Chelation of Cu(II), Zn(II), and Fe(II) by Tannin Constituents of Selected Edible Nuts

Magdalena Karamać

doi:10.3390/ijms10125485

Abstract

The tannin fractions isolated from hazelnuts, walnuts and almonds were characterised by colorimetric assays and by an SE-HPLC technique. The complexation of Cu(II) and Zn(II) was determined by the reaction with tetramethylmurexide, whereas for Fe(II), ferrozine was employed. The walnut tannins exhibited a significantly weaker reaction with the vanillin/HCl reagent than hazelnut and almond tannins, but the protein precipitation capacity of the walnut fraction was high. The SE-HPLC chromatogram of the tannin fraction from hazelnuts revealed the presence of oligomers with higher molecular weights compared to that of almonds. Copper ions were most effectively chelated by the constituents of the tannin fractions of hazelnuts, walnuts and almonds. At a 0.2 mg/assay addition level, the walnut tannins complexed almost 100% Cu(II). The Fe(II) complexation capacities of the tannin fractions of walnuts and hazelnuts were weaker in comparison to that of the almond tannin fraction, which at a 2.5 mg/assay addition level, bound Fe(II) by ~90%. The capacity to chelate Zn(II) was quite varied for the different nut tannin fractions: almond tannins bound as much as 84% Zn(II), whereas the value for walnut tannins was only 8.7%; and for hazelnut tannins, no Zn(II) chelation took place at the levels tested.

Highlights

Phenolic compounds constitute a very diversified group of plant secondary metabolites in terms of structure, molecular weight and physicochemical and biological properties
The content of condensed tannins was 1,261 and 776 mg catechin equivalents (CE)/g for the hazelnut and almond tannin fractions, respectively, whereas the result of this assay for fraction of walnut was several times lower
Protein precipitation capacities of hazelnut and walnut tannin fractions were comparable: 940 and 873 mg tannic acid equivalents (TAE)/g, but a more than tenfold lower value was noted for the almond tannin fraction

Summary

Introduction

Phenolic compounds constitute a very diversified group of plant secondary metabolites in terms of structure, molecular weight and physicochemical and biological properties. The richest source of condensed tannins among the above mentioned nuts is hazelnuts and they comprise (epi)catechin or (epi)gallocatechin subunits [8]. All three flavan-3-ols, which are common in proanthocyanidin structures: (epi)catechin, (epi)gallocatechin and (epi)afzelechin, were identified as subunits of condensed tannins from almonds [11]. Condensed tannins of walnuts are constituted by (epi)catechin units, their content in seeds is seven times lower than in hazelnuts [8]. Fukuda et al [13] and Ito et al [14] identified and isolated more than 20 hydrolysable tannins from the extract of walnuts. Those tannins comprised mainly of ellagitannins, but gallotannins were present

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