Suppression of ESIPT Phenomenon of Flavonoids on Binding Interaction with Double Stranded RNA

The enhanced excited-state intramolecular proton transfer energy barrier of flavonols induced by deprotonation

The barrierless excited-state intramolecular proton transfer (ESIPT) is believed to account for the non-radiative decays of flavonols composed of 5-hydroxyl group. However, the ESIPT mechanisms of flavonol anions have never been elucidated. In this work, by using the time-dependent density functional theory (TDDFT) calculations, we have determined the barrierless ESIPT in kaempferol and galangin, in agreement with their non-emissive properties. In contrast, deprotonation at the position 7 of them is demonstrated to decrease the basicity of proton acceptor and acidity of proton donor in the excited state, largely increasing the ESIPT barrier and leading to the fluorescence emission from the normal state. A further deprotonation of mono-deprotonated kaempferol is inferred to induce blue shifted emission. These results elucidate the nature of emissive flavonol anions and give a deep insight into the optical properties of flavonols in different matrices.

Photophysical Behavior of Plant Flavonols Galangin, Kaempferol, Quercetin, and Myricetin in Homogeneous Media and the DMPC Model Membrane: Unveiling the Influence of the B-Ring Hydroxylation of Flavonols

Flavonols have been studied extensively because of their interesting biological activities and excited-state intramolecular proton transfer (ESIPT) behavior. Galangin, kaempferol, quercetin, and myricetin are structurally related flavonols that differ only in the number of B-ring hydroxyl substituents. In this work, we have carried out a detailed study on the photophysical behavior of these structurally related flavonols in various solvents and a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) small unilamellar vesicles (SUVs) model membrane. We observed that these flavonols exist in different forms in the ground and excited states depending on the nature of the solvent. The weak intrinsic fluorescence of these flavonols gets enhanced in hydrogen-bond-accepting and alcoholic solvents. The phototautomer fluorescence intensity of these flavonols increases significantly in the DMPC membrane compared to water, suggesting ESIPT activation via binding interaction between flavonols and the membrane. According to our findings, both the number of B-ring hydroxy groups and membrane fluidity affect the flavonol binding with the membrane. The steady-state fluorescence intensity, steady-state anisotropy, fluorescence lifetime, and fluorescence anisotropy decay of flavonols were sensitive towards the temperature-induced DMPC membrane phase change. A quenching study has been performed to investigate the location and distribution of flavonols in the DMPC SUVs. Moreover, the antioxidant potential of flavonols in DMPC SUVs has been examined using the DPPH scavenging method. Our results reveal that the B-ring hydroxy groups significantly affect the photophysics, binding affinity, location, distribution, and DPPH scavenging activity of polyhydroxy-flavonols in the DMPC SUVs.