Quinones have multiple reaction positions for nucleophilic addition and have a high potential for redox reactions, therefore, the predicting the reaction mechanism is challenging. Although many methods have been developed to predict reactive positions, in this study, the most reactive position of CARQ and predicted intermediates was selected based on the condensed Fukui function, easily obtained by the natural population analysis of DFT calculations. The indices for the condensed Fukui functions were obtained using UCA-FUKUI software. The values for the atoms of orthoquinone moieties of CARQ and its reaction intermediates are summarized in Table . Regarding the prediction of the reaction mechanism shown in Figure , the 8-position (f + = 0.216) is the most reactive site for the nucleophilic attack on CARQ, where BCysM reacts nucleophilically to form intermediate 3.