Thermodynamics of free radical scavenging activity of deprotonated hibiscetin towards reactive oxygen and nitrogen species

Deprotonated flavonoids are known to be more potent antioxidants than neutral forms. This study examined the reaction enthalpies for the mono-deprotonated forms of the most hydroxylated flavonol, hibiscetin, concerning hydrogen atom transfer (HAT) and sequential proton-loss electron-transfer (SPLET) mechanisms. Since deprotonation is highly prevalent in the aqueous medium, it was chosen as the medium of investigation. Deprotonated hibiscetin prefers to enter the SPLET reaction pathway to HAT or electron transfer. Hence, the sequential double proton loss electron transfer (SdPLET) route offers a stronger justification for the antioxidant activity of hibiscetin in the aqueous phase. The sequential proton loss hydrogen atom transfer (SPLHAT) process can also take place in the aqueous media competitively. The viability of Hibiscetin and mono-deprotonated Hibiscetin in scavenging five reactive oxygen and nitrogen species (ROS and RNS) that are known to damage biological systems in the aqueous solution was assessed by calculating the equilibrium constant and redox potential of electron transfer. The hydroxyl radical exhibited the strongest attraction for the hydrogen atom and electron among the ROS and RNS under consideration. Its equilibrium constants also indicated that hibiscetin and deprotonated hibiscetin scavenge it in a highly advantageous and spontaneous manner. The higher values of the equilibrium constant in the case of mono-deprotonated hibiscetin confirmed its better capacity for scavenging reactive species.