Dicaffeoylquinic Acids – Occurrence, Structure Elucidation, and Characterization as Primary Reference Substances
A widespread subgroup of polyphenolic compounds are the dicaffeoylquinic acids (diCQA). They occur in significant concentrations in e.g. artichoke (Cynara scolymus), coffee (Coffea sp.), mate (Ilex paraguariensis) and coneflower (Echinacea sp.), but have been detected in many other genera such as Eleutherococcus, Lonicera and Artemisia to name only a few. In coffee beverages, the dicaffeoylquinic acids have been shown to be responsible for a large proportion of its antioxidant activity. Six isomeric diCQA exist, i.e. 1,3-, 1,4-, 1,5-, 3,4-, 3,5- and 4,5-dicaffeoylquinic acid, which makes isolation as well as structure elucidation a challenging process. Several approaches to the latter task have been published and the various isomers were distinguished by NMR as well as by their mass spectrometric fragmentation pattern. For analytical purposes of medicinal herbs and food products well characterized reference substances are required but so far only cynarin (1,3-diCQA) has been commercially available. Selection of the correct reference substance is further complicated by the fact that different nomenclature systems exist. As a result the ring numbering of the quinic acid core can differ from IUPAC nomenclature and is inconsistent not only in the scientific literature but also in the product description of commercial suppliers. Especially cynarin is sometimes described as 1,3- or 1,5-diCQA (1,3 corresponds to IUPAC), resulting in different CAS numbers in the product description as well. The poster describes the process of isolation, structure elucidation and characterisation of dicaffeoylquinic acids as primary reference substances for analytical purposes.