Ab initio and density functional theory studies for the explanation of the antioxidant activity of certain phenolic acids

Abstract

Ab initio and density functional theory molecular orbital calculations were carried out at both the HF/6-31 +G(d) and B3LYP/6-31+G(d) levels for the four antioxidants, p-hydroxycinnamic acid derivatives, namely, the p-coumaric, caffeic, ferulic, and sinapinic acid and the corresponding radicals, in an attempt to explain the structural dependency of the antioxidant activity of these compounds. Optimized resulting geometries, vibrational frequencies, absolute infrared intensities, and electron-donating ability are discussed. Both the high degree of conjugation and the extended spin delocalization in the phenoxyl radicals offer explanation for the scavenging activity of the four acids. In structurally related compounds, the calculated heat of formation value in radical formation appears as a meaningful molecular descriptor of antioxidant activity in accordance with experimental data. This becomes more clear at the B3LYP level.

Ab initio conformational study of caffeic acid

“…These structural parameters do not deviate much from the X-ray values found in the literature for caffeic acid [36] (although comparison between results in the solid and in the gas phase must be done with care). Moreover, the values presently obtained for the most stable conformer, CA 1, are in very good accordance with the ones reported by Bakalbassis et al, for the only ground-state geometry calculated by these authors for caffeic acid, at both the HF/6-31 þ G* and B3LYP/6-31 þ G* levels [18]. Potential-energy profiles for internal rotation around different bonds within the caffeic acid molecule -O 7 -C 6 and O 8 -C 5 , C 3 -C 9 , C 10 -C 11 , and C 11 -O 13 -were obtained, along with the respective rotational energy barriers, by scanning the corresponding torsional angles.…”

“…The vibrational spectrum was calculated for all conformers found for caffeic acid (data available from the authors upon request). The calculated wavenumbers for the most stable geometries -CA 1, CA 2 and CA 3 -are comprised in Table 2, along with the corresponding assignment, and show a good overall accordance with the theoretical values reported by Bakalbassis et al, for CA 1 [18]. Also, the agreement between the presently calculated frequencies, using the scaling factors proposed by Scott and Radom [33], and the Raman experimental data found in the literature for this molecule [37 -39] was found to be quite good.…”

“…The atoms are numbered according to Fig. 1 18 . By performing scanning calculations for these same internal rotations without any frozen parameters, a certain amount of cooperativity between the internal rotation of the two phenolic hydroxyls was detected, through small deviations (up to 98) in the initial values of either dihedral upon variation of the other.…”

“…Nevertheless, the reported ab initio molecular orbital calculations on phenolic acid derivatives are very scarce, semiempirical quantum-chemical methods being commonly used instead. In fact, apart from these only a few ab initio studies on phenolic antioxidants is to be found in the literature [14 -18], one of them on several p-hydroxycinnamic acid derivatives, including caffeic acid [18]. However, these studies aim exclusively at the explanation of the structural dependency of the antioxidant activity of this kind of phenolic analogues, focusing only on their most stable geometries.…”

Antioxidant Mechanisms of Polyphenolic Caffeic Acid Oligomers, Constituents of Salvia officinalis

“…In fact, comparing the theoretical coupling constants of the neutral and dissociated catechol radicals (the carboxyl group is always considered dissociated), the participation of the excess electron from the phenolate group obviously enhances the overall resonance stabilization (data not shown). Since application of this theoretical approach for more complex structures is still in its infancy, the only comparable studies were carried out for the calculations of the spin densities of three undissociated semiquinone structures of caffeic acid (34) and calculations of bond-dissociation energies of phenolic protons (35,36).…”