SARS-CoV-2 (COVID-19) has created a global health crisis. This pandemic threatening closely resembles the SARS-CoV outbreak occurred in 2003 (Walls et al., 2020) . However, the present one is highly spreading with extremely higher degree of virulence. As of today (April 1, 2020), it killed 5,11,250 people from a total 10,523,410 infected people in 213 countries (https://www.worldometers.info/ coronavirus/). Global economy has been in a jerk for 4 months because of more period of lockdown all over the globe. Thus the therapeutic/preventive intervention is an immediate requirement and challenging act against this highly stable and frequently mutable viral strain. There are no established preventive/therapeutic measures against this infection. Some of the old drugs are used on the basis of previous experiences from similar kind of infections. Some tests have been performed in some in vitro model with inclusive results. For example, one recent report suggests that remdesivir and chloroquine effectively inhibit this infection in an in vitro experimental model (Cortegiani, Ingoglia, Ippolito, Giarratano, & Einav, 2020; Gao, Tian, & Yang, 2020; Wang et al., 2020) . However, it is now considered that chloroquine has a high level of toxicity and hydroxychloroquine (HCQ) is predicted to be less toxic (Colson, Rolain, Lagier, Brouqui, & Raoult, 2020) . In a trial and survey-type experiment with a very small sample size, it is shown that HCQ treatment is significantly associated with viral load reduction/disappearance in COVID-19 patients and its effect is reinforced by azithromycin (Gautret et al., 2020) . Out of few combination of medication, presently HCQ is being used in COVID-19 cases . However, this drug also has a varied range of toxicity (Cortegiani et al., 2020) . Apart from that, this drug is also used in systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), so strategies should be taken to resolve the issue of sufficient supply of this medicine for these patients also (Yazdany & Kim, 2020) .
When there is almost nothing to do, chloroquine group of drugs are of use, but alarming call has been raised by scientists and clinicians for the wide range of side effects of this drug (Yazdany & Kim, 2020) .
Besides lung, liver and the kidney tissues also face a significant degree of injuries and in this situation use of drugs needs critical analysis and careful approach (Jiang, 2020; Rismanbaf & Zarei, 2020) . Other drugs like remdesivir, lopinavir, ribavirin and ritonavir have shown efficacy to inhibit coronavirus in vitro. Teicoplanin, an antibiotic used to treat staphylococcal infections, has been shown to inhibit MERS-CoV in human cells. So, this drug may be rechecked in the present situation also (Baron, Devaux, Colson, Raoult, & Rolain, 2020) . Molecular docking (MD) results suggest that few drugs, namely sofosbuvir, galidesivir and tenofovir, can be used against SARS-CoV-2 RNA-dependent RNA polymerase inactivation (Elfiky, 2020 Maiti, 2014; Maiti et al., 2017) . Regarding the antiviral role, tea flavonoids have been shown to be strong agent (Li et al., 2020) . Report demonstrates that catechin can prevent influenza A (H1N1) virus infection and gallic acid can inhibit influenza virus infection (You et al., 2018) . In the past three decades several studies suggested that the regular consumption of green tea decreases influenza infection and some cold symptoms. The gargling with tea extracts and/or flavonoids may protect from influenza virus (Furushima, Ide, & Yamada, 2018) . EGCG-fatty acid derivative is important because fatty acid on the phenolic hydroxyl group increases viral and cellular membrane permeability which protects from viral infection (Kaihatsu, Yamabe, & Ebara, 2018) . Preliminary attachment to cellular glycans is a critical step for entry of several human viruses. Some viruses, such as herpes simplex virus type 1 (HSV-1) and hepatitis C virus (HCV), bind to heparan sulfate, whereas others, such as influenza A virus (IAV), bind to sialic acid (Colpitts & Schang, 2014) . The SARS-CoV-2 is also covered by a large number of NAG molecules.
In this background, we have been intended to test different flavonoid effects in the process of SARS-CoV-2 spike glycoproteins binding to its host cell receptor ACE2. In the present bioinformatics, cheminformatics and molecular modeling studies, we initially characterized the nature of nCoV and ACE2 binding and surface contact areas.
The role of specific amino acids and their binding energy have been evaluated. Furthermore, different flavonoids have been docked on two targets; nCoV spike proteins and host cell receptors and all the bindings were carefully characterized. The current study will help to screen some suitable drug that can block the viral binding and further host cell attachment/entry.
The electron microscopic structure of human coronavirus spike glycoprotein (PDB ID: 6vsb) and X-ray diffraction structure of human angiotensin-converting enzyme 2 (ACE2; PDB ID: 4aph) were retrieved from RCSB Protein Data Bank (https://www.rcsb.org/) in PDB format. The structure 6vsb was selected because it was found with number of N-acetyl glucosamine (NAG) attached in different locations of the protein. On the other hand ACE2 (PDB ID: 4aph) was selected as it was a complete tertiary structures with no such viral spike protein contaminations. Other two viral spike protein structures like 6VXX and 6VYB and one spike-ACE2 (receptor) complex structure 6M0J were also retrieved for analysis.
The three-dimensional (3D) structures of different selected tea flavo- nih.gov/). As control or for comparative study, 3D structure of HCQ was also retrieved from PubChem in .sdf format.
Receptor molecules like human coronavirus (COVID-19) spike glycoprotein (PDB ID: 6vsb) and human ACE2 protein (PDB ID: 4aph) were found with numerous water molecules with their structures. The structures were individually dehydrated using PyMol molecular visualization system and edited structures were saved in .pdb format. Additionally, angiotensin II molecule was removed from human ACE2 protein also.
Ligand molecules were retrieved in .sdf format and then they were converted to protein data bank format and saved as .pdb format using
PyMol and used for further analysis. The chemical structures, compound PubChem Compound ID (CID), molecular formula (MF) and molecular weight (MW) of selected ligands were listed in Table 1 .
Basically two different terminologies generally used to define the sol- was used. According to Tian, Chen, Lei, Zhao, and Liang (2018) , protein pockets are those, on infinite cross section at least one will show the larger value then the mouth opening of that pocket.
The MD between human coronavirus spike protein selected ligands and human ACE2 protein selected ligands were performed using 
A large body of evidences has clarified the health benefit of tea components catchin and theaflavin. Tea is a regular beverage that is consumed worldwide and specifically may be more in India, China and other parts of Southeast Asian countries. Moreover, daily food/nutrients intake in these places is based on various vegetable combinations, fruits and green leaves. These are highly enriched with phytochemicals, polyphenols like flavonoids. Earlier our laboratory has demonstrated the anticarcinogenic role of green tea extract (Acharyya, Ali, et al., 2015; Maiti et al., 2017) . It decisively helps in DNA protections by scavenging free radicals and that efficacy was found to be even greater than the known specific radical scavengers (Acharyya, Ali, et al., 2015; Acharyya, Chattopadhyay, et al., 2014) .
According to the relationships between structure and antiviral activity of catechin derivatives, the 3-galloyl and 5 0 -OH group of catechin derivatives appear critical to antiviral activities (Kaihatsu et al., 2018) . In vitro activity of QR-435 green-tea extract against 
The human coronavirus spike glycoprotein (PDB ID: 6vsb) and X-ray (Wrapp et al., 2020) . Among them, S1 region is responsible for surface attachment and bond formation with human cell membrane receptor ACE2. During this attachment a significant conformational change was observed within the spike glycoprotein ( Figure 1 ).
PDB structure 6VXX showed a closed state structure unable to bind with host membrane, whereas in PDB structure 6VYB a raised structure was observed that was an open state conformation of spike glycoprotein (Figure 1b) . Viruses are host-specific particulate. This specificity depends upon the presence of a particular receptor protein (Table 2) 
To understand the solvent accessible area of the selected proteins molecules nCoV2 and ACE2, CASTp server was used, where the internal channel of ACE2 was observed to have protein pocket with a surface area of 2674.824 Å 2 and surface area volume was 2,291.847 Å 3 .
In spike glycoprotein, the solvent accessibility was observed starting from the flexible ACE2 binding region to most rigid central channel with a surface area value of 33706.386 Å 2 and surface area volume of 63378.539 Å 3 (Fig. 3) .
To find out potent viral spike protein destabilizing drug, MD study was performed using PatchDock and Autodock. In Patchdock, best 20 docking results for each interaction were considered for positionspecific docking posture analysis and selection of best one with lowest ACE value calculation. All the selected drug molecules showed higher affinity to viral spike protein in comparison with ACE2 receptor proteins ( F I G U R E 3 Surface topology calculation of COVID-19 spike glycoprotein (nCoV2) and its human receptor ACE2. Surface area and volume is much higher in nCoV2 then ACE2 and the ligand binding pockets were exposed in nCoV2 protein (Li, Hattori, & Kodama, 2011) . Along with this, those components also have different therapeutic and disease protective roles as strong antitoxicant, antioxidant and antiinflammatory agents. Antitumerigenic role of EGCG has also been shown by several laboratories (Acharyya, Ali, et al., 2015; Acharyya, Chattopadhyay, et al., 2014; Maiti et al., 2017) .
According to the nature of viral spike protein attachment with its human receptor protein ACE2, there are several steps behind it. According to Duquerroy, Vigouroux, Rottier, Rey, and Bosch (2005) , showed the lowest Ki value of 6.54 μmol in comparison with TFDG and TFMG which indicated that less amount of drug can inhibit the protein efficiently. A versatile role of EGCG has been demonstrated in the last decade regarding the antiviral effects. EGCG prevented neurotoxicity mediated by HIV-1 proteins gp120 and immune inflammatory responses via IFN-gamma where role JAK/STAT1 signaling has been found to be involved (Giunta et al., 2006) . This suggests that beside the disfavoring role to the viral entry and propagation, EGCG also significantly restricted the cellular inflammatory burst. Role of overproduction of cytokines like IL1, IL-6 and TNF-α and cytokine storm may also be prevented by EGCG. The EGCG can play as inhibitor of hepatitis C and hepatitis B virus entry into the host (Calland et al., 2012; Huang et al., 2014) . Ability to block the several viral entries might indicate the interferences in the viral spike protein destabilization in the preinfusion and postinfusion events. These finding may be validated by the synthetic EGCG-palmitate efficiency against porcine reproductive/respiratory syndrome virus (Zhao, Liu, Li, Yang, & Zu, 2014) . The green tea catechin, EGCG has also been shown to inhibit chikungunya virus infection (Weber, Sliva, Von, Kummerer, & Schnierle, 2015) .
According to our study, EGCG, TFDG and TFMG were the most potent three molecules, which could be used for the COVID-19 treatment as such or after some chemical modification by making it more efficient in interfering the transition between open and closed state of the viral spike. It was proposed in a previous study that gargling with tea catechin extracts may be practiced for the prevention of influenza infection in suspect/sensitive individuals. For the better bioavailability and interaction with different viral components, spike-glycoprotein nanoparticulated catechin derivatives may be used alone or in combination of other bioactive peptides/chitosan/lectins (Hu et al., 2012) . To serve promptly this bioinformatics, MD and virtual binding/inhibition study was finished with great pace. We believe that our present work will add some new findings in the present emergent situation developed pandemic COVID-19 outbreak.
This work was partially funded by Department of Science and Technology, Government of West Bengal, India.