Tannins from Amazonian tree barks as potential natural antioxidants: An alternative to synthetic phenolic compounds

Synthetic antioxidants are widely used to prevent oxidation. However, due to their associated environmental and health risks, the demand for natural alternatives is increasing. This study evaluated the phytochemical profile and antioxidant properties of tannins extracted from the bark of five tree species from the Amazon rainforest: Byrsonima spicata, Croton matourensis, Myrcia splendens, Tapirira guianensis, and Vismia guianensis. The main objective was to evaluate the viability of these tannins as alternatives to synthetic phenolic antioxidants. Bark samples were collected from secondary forests in the state of Pará, Brazil. Colorimetric assays showed that, except for C. matourensis, all extracts contained higher levels of condensed tannins, followed by flavonoids. M. splendens, V. guianensis, and B. spicata exhibited the highest condensed tannin concentrations (841.8, 544.5, and 503.6 mg catechin equivalents g^-1 of bark extract, respectively). These tannins demonstrated strong antioxidant activity against the 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH) free radical, with inhibition rates of 95.4 % (B. spicata), 95.2 % (T. guianensis), and 92.1 % (M. splendens), surpassing both the commercial tannin (69.6 %) and the synthetic antioxidant butylated hydroxytoluene (BHT, 87.9 %). Six phytochemicals identified via high-performance liquid chromatography (HPLC) were further evaluated using in silico tools for biological activity and toxicity predictions. Gallic, caffeic, ferulic, and o-coumaric acids showed potential for inhibiting enzymes involved in melanin synthesis in fruits, aiding in their preservation. Catechin was found to be non-toxic (oral LD₅₀ = 10,000 mg.kg^-1) and may inhibit enzymes related to cellular damage and oxidative stress-related diseases.