Radical reactions between thiols and quinones have been reported. A radical mechanism can be proposed using the Fukui function (f 0 ) as an indicator for this reaction (Figure and Table in section 3 of Supporting Information). It is important to note that a one-electron transfer reaction from thiol to quinone is required in the absence of exogenous radical species. However, based on the calculated ionization potential (Adiabatic IP = 149.8 kcal/mol for BCysM) and electron affinity (Adiabatic EA = 92.5 kcal/mol for CARQ), initiating a one-electron transfer reaction between both starting materials might be less progressive than the nucleophilic addition of thiols.