculations in this work were studied in the gas and water phase, to evaluate the reactivity and effect of water solvent on the obtained parameters [23,24].

Results and discussion
The tautomerism is one of the most studied cases, both theoretically and experimentally. We have optimized and geometric analysis of four different tautomers for each configuration of favipiravir by DFT method with B3LYP functional and 6-31G basis set. Intra or intermolecular hydrogen bonding is also an important parameter of molecule, which usually leads to shortening of the C−OH bond compared to bond length of C−F, for compounds studied. Based on the gotten  The relationship between the drug properties and various biological processes is based on electronic properties of compounds. There is strong evidence from the literature that the energy gap and dipole moment are good inherent indicators of the transport or binding ability of drugs [23][24][25][26]. The energy gap is the dominant factor for determining the inhibitory efficacy of favipiravir, the dipole moment is roughly less influential than the energy gap. The fluorine substitution by hydroxil group leads to change the energy gap (Eg), which is energy difference between the HOMO and LUMO levels that in turn alters the electronic properties. , we note that the compounds have large dipole moment and this is a basic criteria for the interaction between drug and receptor. It is essential to analyze the density of state (DOS) and the HOMO and LUMO distribution patterns of the tautomers formations before and after substituting F atoms with OH group to more know the changes in molecular properties.
The DOS with various sites of H-atom, we observed that the DOS structure and electronic band gap are affected by changing the locations of the H-atom. Where the origin of the energy gap depends on the structural configuration of H-atom and the double bond of tautomers. Figure 3 show the HOMO and LUMO distribution and the DOS analysis which consists of the energy difference for the HOMO-LUMO, for detect the effects of tautomerism process on the molecular orbitals of all Based on the computational method, the total energies data would be of importance of assessment of the stability of favipiravir compounds and for choosing of the greatest stable structures. All the parameters of the compounds studied in the gas phase and water solvent in Table  1.  Table 1 The HOMO energies (E HOMO ), LUMO energies (E LUMO ), energy gap (Eg), dipole moment (DM), and total energy (E total ) were calculated at B3LYP/6-31G level of theory for compounds under study in the gas phase and water solvent. The UV-Vis spectra, maximum absorption wavelengths (λ max ) and oscillator strengths (f) for tautomers before and after replacing F atom with OH were simulated with TD-DFT at B3LYP/6-31G level of theory using the gas-phase optimized geometries. The absorption bands were centered at 297.45-374.98 nm range for F and OH-compound. The simulated UV-Vis spectra agree well with the interaction energies and HOMO-LUMO gap. All the molecular results of the electronic spectrum are collected in Table 2. The UV-Vis spectra for all compounds are shown in Figure 4.

Table 2
Calculated maximum wavelengths (λ max ) , and oscillator strength (f) of all the compounds in the gas phase.  The obtained values of tautomers indicate that Fb4 with OH could interact in stronger mode than other Fa and Fb structures, however, these parameters could be fully validated only by further experimental studies to synthesize them.

Conclusion
In summary, we studied the synthesis, electronic and optical features of favipiravir compounds by using DFT and TD-DFT calculations. Depending on positions of H-atom, change in the electronic properties are observed. From the obtained results, the inhibitory efficacy of the RNA polymerase enzyme by a tautomers of drug has been shown to be mainly determined by the HOMO-LUMO energy gap of the inhibitors, as the energy gap of the inhibitors is an inherent chemical reactivity, with lesser dependencies on the dipole moment. Fb4 is the most active tautomer based on the energy gap values and the good stable based on the structural analysis results with hydroxyl (OH) group, the substitutional hydroxyl would be promising candidate. Finally, favipiravir could be considered for showing the inhibitory efficacy, but further investigations are still required to examine different sides of favipiravir use in COVID-19.