phloroglucinolysis

Phloroglucinolysis is an analytical degradation method used to characterize proanthocyanidins (condensed tannins), a class of polymeric flavonoids found in many plants. The method relies on acid-catalyzed cleavage of interflavanyl linkages in proanthocyanidin polymers in the presence of phloroglucinol. Under these conditions, the polymer breaks into short flavan-3-ol units and phloroglucinol adducts of the extension units. The products are typically analyzed by high-performance liquid chromatography (HPLC) or LC-MS to determine the composition of terminal units, such as catechin and epicatechin, and the phloroglucinol adducts derived from extension units.

Usually performed in a methanolic, acidic medium containing phloroglucinol, the mixture is heated briefly to promote cleavage of C–C interflavanyl bonds. Phloroglucinol acts as a nucleophile that traps reactive carbocation intermediates formed on the extension units, yielding phloroglucinol ether adducts, while terminal units are released as free flavan-3-ols.

From the detected products, researchers estimate the mean degree of polymerization (mDP) and infer structural features of the original proanthocyanidin polymer, such as the ratio of terminal to extension units and the relative abundance of catechin- versus epicatechin-derived components. The method is widely used in food science, enology, and plant polyphenol research to characterize tannin composition in samples such as grape seeds, wines, and bark extracts. It complements other depolymerization methods, offering complementary information to thiolysis and other analyses.

Limitations include variable reactivity of certain linkages, sensitivity to galloylation or glycosylation, and potential underestimation of higher DP polymers in some sample types. Accurate interpretation relies on appropriate calibration with standards and awareness of the method's assumptions. Phloroglucinolysis has become a standard tool in polyphenol analysis since its introduction in the 1990s.