Phlorotannins may be considered structurally the most simple tannin group, but their relatively scarce distribution in the plant kingdom has hindered major advances in their isolation and identification techniques
Phlorotannins are found mainly in marine organisms such as brown algae and they are structurally perhaps the most simple tannin group. Individual phlorotannins are composed of two or more phloroglucinol (Fig. 1a) units that are attached to each other via C-C or C-O-C bonds, thus yielding oligomers such as the tetrameric phlorotannin (Fig. 1b). Further structural variations may include additional OH-groups in the molecules or additional bonds between the monomers. This structure-based definition very clearly differentiates phlorotannins from other types of phenolic compounds.
Fig. 1. Structures of phloroglucinol (a), the phlorotannin building unit, and a tetrameric phlorotannin (b) consisting of four phloroglusinol units.