The reactivity of cinnamic acid derivatives as lignin precursors

Abstract

Objectives. Cinnamic acid derivatives belong to a large class of phenolic compounds, which are widely distributed in plants and have high potential for use in the medical and industrial fields. They have various useful practical properties, e.g., antioxidant, anti-inflammatory, antiplatelet, and anti-melanogenic properties. Hydroxycinnamic acids are of particular interest as phenylpropanoids, which are the starting compounds of lignin. The aim of this work was to study the electronic structure and analyze the reactivity of the simplest representatives of phenylpropanoids formed during the biosynthesis of the coumaric (p-hydroxycinnamic), caffeic (3,4-dihydroxycinnamic), ferulic (3-methoxy-4-hydroxycinnamic), sinapic (3,5-dimethoxy-4- hydroxycinnamic), and 3,4-dimethoxycinnamic acids. These acids are the biogenetic precursors of most other phenolic compounds (coumarins, melanins, lignins, and flavonoids) and are found in almost all higher plants.Methods.Calculations with full optimization of the geometric parameters were performed using the original Hartree–Fock theory and hybrid density functional method. All calculations were performed using the Firefly program.Results.A comparative quantum chemical calculation of the geometric parameters of hydroxycinnamic acid molecules was conducted via two methods, and the values of the charges on atoms according to Mulliken were determined. It was found that with the addition of hydroxyl and methoxy substituents at the meta and para positions relative to the carboxyl fragment, the electron density shifts toward the benzene ring, and the symmetry of the molecule decreases. Additionally, in these structures, there is π,π-conjugation of the carboxyl fragment of the –СН=СНСООН molecule with the aromatic ring, which significantly affects the geometric configuration of the molecule. The maximum positive charge is concentrated on the C9 atom, while the maximum negative charge is on the oxygen atoms belonging to the methoxy substituents and the hydroxyl group, which confirms the role of oxygen atoms in the chemical transformations of acids.Conclusions.Two different methods were used to calculate the geometric, electronic, and energy parameters and electrophilicity indices of the studied hydroxycinnamic acids in the gas phase. The obtained values were consistent (within the limits of error) with the experimental data as well as the results described in earlier works’ calculations by other methods.