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. 2015 Jan 16;13(1):509-28.
doi: 10.3390/md13010509.

Profiling of the molecular weight and structural isomer abundance of macroalgae-derived phlorotannins

Natalie Heffernan  1 , Nigel P Brunton  2 , Richard J FitzGerald  3 , Thomas J Smyth  4
Affiliations

Profiling of the molecular weight and structural isomer abundance of macroalgae-derived phlorotannins

Natalie Heffernan et al. Mar Drugs. .

Abstract

Phlorotannins are a group of complex polymers of phloroglucinol (1,3,5-trihydroxybenzene) unique to macroalgae. These phenolic compounds are integral structural components of the cell wall in brown algae, but also play many secondary ecological roles such as protection from UV radiation and defense against grazing. This study employed Ultra Performance Liquid Chromatography (UPLC) with tandem mass spectrometry to investigate isomeric complexity and observed differences in phlorotannins derived from macroalgae harvested off the Irish coast (Fucus serratus, Fucus vesiculosus, Himanthalia elongata and Cystoseira nodicaulis). Antioxidant activity and total phenolic content assays were used as an index for producing phlorotannin fractions, enriched using molecular weight cut-off dialysis with subsequent flash chromatography to profile phlorotannin isomers in these macroalgae. These fractions were profiled using UPLC-MS with multiple reaction monitoring (MRM) and the level of isomerization for specific molecular weight phlorotannins between 3 and 16 monomers were determined. The majority of the low molecular weight (LMW) phlorotannins were found to have a molecular weight range equivalent to 4-12 monomers of phloroglucinol. The level of isomerization within the individual macroalgal species differed, resulting in substantially different numbers of phlorotannin isomers for particular molecular weights. F. vesiculosus had the highest number of isomers of 61 at one specific molecular mass, corresponding to 12 phloroglucinol units (PGUs). These results highlight the complex nature of these extracts and emphasize the challenges involved in structural elucidation of these compounds.

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Figures

Figure 1
Figure 1
Total ion chromatogram (TIC) of reverse phase (RP) flash chromatography-enriched fractions of (a) Fucus serratus; (b) Fucus vesiculosus; (c) Himanthalia elongate; and (d) Cystoseira nodicaulis. Prominent peaks are labeled with retention time (on the left or on top) along with observed m/z (on the right or below the retention time).
Figure 2
Figure 2
Chemical structures of a selection of phlorotannins, illustrating varying bonding mechanisms between phloroglucinol units.
Figure 3
Figure 3
UPLC-MS/MS MRM ion chromatograms of reversed-phase (RP) flash chromatography enriched fraction of Fucus serratus for deprotonated molecules [M − H]− of 745.3, 869.3, 993.4, 1117.4, 1242.4, 1365.4, and 1489.4 m/z, which correspond to phlorotannins of 6, 7, 8, 9, 10, 11, and 12 phloroglucinol units, respectively.
Figure 4
Figure 4
UPLC-MS/MS MRM extracted ion chromatogram of Fucus serratus phlorotannin isomers at the selected deprotonated molecules [M − H]− of 1490 m/z, which corresponds to phlorotannins consisting of 12 PGUs in size.
Figure 5
Figure 5
UPLC-MS total percentage peak intensity for individual molecular ions corresponding to phlorotannins of between 3 and 16 phloroglucinol units (PGUs) enriched from four species of macroalgae. The number of phlorotannin isomers detected at each molecular weight correspond to phlorotannins detected fromin extracts from each brown algal species are highlighted with bold numbers. Fucus serratus (FS), Fucus vesiculosus (FV), Cystoseira nodicaulis (CN), Himanthalia elongata (HE).
Figure 6
Figure 6
Chemical structure of a range of isomers (fucotetraphlorethol A–G), with degrees of polymerization of 6, demonstrating the potential for large numbers of isomers.
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