Reactions of nicotine and the hydroxyl radical in the environment: Theoretical insights into the mechanism, kinetics and products.

Nicotine (NCT) is a prevalent and highly poisonous tobacco alkaloid found in wastewater discharge. Advanced oxidative processes (AOP) are radical interactions between harmful pollutants and ambient free radicals that, theoretically, result in less toxic compounds. For a better understanding of the chemical transformations and long-term environmental effects of toxic discharges, the study of these processes is crucial. Here, quantum chemical calculations are used to investigate the AOP of the NCT in aqueous and lipidic environments. It was found that NCT interacted with HO• in polar and nonpolar media, with an overall rate constant koverall = 106 - 1010 M-1 s-1. The computed kinetic data are reasonably accurate as seen by the comparison with the experimental rate constant in water (pH = 7.0), which results in a kcalculated/kexperimetal ratio of 1.4. The hydrogen transfer (C7, C9, C12)-single electron transfer pathways are the main mechanisms for the HO• + NCT reaction in pentyl ethanoate solvent to form the cations as the primary products of the two-step reaction. However, in aqueous environments, the degradation of NCT by HO• radicals increases with increasing pH levels. It is predicted that oxidation products are less toxic than nicotine itself, especially in an aqueous environment with a pH < 7.0.

The hydroperoxyl antiradical activity of natural hydroxycinnamic acid derivatives in physiological environments: the effects of pH values on rate constants.

Hydroxycinnamic acid derivatives (HCA) are a type of phenolic acid that occurs naturally. HCA are widely known for their anti-inflammatory, anti-cancer, and especially antioxidant capabilities; however, a comprehensive study of the mechanism and kinetics of the antiradical activity of these compounds has not been performed. Here, we report a study on the mechanisms and kinetics of hydroperoxyl radical scavenging activity of HCA by density functional theory (DFT) calculations. The ability of HCA to scavenge hydroperoxyl radicals in physiological environments was studied. The results showed that HCA had moderate and weak HOO˙ antiradical activity in pentyl ethanoate solvent, with the overall rate constant k overall = 8.60 × 101 - 3.40 × 104 M-1 s-1. The formal hydrogen transfer mechanism of phenyl hydroxyl groups defined this action. However, in water at physiological pH, 2-coumaric acid (1), 4-coumaric acid (2), caffeic acid (3), ferulic acid (4), sinapic acid (5) and 4-hydroxyphenylpyruvic acid (7) exhibit a significant HOO˙ antiradical activity with k total = 105 - 108 M-1 s-1 by the electron transfer reaction of the phenolate anions. Following a rise in pH levels in most of the studied substances, the overall rate constant varied. The acid 5 exhibited the highest HOO˙ radical scavenging activity (log(k overall) = 4.6-5.1) at pH < 5; however, at pH = 5.4-8.8, the highest HOO˙ radical scavenging activity were observed for 3 with log(k overall) = 5.2-5.7. At pH > 6.2, acids 2, 3, 4, and 5 presented the largest radical scavenging activity. By contrast, acid 3-coumaric acid (8) had the lowest antiradical activity at most pH values. Thus, the hydroperoxyl radical scavenging activity in pentyl ethanoate follows the order 3 > 5 > 1 ∼ 2 ∼ 4 ∼ 6 (homovanillic acid) ∼ 7 > 8, whereas it follows the order 3 > 2 ∼ 4 ∼ 5 > 6 ∼ 7 > 1 > 8 in water at pH = 7.40. The activity of 1, 2, 3, 4, 5, 6, and 7 are faster than those of the reference Trolox, suggesting that these HCA could be useful natural antioxidants in the aqueous physiological environment.