Protein production. The DNA sequence encoding PL pro SARS-CoV-2 protease was synthetized (optimized for expression in Escherichia coli) and cloned into a pD431-SR vector between XbaI and SmaI restriction sites, by Atum (Newwark, CA, USA). The protein sequence was preceded by an N-terminal TEV protease site (ENLYFQ↓G/S) followed by a 6-His purification tag. E. coli BL21 (DE3) cells were transformed using the plasmid described above and the CaCl2 method and colonies were selected on LB-agar-Kanamycin plates. A single colony was picked and grew over night in LB media and then used to inoculate a 400 mL LB culture. This was grown at 37 ºC, 250 rpm, until an optical density at 600 nm of 0.7 (O.D.600) was reached and then induced for 18h at 16 ºC (250 rpm) by adding 0.20 mM IPTG. The bacterial pellet was collected after 9,000 rpm centrifugation and resuspended in 30 mL of 50 mM Tris-HCl pH 8.0, 150 mM NaCl, 20 mM imidazole (resuspension buffer). An Ultrasonic Cell Disruption Sonifier 450 (Branson) was used to sonicate the cells (on ice) with 8 pulses of 2 mins (Output Control 7 and 50 % Duty Cycle).