Tannins represent a large sub-class of polyphenolic compounds ubiquitously present in plants. Naturally occurring gallotannins (GTs) are widely studied due to their multiple prophylactic and therapeutic effects [1]. GTs are basically composed of a central carbohydrate core that is esterified with gallic acid. The most common carbohydrate is d-glucose, although other monosaccharides are present as well. GTs are extensively studied as they exhibit diverse biological activities ranging from antioxidant, antimicrobial, antiviral, anti-inflammatory, immune-modulatory to anticancer effects [2‑4]. As a part of our ongoing studies on the synthesis of biologically important saccharides, as potential drug candidates we have synthesized a new series of GTs and evaluated their biological activities in various experimental systems. Antioxidant, antibacterial and anti-biofilm activities of structurally different GTs derived from branched-chain d-lyxose, d-ribose, l-rhamnose, d-mannose and d-fructose, together with three unnatural analogues [5] of 1,2,3,4,6-penta-O-galloyl-d-glucose (PGG) were examined and mutually compared.

Based on the experimental results obtained in this study it can be concluded that studied compounds are excellent antioxidants and radical-scavenging agents. They exhibited only moderate activity against gram-positive pathogens such as S. aureus and E. faecalis and low antimycobacterial activity. However, they were efficient inhibitors and disruptors of S. aureus biofilms in sub-MIC concentrations and interacted with quorum-sensing system in Chromobacterium violaceum. Moreover, they are non-toxic to eukaryotic cells, thus are promising candidates for further studies in developing new antimicrobial agents.