Electrostatic potential mapping of ascorbic acid and dimethoxyaniline for the antioxidant evaluation capacity

We have here presented an extended theoretical and experimental study of hydroxyl radical (HR) trapping assay by ascorbic acid (AA). 2,4-Dimethoxyaniline (DMA) in the presence of hydrogen peroxide and horseradish peroxidase (HRP) produces an imine form of 2,4-DMA (DMA*), a violet colored product with an UV-Vis absorption maximum at 540 nm. The formation of the violet colored product was inhibited by the addition of AA. Fukui functions and dual descriptor enumerating the local and global electron densities of the atoms on the DMA, AA, and HR systems were calculated. The preferential electron density pattern on DMA, AA, and HR was assessed by left and right Fukui functionals at constant external potential. Highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO- LUMO) gap for DMA and AA were found to be 8.834 and 10.908 eV, respectively, and singly occupied molecular orbital- highest occupied molecular orbital (SOMOHOMO) gap for HR was 16.872 eV. The global softness for DMA, HR, and AA was 12.99, 6.10, and 11.70, respectively. The electronic transitions (n to pi* and pi to pi*) of the proposed colored product were simulated by time dependent-density functional theory (TD-DFT) calculation with its correlation with the practical UV-Vis data. Natural bond order analysis (NBO) and electrostatic potential (ESP) providing theoretical information on the relative electron density over the atomic position of AA and DMA were presented. This article can provide a unique way of understanding the position of HR radical attack on AA and DMA.