Abstract
Tannins are natural compounds that have historically been used in the tanning of leather. In the scientific literature, one finds many reports of their possible beneficial health effects, although these are not always unequivocally confirmed. In order to gain a better insight into their proposed anticancer potential, we studied the scavenging capacity of the basic tannin building blocks against various chemical carcinogens of the epoxy type. The reactivity of gallic acid, ellagic acid, and epicathechin was examined using quantum mechanical calculations at the Hartree-Fock level of theory in conjunction with flexible basis sets and implicit solvation models. The monomeric tannin building blocks exhibited significant scavenging potential, with epicatechin presenting the best scavenger, thus encouraging and guiding future experimental studies of their anticarcinogenic properties.
Highlights
Tannins are secondary plant metabolites from the class of polyphenols that are found in various plant tissues
Proanthocyanidins are composed of different cathechin moieties, and complex tannins may consist of a combination of all three types of basic units
We focused on tannin basic building blocks for two main reasons: (1) simulations of reactions with complex higher tannin structures would be extremely CPU-time expensive, while it could even happen that optimization would not be possible because of the large number of heavy atoms; (2) at least in the case of hydrolyzable tannins, many of the more complex compounds are hydrolyzed to their basic building blocks in the digestive tract, e.g., epigallocatechin gallate is hydrolyzed into gallic acid and epigallocatechin,[14,15] while many of the ellagitannins release ellagic acid as one of their hydrolysis products
Summary
Are difficult to separate from other factors.[18]. The selected toxins either are important industrial chemicals or are naturally occurring in various food sources The genotoxicity of these toxins varies widely, from the most genotoxic aflatoxin B1, which, next to its carcinogenicity, possesses acute toxicity, to compounds such as styrene oxide or propylene oxide, for which carcinogenicity for humans has not been proven.[23−25] Apart from their presence in human environments, these toxins were selected based on the fact that they either contain an epoxy group or are metabolically transformed into ultimate carcinogens containing this group via the action of different enzymes from the enzyme superfamily of cytochromes P450 (P450 3A4 for aflatoxin B1, P450 2E1 for styrene, vinyl chloride, and acrylonitrile). All toxins, apart from β-propiolactone, share an epoxy ring, to which different functional groups are attached These toxins range from the simplest ethylene oxide (Figure 2b) to the most complex aflatoxin B1 exo-8,9-epoxide (Figure 2a) over compounds possessing functional groups with various electronic and steric properties. N2 of reaction the epoxy group, the first (rate-determining) step is the attack of the nucleophile (the negatively charged oxygen atom of the ionized tannin building block) with simultaneous epoxy ring opening, while the second (faster) step is the protonation of the obtained product.[28−30,37] the SN2 mechanism and the reaction proceeding at the sterically less hindered carbon atom of the epoxy ring was assumed (Figure 3).[26]
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