Molecular docking allows us to see the strength of ligand-receptor interaction.  In using this method, it was found that quercetin showed significant inhibition of 3CLpro and PLpro, two viral proteases of SARS-CoV-2, with docking binding energies of −6.25 and −4.62 kcal/mol, respectively.  This bonding demonstrates a potential for quercetin to interfere with SARS-COV-2 replication, since the viral proteases 3CLpro and PLpro play a crucial role in this process.  From this, they looked at the interaction that quercetin has on the main SARS-CoV-2 protease, 6LU7.  Here, it was found that quercetin formed hydrogen bonds with many of the amino acids within the active site of the protease, including His164, Glu166, Asp187, Gln192, and Thr190.
It was also found that there was a strong interaction between quercetin and the S spike protein, particularly at the S protein‐ACE2 receptor interface.  



Using molecular docking to determine potential mechanism of quercetin to mitigate COVID-19

This study looked to evaluate the effect of quercetin on COVID-19 treatment, investigating if a daily supplementation of quercetin, vitamin C, and bromelain (QCB) would alter the treatment outcomes of COVID patients.  Ultimately, they found that this supplementation had a positive effect on the outcome of treatment.  

Treatment of COVID-19 Patients with Quercetin: A Prospective, Single - Centre, Randomized, Controlled Trial

The flavinoid quercetin is found in many fruits and vegetables, including berries, capers, cilantro, apples, and onions, but is not produced by the human body. It has been found to have antiviral, anti‐atopic, pro‐metabolic, and antinflammatory effects, and it is a promising therapeutic treatment for COVID-19.

Chemical structure of quercetin:



Bowdoin College

- Many naturally occurring polyphenols are inexpensive to produce and have low risk for development of toxicity, making these compounds good candidates for preventive treatment to decrease viral infectivity and to dampen the risk of a virus-induced inflammatory storm.
- At the molecular level, polyphenols hold promise as inhibitors of viral proteases involved in viral replication due to their general affinity to proteins via hydrogen bonding and their low risk of toxic effects. 
- The same may hold true for binding of polyphenols to S protein, although pre-clinical and clinical studies are required to strengthen existing evidence. Another point that should be taken into account is the proper formulation for these polyphenol-based nutraceuticals.

Potential use of polyphenols in the battle against COVID-19

Derosa, G., Maffioli, P., D'Angelo, A>, Pierro, F. (2020). A role for quercetin in coronavirus disease 2019 (COVID‐19)  Phytottherapy.  https://onlinelibrary.wiley.com/doi/full/10.1002/ptr.6887

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