Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Honorata M. Ropiak, Peter Lachmann, Aina Ramsay, Rebecca J. Green, Irene Mueller-Harvey, Jamshidkhan Chamani.
A diverse panel of condensed tannins was used to resolve the confounding effects of size and subunit composition seen previously in tannin-protein interactions. Turbidimetry revealed that size in terms of mean degree of polymerisation (mDP) or average molecular weight (amw) was the most important tannin parameter. The smallest tannin with the relatively largest effect on protein aggregation had an mDP of ~7. The average size was significantly correlated with aggregation of bovine serum albumin, BSA (mDP: r = -0.916; amw: r = -0.925; p<0.01; df = 27), and gelatin (mDP: r = -0.961; amw: r = -0.981; p<0.01; df = 12). The procyanidin/prodelphinidin and cis-/trans-flavan-3-ol ratios gave no significant correlations. Tryptophan fluorescence quenching indicated that procyanidins and cis-flavan-3-ol units contributed most to the tannin interactions on the BSA surface and in the hydrophobic binding pocket (r = 0.677; p<0.05; df = 9 and r = 0.887; p<0.01; df = 9, respectively). Circular dichroism revealed that higher proportions of prodelphinidins decreased the apparent α-helix content (r = -0.941; p<0.01; df = 5) and increased the apparent β-sheet content (r = 0.916; p<0.05; df = 5) of BSA.
Condensed tannins (CT, syn. proanthocyanidins, Fig 1) occur as polyphenolic oligomers and polymers in many fruits and in some vegetables , medicinal plants  and forage legumes . Some CT can have positive impacts on animal nutrition, health and welfare , and there is now also considerable interest in their anthelmintic effects against gastrointestinal nematodes [4–9]. Recent research has shown that the integrity of the parasitic nematode cuticle becomes distorted after exposure to CT . This cuticle is largely composed of collagen-like proline-rich proteins and structural proteins (cuticlins) [11, 12]. CT have a high affinity to proline-rich proteins , which may explain the effect of CT on nematode cuticles.
A large panel of CT with different structural features was isolated from a diverse set of plants in order to cover a wide range of average oligomer/polymer sizes and structures. The mean degree of polymerisation and average molecular weight correlated very significantly to the efficacy of CT to aggregate BSA and gelatin in a turbidimetry study. The average size of the CT, rather than the hydroxylation pattern or stereochemistry of the flavan-3-ol subunits, was most important for aggregation. The data can be approximated with two linear fits, which intersected at an mDP of ~7 or ~2000 Da. This means that the smallest CT with the relatively largest effect on protein aggregation would have an mDP of ~7. A similar trend was observed in a procyanidin-BSA study by ITC where a CT hexamer of 1721 Da had an optimal binding stoichiometry . Interestingly, the greatest effect on bioactivity was also observed at around ~2000 Da in two unrelated studies. Immunological effects of CT showed that mDP of 6.5 and 9.1 had the greatest effect on the activation of porcine γδ T-cells compared to CT of lower mDP values . Similarly, the highest inhibition of Hepatitis C virus RNA expression was observed with mDP 7.7 in tests that had explored mDP values from ~1 to 14 . It remains to be seen what the mechanisms are behind this tannin threshold and whether this applies more widely across different biological systems. This type of information may also prove helpful for breeding new plant varieties with highly active CT as nutraceuticals .