Advancing quantification methods for polyphenols in brown seaweeds—applying a selective qNMR method compared with the TPC assay

Abstract

Introduction Brown seaweeds are a sustainable biomass with a potential for various industrial applications. Polyphenols are an important contributor to this potential.

Objective The aim was total quantification of polyphenols in brown seaweeds from different tidal zones, using a selective 1H quantitative NMR (qNMR) method, comparing the results with the colorimetric Folin–Ciocalteu total phenolic content (TPC) assay.

Method qNMR was performed with integration of selected peaks in the aromatic region (7–5.5 ppm). Deselection of non-polyphenolic 1H signals was based on information from 2D (1H-13C, 1H-15N) NMR spectra. 13C NMR phlorotannin characterisation facilitated the average number of protons expected to be found per aromatic ring used for the 1H quantification.

Results Selective qNMR and the TPC assay showed similar results for the three sublittoral growing species from the Laminariaceae; lower amounts for Laminaria hyperborea and Laminaria digitata (qNMR: 0.4%–0.6%; TPC: 0.6%–0.8%, phloroglucinol equivalents (PGE), dry weight (DW)) and higher amounts for Saccharina latissima (qNMR: 1.2%; TPC: 1.5%, PGE, DW). For the eulittoral Fucaceae, Fucus vesiculosus (qNMR: 1.1%; TPC: 4.1%; PGE, DW) and Ascophyllum nodosum (qNMR: 0.9%; TPC: 2.0%; PGE, DW), the TPC results were found to be up to three times higher than the qNMR results. The 13C NMR characterisation showed the highest phlorotannin polymerisation degree for F. vesiculosus.

Conclusion The TPC assay provided similar polyphenolic amounts to the selective qNMR method for sublittoral species. For eulittoral growing species, the TPC method showed amounts up to three times higher than the qNMR method—most likely illustrating the lack of selectivity in the TPC assay.