A Comparative Analysis of Polyphenol Content and Biological Potential of Quercus petraea Matt. and Q. pubescens Willd. Bark Extracts

Quercus wood (oak wood) is a valuable resource, which has led to the intense processing of oak trees by the forestry industry. As a result, large amounts of forestry by-products (bark and leaves) are left in the woods, considered valueless. Thus, the aim of this study was to evaluate and compare the phytochemical profile and potential biological activities of Q. petreaea Matt. and Q. pubescens Willd. bark extracts. The extracts were obtained by microwave (MAE) and ultrasound-assisted extraction (UAE) with water (100%) and ethanol:water (70:30). These extracts were then characterized in terms of the total polyphenolic and tannin contents using the Folin–Ciocâlteu method. Their antioxidant properties were determined by observing the neutralizing effects of the extracts against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) radicals. The antimicrobial effect was tested on Gram-positive and Gram-negative bacterial strains and three fungi from the Candida genus. Cellular counts were measured to determine the cytotoxic effects of the extracts on HEK 293T cell lines. Moreover, spectrophotometrical assays were performed to assess the inhibitory effects of the extracts against the enzymatic activity of α-glucosidase, tyrosinase, and acetylcholinesterase. The MAE resulted in higher yields of polyphenolic compounds and tannins compared to the UAE bark extracts. All of the experimental variants exhibited free-radical-neutralizing properties, especially Q. petraea extracts. Q. petraea extracts also had a more efficient antibacterial effect, especially against Gram-positive bacteria and K. pneumoniae. Antifungal activity was highlighted against C. krusei. Cell counts indicated a cytotoxic effect of the tested extracts against HEK 293T cells. The tested extracts inhibited the activity of α-glucosidase, tyrosinase, and acetylcholinesterase, indicating the potential use of these extracts as antidiabetic, neuroprotective, and skin-protecting agents. These findings highlight the untapped therapeutic potential of the bioactive compounds found in the bark of Q. petraea and Q. pubescens.