Hardware
Computations were done using mainly Hewlett Packard Z230 workstation with intel Xeon CPU-1225 3.2 GHz, RAM 8 Gb, Windows 7 Professional 64-bit (compA) and FK BY workstation with Intel Core i7, 3 GHz, RAM 8 Gb, Windows 10 64-bit (CompF) as well as various laptops with worse parameters. Inverse high-throughput virtual screening (inv HTVS) data processing was done using Hewlett Packard Probook 450 notebook with Intel Core i5-3230M 2.6 GHz, RAM 4 GB working under 64-bit Windows 7. Information from web resources www.stackoverflow.com, www.geeksforgeeks.org, www.support.microsoft.com, https://github.com, www.planetaexcel.ru and others was used to realize coding for the helper software creation.
Dataset preparation
The 461 SARS-CoV-2 protein 3D structures files with code range from 5R7Y to 7LKV, available to the beginning of 2021 (now at the Jan 2022 ~1700 are available), were taken from Protein Data Bank database (https://www.rcsb.org ). Supplementary Table S1 lists PDB accession numbers and names for proteins under consideration. Only records about AAs atom coordinates belonging to chain A were used for further processing, but the rest of information was removed. Further formatting was done using standard MGL Tools [15] Python script prepare_receptor4.py resulting in a set of protein files in ready-to-use pdbqt format. Ligand 3D or, if unavailable, 2D structures files were taken from Pubchem (https://pubchem.ncbi.nlm.nih.gov) and BioGem (https://pdt.biogem.org) [16] databases. Smiles codes and substructure search options of the databases were used to find structures with α,β-unsaturated carbonyl and few others electrophilic motifs. Structures for compounds with trivial names were selected preferably. Finally, a library of ~2000 compounds was collected. Open Babel 2.4.1 software [17] was used for conversion of the selected files from both sdf and smi formats to .pdb format using 3D-structure generation and minimization commands (--gen3d and -ff uff -n 800 -cg -c 1e-8, respectively). Then the files were formatted using standard MGL Tools Python script prepare_ligand4.py resulting in ligand files in pdbqt format.
Description of selected ligands set
The 1788 structures of compounds were used. Majority of them were α,β-unsaturated carbonyl-bearing linear esters and lactones, ketones and amides which can be found in plants or other natural sources. Such structures were selected because they were reported to make covalent bonds with nucleophilic atoms of AAs in various proteins. In particular, α,β-unsaturated amide motif has become a classic for design of approved drug covalent inhibitors targeting Cys residues in kinases [18]. α,β-Unsaturated lactones are reported to attack Cys [22][ 23], His [22] and even Lys [22], whereas similar properties are monitored for linear esters too [24]. α,β-unsaturated ketones were reported to bind with Cys [25] and His [24]. Previous reports demonstrated examples of covalent inhibition of coronaviral enzymes by such electrophilic natural compounds, e.g. MPro by unsaturated lactone andrographolide [25], artecanin [12] and oridonin [21].
Docking and docking-based data processing
Docking-based virtual screening was performed using Autodock Vina [19]. Parameters for grid box (4x4x4 cubic nm, centered on the center of each individual protein pdbqt file), exhaustiveness (12) and number of models (5) were identical for all calculations.
The docking results were processed using python script process_VinaResults.py and the best poses were selected. For further analysis of predicted protein-ligand complexes Python script binana.py [20] was used. The script was modified to save protein-ligand complex data in one file and to operate with more than 1000 AAs numbers.
Parameters description
Various parameters for each protein-ligand interaction were tabulated and analyzed, in particular, binding energy (Ebind, kcal/mol), AAs and their atoms within 0,4 nm from atoms of a ligand (close contacts), ligand codes, PDB codes and trivial names for proteins etc. Only interactions with Ebind of -6.9 or less were taken for detailed analysis. The colocalization of ligands electrophilic atoms (selected manually) with nucleophilic atoms of Cys (-SH, SG in pdbqt files) and His (any of imidazole N-atoms, ND1 or NE2) residues within 0.4 nm was analyzed to guess whether the interaction have a potential to covalent bond formation (“distance criterium”).
The number of different protein-ligand complexes meeting the distance criterium for a certain amino acid residue (Cys was used as default) and Ebind criterium for the same protein was also used as a main criterium to highlight repeatability of such type of interaction in spite of variability between different PDB structures (“repeatability criterium”). Trivial names for hit ligands were retrieved from aforesaid databases. In the cases of absence of the names, trivial names of similar compounds with suffix -like were mentioned; the similarity was determined based on “Substructure” or, if not, “Similarity” functions of Pubchem database.
Also, two more parameters were calculated for more effective interaction assessment: Erelative and Eaverage. The relative binding energy (Erelative, kcal/mol×atom) of protein-ligand interactions was calculated using the following equation:

where the number of ligand atoms was taken from the corresponding ligand.pdbqt file for a calculated interaction, Ebind is the minimal binding energy for this interaction. The criterium depict effectivity of a ligands’ atoms “usage” to bind with a certain protein and should be less than -250 (“Erelative criterium”).
The average binding energy (Eaverage, kcal/mol×complex) was evaluated as follows:

where denominator is the number of complexes meeting all criteria for the same protein and numerator is the sum of this complexes minimal binding energies. The parameter allows for a more accurate assessment of the formation a bond between a given ligand and protein possibility. We assume that the closer Eaverage to Ebind, the better the ligand binds to various protein structures of the same protein, and therefore the greater probability of the ligand effective binding to the protein in practice.
All the procedures in the workflow described above were semi-automatically done using an original helper program tool named FYTdock, which is based on aforesaid known Python scripts and few original scripts as well as Microsoft Excel book files with macroses (see Supplementary Video_1 and Video_2).