The clinically established gold drug Auranofin was reacted individually with a group of representative proteins, namely ubiquitin, ribonuclease A, carbonic anhydrase, haemoglobin and superoxide dismutase, and adduct formation was monitored in the various cases by ESI-MS analysis. We found that the reaction is highly selective for solvent exposed free cysteines that are modified through coordination of the AuPEt₃ ⁺ fragment. Indeed, ESI-Q-TOF MS spectra carried out on protein samples incubated with a three fold molar excess of Auranofin allowed direct detection of the native proteins bearing bound AuPEt₃ ⁺ fragments in the cases of carbonic anhydrase and haemoglobin. At variance, the two proteins that do not possess any free cysteine residue, i.e. ubiquitin and ribonuclease A, were unable to bind the gold fragment. In the case of superoxide dismutase, adduct formation is hindered by the scarce solvent accessibility of the free cysteine residue. These findings were further confirmed by a series of competition binding experiments with ebselen, a potent and selective cysteine-modifying reagent; we observed that pre-treatment with ebselen prevents the binding of the AuPEt₃ ⁺ fragment to both carbonic anhydrase and haemoglobin.

The COVID-19 pandemic poses an unprecedented challenge for the rapid discovery of drugs against this life-threatening disease. Owing to the peculiar features of the metal centers that are currently...

The tetrapeptide Ac-Gly-[Cys-Sec]-Gly-NH₂, reproducing the C-terminal motif of the selenoenzyme thioredoxin reductase, was designed and synthesized, and its reactions with a few medicinally relevant gold(I,III) compounds investigated by ESI-MS. Remarkably, the main reaction products could be unambiguously identified providing valuable insight into the likely mechanisms of enzyme inhibition by gold compounds.