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Table 1 Tannin-PRP binding specificity

From: Salivary proline-rich protein may reduce tannin-iron chelation: a systematic narrative review

Reference Method Tannin type PAC-TA comparison? Conditions of assay Outcome Mechanism agreement
[48] NMR B2, PGG, TGG, PAC monomer, epicatechin yes 40 mM B2, other assays 50 mM; either 0.5 ml of 2 mM or 4 mM PRP from mouse; pH 3.8 N-terminal proline residues linked to amide and amino structures bind tannin, then secondary interactions with galloyl groups changes structure of open conformation around PRP (specific binding) Yes
[49] NMR PAC as B1, B3, C2 No 0.5-20 mM PRP (IB9) with 15.7 mM tannin; pH 3.5 Tannin-PRP binding is specific to a certain concentration of tannin; then becomes random Yes
[50] ESI-MS EgCG, ECG, B2, B2 3-O gallate, reserpine No 1:10 ratio protein: polyphenol; pH 3.2 Tannin-PRP binding is specific; PRP-reserpine did not bind (similar structure to studied tannins) Yes
[51] ESI-MS; DLS, SAXS EgCG No 0.336 mM (1–3.5 mg/ml) PRP (IB5); 2:1 protein: polyphenol; pH 5.5 Tannin-PRP interaction is specific and dependent on tannin interactions; PRP sites for tannin binding are independent and have free energy; at a threshold, multidendate tannin crosslinks strengthen tannin-PRP bonds Yes
[43] DLS, ITC EgCG No 6.4 or 12.8 EgCG with 0.25–2 mg IB5; pH 3.5 Tannin-PRP interaction is concentration dependent; there is slow and specific binding of tannins followed by rapid and non-specific aggregation as tannin-PRP binding sites are saturated. Yes
[52] In vitro digestion, SDS-PAGE, HPLC EgCG No 0.05–0.5 mM EgCG, Human salivary PRPs; protein: tannin ratio 3:1; pH gastric 2.07; duodenal pH 7.8 Preferential tannin-PRP binding compared to lipase, alpha amylase, chymotrypsin, trypsin, lactase Yes
[86] DLS, ITC EgCG No 1:5 ratio saliva: wine in 1% TFA compared to physiological conditions Salivary PRP ‘moderately’ bound tannins Yes
[47] NMR, DLS EgCG, EGC, PGG Yes 20 mM polyphenol with 2 mM mouse PRP; pH 3.8 There is preferential binding of tannin to proline residues of PRPs vs. alternative amino acids Yes
  1. NMR Nuclear magnetic resonance imaging, ESI-MS electrospray ionization mass spectrometry, DLS dynamic light scattering, SAXS small angle X-ray scattering, ITC isothermal titration calorimetry, SDS-PAGE sodium dodecyl polyacrylamide gel electrophoresis, HPLC high performance liquid chromatography. B1, B2, B3: proanthocyanidin B1, B2, B3, PGG pentagalloylglucose, TGG tetragalloylglucose, PAC proanthocyanidin, EgCG epigallocatechin gallate, ECG epigallocatechin, PRP proline-rich protein, TA tannic acid