Density Functional Theory Studies on the Chemical Reactivity of Allyl Mercaptan and Its Derivatives

Abstract

On the basis of density functional theory (DFT) at the B3LYP/cc-pVQZ level with the C-PCM solvation model, a comparative analysis of the reactivity of the garlic metabolites 2-propenesulfenic acid (PSA) and allyl mercaptan (AM, 2-propene-1-thiol) was performed. In particular, the thermodynamic descriptors (BDE, PA, ETE, AIP, PDE, and G_acidity) and global descriptors of chemical activity (ionization potential (IP), electron affinity (EA), chemical potential (μ), absolute electronegativity (χ), molecular hardness (η) and softness (S), electrophilicity index (ω), electro-donating (ω^-) and electro-accepting (ω⁺) powers, and Ra and Rd indexes) were determined. The calculations revealed that PSA is more reactive than AM, but the latter may play a crucial role in the deactivation of free radicals due to its greater chemical stability and longer lifetime. The presence of a double bond in AM enables its polymerization, preserving the antiradical activity of the S-H group. This activity can be amplified by aryl-substituent-containing hydroxyl groups. The results of the calculations for the simplest phenol-AM derivative indicate that both the O-H and S-H moieties show greater antiradical activity in a vacuum and aqueous medium than the parent molecules. The results obtained prove that AM and its derivatives can be used not only as flavoring food additives but also as potent radical scavengers, protecting food, supplements, cosmetics, and drug ingredients from physicochemical decomposition caused by exogenous radicals.

Keywords: 2-propenesulfenic acid; DFT method; allyl mercaptan; chemical activity descriptors; garlic metabolites; thermodynamic descriptors.

Scheme 1

Selected chemical reactions that occur in garlic under its homogenization, leading to conversion of alliin to allicin, 2-propenesulfenic acid, and allyl mercaptan.

Scheme 2

Peroxyl radical scavenging by (A) allicin, (B) 2-propenesulfenic acid, and (C) allyl mercaptan.

Figure 1

The optimized geometries of PSA in water (A) and AM in water (B), a vacuum (C), and benzene (D), evaluated by the DFT method at the B3LYP/cc-pVQZ theory level and in the C-PCM solvation model.

Figure 2

(A) HOMO–LUMO energies and frontier orbitals of PSA in water medium, calculated using the DFT B3LYP/cc-pVQZ level of theory and the C-PCM solvation model. (B) Contour surface of electrostatic potential of PSA.

Figure 3

(A) HOMO–LUMO energy levels and frontier orbitals of AM in water medium, calculated using the DFT B3LYP/cc-pVQZ level of theory and the C-PCM solvation model. (B) Contour surface of electrostatic potential of AM.

Figure 4

Optimized geometries of phenol–AM derivative (AMD), dimers (2), and trimers (3) of AM in the polymerized A and B forms evaluated by the DFT method at the B3LYP/cc-pVQZ theory level in a vacuum.

Figure 5

(A) Projected AM polymer that protects active thiol S-H groups and (B) AM polymer obtained by Xi et al. [24].

Figure 6

Projected phenol–AM derivative (AMD).