Reactions of chlorophyll with hydroxyl radicals via RAF, HAT and SET mechanisms: A theoretical study

To understand the scavenging action of chlorophyll (Chla) found in most of the vegetables towards hydroxyl (\({{\text{OH}}}^{\bullet }\)) radicals, its reactions with \({{\text{OH}}}^{\bullet }\) radicals via RAF, HAT, and SET mechanisms have been investigated theoretically using two layer ONIOM [M06-2X/6-31G(d) (High):M06-2X/3-21G (Low)] method and M06-2X/6–311 + G(d,p) level of density functional theory. The molecular electrostatic potential (MEP), highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots, HOMO–LUMO energy gap, global hardness (η), global softness (S), electronegativity (χ), and electrophilicity index (ω) of Chla molecule were computed and analyzed to determine its stability and reactive sites. It is found that RAF and HAT reactions are exergonic in both gaseous and aqueous media whereas SET reactions are endergonic in both media. However, all the RAF, HAT and SET reactions studied here are found to be more favourable in aqueous media vs. gas phase. The rate constants of RAF reactions at different sites are found to be of the order of ~ 6.2 × 10⁷–1.8 × 10¹⁰ s⁻¹ indicating that RAF reactions would be appreciably fast. This study concludes that chlorophyll can efficiently scavenge \({{\text{OH}}}^{\bullet }\) radicals preferably via RAF and HAT mechanisms and intake of water with chlorophyll can enhance its scavenging actions.