Experimental and DFT studies on the antioxidant activity of a C-glycoside from Rhynchosia capitata

Rhynchosia capitata (=Glycine capitata) Heyne ex roth, was found to possess polyphenolics including flavonoids, which acts as potential antioxidant. The study of ethanolic extract of roots and leaves reveals that the leaves possess high polyphenolics including flavonoids than roots. This was also confirmed by DPPH radical scavenging activity. Leaf powder of the plant was extracted with different solvents by soxhlet apparatus in the order of increasing polarity. The DPPH scavenging activity of methanol fraction was found to be high compared to the crude extract and other fractions. Nitric oxide scavenging activity was dominant in chloroform fraction compared to methanol fraction. Presence of flavonoids especially vitexin, a C-glycoside in methanol and chloroform fractions were confirmed by high pressure thin layer chromatography (HPTLC) analysis. The structural and molecular characteristics of naturally occurring flavonoid, vitexin was investigated in gas phase using density functional theory (DFT) approach with B3LYP/6-311G(d,p) level of theory. Analysis of bond dissociation enthalpy (BDE) reveals that the OH site that requires minimum energy for dissociation is 4′-OH from B-ring. To explore the radical scavenging activity of vitexin, the adiabatic ionization potential, electron affinity, hardness, softness, electronegativity and electrophilic index properties were computed and interpreted. The nonvalidity of Koopman’s theorem has been verified by the computation of Eo and Ev energy magnitudes. Interestingly, from BDE calculations it was observed that BDE for 4′-OH, 5-OH and 7-OH are comparatively low for vitexin than its aglycone apigenin and this may be due to the presence of C-8 glucoside in vitexin. To substantiate this, plot of frontier molecular orbital and spin density distribution analysis for neutral and the corresponding radical species for the compound vitexin have been presented.

The aim of present investigation is to discover novel source of naturally occurring antioxidants and to study the radical scavenging activity of aerial parts of Rhynchosia capitata by DPPH assay. Quantification of polyphenolics and flavonoids are carried out by Folin–Ciocalteu method and colorimetric method. Further, presence of flavonoids is ensured by HPTLC technique. In the current research, DFT model chemistry 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-glycoside vitexin present in the plant. To enlighten the ability of their antioxidant activity, the OH BDE and other electronic properties along with HOMO and LUMO energies, electron affinity (EA), ionization potential (IP), softness, hardness, electronegativity and electrophilicity index have been illuminated. Validity of Koopman’s theorem is also verified. Spin density distribution of neutral and radical species have also been considered. The computed molecular descriptor values are compared and analyzed with reference to its aglycone apigenin.Figure optionsDownload as PowerPoint slideHighlights
► Presence of flavonoids in aerial parts of R. capitata was confirmed by HPTLC.
► Highly polar methanol fraction exhibited high oxide radical scavenging activity.
► High reactivity of B-ring in vitexin was confirmed by DFT.
► Presence of C-8 glucoside in vitexin reduces its BDE compared to aglycone apigenin.
► Vitexin is capable of donating electrons and acts as good radical scavenger.