Antioxidant potential of orientin: A combined experimental and DFT approach

• FRAP and antihemolytic assay prove leaves of R.capitata act as a good antioxidant.
• High reactivity of B-ring in orientin was confirmed by MEP mapping.
• Electron density distributions of reactive sites are analyzed.
• Presence of intramolecular hydrogen bond between 3′-O and 4′-H reduce BDE of 3′-OH.
• Orientin is capable to donate electrons, act as potential antioxidant than vitexin.

The antioxidant activity of the bioactive fractions obtained from the leaves of Rhynchosia capitata is evaluated for its capacity to reduce ferric ions. In vitro antihemolytic analysis for the separated erythrocytes of Wistar rat blood cells exhibits maximum inhibition value for ethyl acetate (1202.55 ± 9.46) than ethanol fraction (424.57 ± 12.04). Gas and solvent phase studies of structural and molecular characteristics of C-glycosyl flavonoid, orientin present in the bioactive fraction of R. capitata is investigated through hydrogen atom transfer mechanism (HAT) using DFT/B3LYP/6-311G(d,p) level of theory. Interestingly, the intramolecular hydrogen bonding formed between 3′-O and 4′-H makes 3′-OH as the active site which is supported by its bond dissociation energy values. The computed values of the adiabatic ionization potential, electron affinity, hardness, softness, electronegativity and electrophilic index indicate that orientin possess good radical scavenging activity. In this study, role of molecular electrostatic potential and electron density distribution map in predicting the importance of B-ring are analyzed and reported. Spin density distribution analysis for the radicals is formed by summing of spin on rings A, B and C. The most active system able to transfer a hydrogen atom is orientin compared to vitexin and the bond dissociation enthalpy follows the order benzene > ethyl acetate > water.

The aim of present investigation is to discover novel source of naturally occurring antioxidants and to identify the bioactive solvent fraction of Rhynchosia capitata. Antihemolytic activity on the separated erythrocytes of Wistar rat blood cells for ethyl acetate fraction exhibits good inhibition values. Density functional theory is used to compute relevant electronic properties with the purpose of generating accurate information which facilitates better understanding of the antioxidant behavior exhibited by the C-glucoside orientin present in the plant. To enlighten the ability of their antioxidant activity, the OH BDE, frontier molecular orbitals (FMO), molecular electrostatic potential (MEP), electron density distribution map and electronic properties such as electron affinity (EA), ionization potential (IP), softness (S), hardness (η), electronegativity (χ) and electrophilicity index (ω) have been illuminated. Spin density distribution of neutral and radical species have also been considered.Figure optionsDownload as PowerPoint slide