A comprehensive DFT-based study of the antioxidant properties of monolignols: Mechanism, kinetics, and influence of physiological environments.

Monolignols, p-coumaryl alcohol (CouA), coniferyl alcohol (ConiA), and sinapyl alcohol (SinA), are the fundamental materials for lignin biosynthesis, a major component of lignocellulosic biomass. In the present study, we report a comprehensive analysis of the antioxidant properties of monolignols, using density functional theory (DFT) calculations. Under model physiological conditions, monolignols demonstrated a high hydroperoxyl radical scavenging capacity in polar media, with overall rate constants (k_overall) ranging from 5.80 × 10⁶ to 1.15 × 10⁷ M^-1 s^-1. In contrast, this activity was less pronounced in lipid media, with k_overall in the range of 2.66 × 10² to 2.61 × 10⁴ M^-1 s^-1. The single electron transfer (SET) mechanism was found to play a decisive role in water at physiological pH and under basic conditions, whereas the formal hydrogen transfer (FHT) mechanism was the exclusive pathway in aqueous acid conditions and lipid media. Furthermore, the monolignols ConiA and SinA, demonstrated a strong capacity to chelate Cu(II) and Fe(III) ions in water, with apparent equilibrium constants in the range of 9.21 × 10¹⁴ to 5.93 × 10²¹ M^-1 s^-1. Their complexes were also found to be highly effective in blocking the reduction of Cu(II)-to-Cu(I) and Fe(III)-to-Fe(II) via the ascorbic acid anion pathway.