Insight into the antioxidant and antiradical properties of colorotane sesquiterpenes extracted from Warburgia ugandensis: theoretical evaluation

Abstract

The present work is focused on the examination of two biological activities (antioxidant and antiradical properties) of colorotane sesquiterpenes extracted from Warburgia ugandensis. The geometrical optimizations of structures studied were performed in gas phase and water at B3PW91/6-311+G(d,p)// B3PW91/6-31+G(d,p) level under stereochemical constraints. The stability of chemical systems has been elaborated according to thermodynamic and kinetic approaches. Additional descriptors referred to global reactivity descriptors (electronegativity, global hardness, global softness, and electrophilicity index) have also been integrated. Predictions of antioxidant power were exclusively based on the calculation of the enthalpy change of the single-electron transfer (SET) mechanism named IP. Electrodonating (ω−), electroaccepting (ω+) powers, donor Rd and Ra indexes are used as antiradical descriptors. Our results demonstrated that 1,4,4a,5,6,7,8,8a-octahydro-1,4-dihydroxy-4a,5,5,8a-tetramethylnaphthalene-1,2-dicarbaldehyde in the absolute configuration (1R,4R) bearing one hydrogen bond exhibits the highest antioxidant profile. This compound that exhibited the lowest hardness values η (1.517 eV) has shown to be the less stable. Collectively, all colorotane sesquiterpenes have exhibited a good electron donor than sodium (Na). The location of the vitamin C above the colorotane sesquiterpenes in the donor-acceptor map illustrates better antiradical behaviour than the vitamin C in the gas phase. But, these compounds have presented worse antiradical behaviour with respect to gallic acid in the gas phase. In water, the Rd of the compound examined is higher than that of gallic acid (0.178 eV) with exception of 1,4,4a,5,6,7,8,8a-octahydro-1,4-dihydroxy-4a,5,5,8a-tetramethylnaphthalene-1,2-dicarbaldehyde in the absolute configuration (1R,4R) that have shown to be the best antiradical activity due to its highest Ra value (0.904 eV) and lowest Rd value (0.169 eV).