Tannin-protein Interactions Research Articles

Alcohol-Induced Conformation Changes and Thermodynamic Signatures in the Binding of Polyphenols to Proline-Rich Salivary Proteins.

The first contact of polyphenols (tannins) with the human body occurs in the mouth, where they are known to interact with proline-rich proteins (PRPs). These interactions are important at a sensory level, especially for the development of astringency, but affect also various other biochemical processes. Employing thermodynamic measurements, fluorescence and CD spectroscopy, we investigated the binding process of the prototypical polyphenol ellagic acid (EA) to different IB-PRPs and BSA, also in the presence of ethanol, which is known to influence tannin-protein interactions. Binding of EA to BSA and the small peptide IB7-14 is weak, but very strong to IB9-37. The differences in binding strength and stoichiometry are due to differences in the binding motifs, which also lead to differences in the thermodynamic signatures of the binding process. EA binding to BSA is enthalpy-driven, whereas binding to both IB7-14 and IB9-37 is entropy-driven. The presence of 10 vol.% EtOH, as present in wines, increases the binding constant of EA with BSA and IB7-14 drastically, but not that with IB9-37; however, it changes the binding stoichiometry. These differences can be attributed to the effect of EtOH on the conformation dynamics of the proteins and to changes in hydration properties in alcoholic solution.

Arabinogalactan proteins and polysaccharides compete directly with condensed tannins for saliva proteins influencing astringency perception of Cabernet Sauvignon wines

Wine astringency is thought to be due to salivary protein precipitation; however, the actual mechanism is not well-defined. This study aimed understand the relationship between whole polysaccharide extracts, produced with and without enzyme maceration, and the saliva protein-tannin precipitation reaction. Polysaccharides were analyzed in the context of salivary protein–tannin interactions using gel electrophoresis, quantitative 1H proton nuclear magnetic resonance (qHNMR), size separation chromatography, immunochemistry, and sensory analysis. Polysaccharide addition reduced saliva protein concentration in tannin-saliva protein-polysaccharide mixtures, indicating that native-wine polysaccharides compete with condensed tannins for salivary protein as ligand partners. qHNMR showed that tannin levels were increased by adding polysaccharides, suggesting that in these conditions, polysaccharides interact with saliva proteins via competitive protein-polysaccharide complex formation. Polysaccharides from non-enzyme-treated wines had threshold concentration of 121 mg/mL versus 86 mg/ml for enzyme-treated as detected by a sensory panel. Enzyme-treated polysaccharides changed astringency perception at a lower concentration than non-enzyme-treated polysaccharides.

Relationship between salivary lubrication and temporal sensory profiles of wine mouthfeel and astringency sub-qualities

Wine mouthfeel plays an important role in sensory quality. Astringency, as an important aspect of mouthfeel, is composed of distinct sub-qualities associated with tannin-salivary colloidal interactions and loss of oral lubrication. Perception of wine mouthfeel is temporal – the sensory intensity of wine mouthfeel attributes changes over the oral process of evaluation. It remains unclear whether these changes in sensory are relevant to distinct physical processes. This study took sensory-instrumental and multivariate approaches to provide mechanistic insights into the wine mouthfeel during the temporal evaluation process. The sample set was created by doctoring ethanol, tannin, pH and mannoprotein in a Malbec. Wines were sensorily evaluated by a progressive profiling method and instrumentally tested using a soft-tribology protocol involving saliva. Results indicate that the scores of mouthfeel attributes are dependent on the interactive effect of matrix components. The decaying rate of the score for roughness after expectoration correlates with those for drying and pucker. The multivariate analyses indicate that tannin-protein interaction and high boundary friction in tribology are important in explaining the perception of all astringency sub-qualities. At the end of the evaluation, the boundary friction becomes less important in explaining the score of drying/roughness, while pH becomes less important in explaining the score of pucker. It is inferred that, after expectoration, saliva secretion recovers the oral lubrication; astringency sub-qualities are contributed less from the frictional mechanism but more from an alternative perceptive mechanism. The outcome could assist the wine industry in better managing the wine matrix to attain preferable mouthfeel characteristics.

Enzymatic modification of oat globulin enables covalent interaction with procyanidin B2

The effect of enzyme treatment on protein–tannin interactions was investigated using up-to-date analytical approaches for improving their physical properties. The formation of ligands between procyanidin B2 and native oat globulin (OG) was observed to be affected by the ratio of procyanidin B2 to OG and the availability of tryptophan. For the transglutaminase-treated OG, the results obtained from circular dichroism (CD) and size exclusion chromatography (SEC) revealed that procyanidin B2 acted as an acyl acceptor in the process of OG deamidation. Procyanidin B2 also inhibited the non-covalent protein–protein interactions occurring between the aromatic side-chains or sedimentation of tryptophan aggregates. For trypsin-treated OG, procyanidin B2 interacted with phenylalanine and the tryptophan side-chain of OG. The inhibition of procyanidin B2 towards protein–protein aggregation was proved by the observation of CD, SEC and asymmetric flow field-flow fractionation.

Inhibition of LPMOs by Fermented Persimmon Juice.

Fermented persimmon juice, Kakishibu, has traditionally been used for wood and paper protection. This protective effect stems at least partially from inhibition of microbial cellulose degrading enzymes. The inhibitory effect of Kakishibu on lytic polysaccharide monooxygenases (LPMOs) and on a cocktail of cellulose hydrolases was studied, using three different cellulosic substrates. Dose dependent inhibition of LPMO activity by a commercial Kakishibu product was assessed for the well-characterized LPMO from Thermoascus aurantiacus TaAA9A, and the inhibitory effect was confirmed on five additional microbial LPMOs. The model tannin compound, tannic acid exhibited a similar inhibitory effect on TaAA9A as Kakishibu. It was further shown that both polyethylene glycol and tannase can alleviate the inhibitory effect of Kakishibu and tannic acid, indicating a likely mechanism of inhibition caused by unspecific tannin–protein interactions.

Wine astringency: more than just tannin-protein interactions.

Red wines are characterized by their astringency, a very important sensory attribute that affects the perceived quality of wines. Three mechanisms have been proposed to explain astringency, and two theories describe how these mechanisms work in an integrated manner to produce tactile sensations such as drying, roughening, shrinking and puckering. The factors involved include not only tannins and salivary proteins, but also anthocyanins, grape polysaccharides and mannoproteins, as well as other wine matrix components that modulate their interactions. These multifactorial interactions could be responsible for different sensory responses and therefore need to be further studied. This review presents the latest advances in astringency perception and its possible origins, with special attention on the interactions of components, their impact on oral perception and the development of astringency sub-qualities. Future research efforts should concentrate on understanding the mechanisms involved as well as on the limiting factors related to the conformation and stability of the tannin-salivary protein complexes. © 2021 Society of Chemical Industry.

Effect of the nixtamalization process on the protein bioaccessibility of white and red sorghum flours during in vitro gastrointestinal digestion

Protein bioaccessibility is a major concern in sorghum (Sorghum bicolor L. Moench) due to potential interactions with tannins affecting its nutritional value. Technological treatments such as boiling or alkaline cooking have been proposed to address this problem by reducing tannin-protein interactions. This research aimed to evaluate the impact of nixtamalization in the protein bioaccessibility from two sorghum varieties (red and white sorghum) during in vitro gastrointestinal digestion. Nixtamalization increased protein bioaccessibility in the non-digestible fraction (NDF) (5.26 and 26.31% for red and white sorghum, respectively). However, cooking showed a higher permeation speed of protein from red sorghum flours at the end of the intestinal incubation (9.42%). The SDS-PAGE profile of the digested fraction (DF) at 90 min of intestinal incubation indicated that, for red sorghum, cooking allows the formation of α and γ-kafirins while nixtamalization increase α-kafirin release. Principal Components Analysis (PCA) showed the association between nixtamalization and dissociation of δα kafirin complexes and increased protein content in the digestible fraction. In silico interactions indicated the highest biding energies for (+)-catechin and kafirin fractions (β-kafirin: −7.0 kcal/mol; γ-kafirin: −5.8 kcal/mol, and δ-kafirin: −6.8 kcal/mol), suggesting a minor influence of depolymerized proanthocyanidin fractions with sorghum proteins as a result of the nixtamalization process. In conclusion, nixtamalization increased the bioaccessibility of sorghum proteins, depolymerizing condensed tannins, and breaking protein-tannin complexes. Such technological process improves the nutrimental value of sorghum, supporting its inclusion in the human diet.

Rheological study of tannin and protein interactions based on model systems.

The interaction between wine tannins and saliva proteins is responsible for wine astringency perception, producing a depletion of salivary proteins and changes on oral friction. In sensorial terms, astringency is described as a dryness and puckering sensation in the mouth, which is related to the "structure" or "body" of red wines. However, these last descriptors, as structure or body, are perceived during wine tasting and commonly related to wine viscosity. To address these differences on sensory response, we hypothesize that tannin-protein interactions could be a key factor involved in the viscosity of red wines/saliva mixtures, just as they are for astringency. We used a rheological method to study the impact of tannin-protein interaction on the viscosity of model wine-saliva systems. Mixtures of model saliva based on mucin and typical astringent compounds, as commercial tannins and gallic acid, were evaluated for their rheological behavior. The viscometric flow of the fluid mixtures was determined, and subsequently, the viscosity was evaluated at a shear rate of 60 s-1 . It was observed that red wines/saliva mixtures exhibit non-Newtonian flow and ascending tannin doses led to an increase in the apparent viscosity. Nephelometric analysis demonstrate that tannin-mucin aggregates were formed, which suggests that these complexes were potentially responsible for the viscosity increases, modifying the rheological behavior of these mixtures. Results from this work propose that tannin-protein interactions are also involved in the underlying mechanism of thickness perception of red wines and rheology could be a complementary instrumental technique for wine mouthfeel characterization.

Development of a direct ESI-MS method for measuring the tannin precipitation effect of proline-rich peptides and in silico studies on the proline role in tannin-protein interactions

Tannins are a heterogeneous class of polyphenols that are present in several plants and foods. Their ability to interact and precipitate proline-rich proteins leads to different effects such as astringency or antidiarrheal activity. Thus, evaluation of the tannin content in plant extracts plays a key role in understanding their potential use as pharmaceuticals and nutraceuticals. Several methods have been proposed to study tannin-protein interactions but few of them are focused on quantification. The purpose of the present work is to set up a suitable and time efficient method able to quantify the extent of tannin protein precipitation. Bradykinin, chosen as a model, was incubated with increasing concentrations of 1,2,3,4,6-penta-O-galloyl-β-d-glucose and tannic acid selected as reference of tannic compounds. Bradykinin not precipitated was determined by a mass spectrometer TSQ Quantum Ultra Triple Quadrupole (direct infusion analysis). The results were expressed as PC50, which is the concentration able to precipitate 50% of the protein. The type of tannin-protein interaction was evaluated also after precipitate solubilisation. The involvement of proline residues in tannin-protein interactions was confirmed by repeating the experiment using a synthesized peptide (RR-9) characterized by the same bradykinin sequence, but having proline residues replaced by glycine residues: no interaction occurred between the peptide and the tannins. Moreover, modelling studies on PGG-BK and PGG-RR-9 were performed to deeply investigate the involvement of prolines: a balance of hydrophobic and H-bond contacts stabilizes the PGG-BK cluster and the proline residues exert a crucial role thus allowing the PGG molecules to elicit a sticking effect.

Influence of valoneoyl groups on the interactions between Euphorbia tannins and human serum albumin

Tannins belonging to plant polyphenols, are large group of compounds with diverse biological activity. Many of them are being studied as potential natural medicine due to their antioxidant, antiviral, antibacterial or anticancer properties. However, so far little is known about the structural and functional relations in protein-tannin interactions, in particularly the role of valoneoyl groups in tannin structure. In this study we first investigated the mechanisms of interaction 1,2-di-O-galloyl-4,6-valoneoyl-β-D-glucose (Tannin 1), 2-O-galloyl-4,6-valoneoyl-β-D-glucose (Tannin 2), 3-O-galloyl-1,2-valoneoyl-β-D-glucose (Tannin 3) isolated from Euphorbia plants with human serum albumin (HSA). To get more detailed information about nature of albumin-tannin interactions besides standard Trp fluorescence quenching analysis we used also transmission electron microscopy (TEM), circular dichroism (CD) and fluorescence labelling (ANS dye) techniques. It was shown that all the tannins strongly interacted with HSA and quenched the tryptophan amino acid (Trp) fluorescence but slightly changed protein secondary structure (circular dichroism CD analysis). TEM demonstrated that all used compounds formed complexes with HSA. Tannin 3 most strongly quenched HSA fluorescence and changed protein dynamic as well as had the highest binding constant (12.4 ± 1.1 × 1013M−1s−1 in comparison with 7.0 ± 0.38 × 1013M−1s−1 for tannin 1 and 8.6 ± 1.1 × 1013M−1s−1 for tannin 2). For tannin 1 that was the largest of the studied compounds was observed the weakest influence on the fluorescence parameters, probably due to the effect of steric hindrance limiting interaction with albumin Trp pocket. On the other hand T1 induced the strongest changed secondary structure of HSA in comparison to another studied tannins.Our results demonstrated that all used tannins interact with albumin by complex formation but in the manner depends on chemical structure and flexibility of studied compounds.

Effect of a commercial tannin on the sensorial temporality of astringency

Astringency is a tactile sensation that is generated by a reduction in lubrication in the oral cavity and is generally attributed to the interaction of procyanidins or condensed tannins with salivary proteins. Several factors influence tannin-protein interactions, such as pH, alcohol, sweetness, oxygen and polyphenol content. A scarcely studied factor is the effect of the tannin content on the perception of astringency. The objective of this study was to evaluate the effect of different concentrations of commercial oenological tannin (COT) on the timing of the perception of astringency. For this model, a vinous solution enriched with three concentrations of COT was used. The samples were subjected to a storage period of three months. Additionally, a panel was trained in the perception of astringency in red wines using a method of temporal dominance of sensations (TDS). Astringency descriptors were selected, and the TDS method was used to characterize the astringency. The samples were evaluated using traditional descriptors in TDS and astringency descriptors in TDS.In traditional TDS curves, treatments with higher concentrations of COT showed a higher and more persistent dominance index in the descriptor astringency. Moreover, the stimulus duration variable increased as the COT level increased. Likewise, temporary astringency was dominant over alcohol. For astringency TDS, at low concentrations of COT, the soft and adhesive descriptors were dominant, whereas at high COT concentrations, aggressive and drying were perceived as the dominant descriptors. An increasing concentration of tannin in the vinous solution generated an increased duration and dominance of astringency and reduced the duration of the sensation of alcohol. Finally, the type of perceived astringency was closely related to the tannin concentration.

Interaction of α-synuclein with Rhus typhina tannin – Implication for Parkinson’s disease

The etiology of Parkinson’s disease (PD) relates to α-synuclein, a small protein with the ability to aggregate and form Lewy bodies. One of its prevention strategies is inhibition of α-synuclein oligomerization. We have investigated the interaction of α-synuclein and human serum albumin with 3,6-bis-О-di-О-galloyl-1,2,4-tri-О-galloyl-β-d-glucose (a tannin isolated from the plant Rhus typhina). Using fluorescence spectroscopy method we found that this tannin interacts strongly with α-synuclein forming complexes. Circular dichroism analysis showed a time-dependent inhibition of α-synuclein aggregation in the presence of the tannin. On the other hand, 3,6-bis-О-di-О-galloyl-1,2,4-tri-О-galloyl-β-d-glucose had a much stronger interaction with human serum albumin than α-synuclein. The calculated binding constant for tannin-protein interaction was considerably higher for albumin than α-synuclein. This tannin interacted with albumin through a “sphere of action” mechanism. The results lead to the conclusion that 3,6-bis-О-di-О-galloyl-1,2,4-tri-О-galloyl-β-d-glucose is a potent preventive compound against Parkinson’s disease. However, this tannin interacts very strongly with human serum albumin, significantly reducing the bioavailability of this compound.

Replacing alfalfa silage with tannin-containing birdsfoot trefoil silage in total mixed rations for lactating dairy cows

Two lactation trials were conducted comparing the feeding value of silages made from birdsfoot trefoil (BFT, Lotus corniculatus L.) that had been selected for low (BFTL), medium (BFTM), and high (BFTH) levels of condensed tannins (CT) to an alfalfa silage (AS) when fed as the principal forage in total mixed rations. Diets also included corn silage, high-moisture shelled corn, soybean meal, soy hulls, and supplemental fat. In trial 1, 32 lactating Holstein cows were blocked by days in milk, assigned to treatment sequences in 8 balanced 4 × 4 Latin squares, and fed 50% dietary dry matter from AS or 1 of 3 BFT silages containing 0.6, 1.2, or 1.7% CT. Diets averaged 17.5 to 19.5% crude protein and 26% neutral detergent fiber on a dry matter basis. Data were collected over the last 2 wk of each 4-wk period. Intakes were 1.3 to 2.8 kg of dry matter/d greater on BFT than on AS and cows gained 0.5 kg of body weight/d on BFT diets while losing 0.14 kg of body weight/d on the AS diet; this resulted in greater milk per dry matter intake (DMI) on AS. Linear effects indicated true protein yield and milk urea nitrogen declined with increasing CT concentration and quadratic effects indicated DMI, energy-corrected milk, and fat yield were increased at intermediate CT concentration. True protein yield and apparent N-efficiency were greater, and milk urea nitrogen lower, on all BFT diets than on AS. In trial 2, 50 lactating Holstein cows were fed a covariate AS diet for 2 wk and then blocked by parity and days in milk and randomly assigned to 1 of 5 diets that were fed continuously for 12 wk. Diets contained (dry matter basis) 48% AS, 16% AS plus 32% of 1 of 3 BFT silages with 0.5, 0.8, or 1.5% CT, or 48% of an equal mixture of each BFT silage. Diets averaged 16.5% crude protein and 30% neutral detergent fiber. Intake and milk yield tended to be lower on AS than BFT, but body weight gains averaged 0.6 kg/d on all diets. Cows fed any of the BFT silages had reduced milk urea nitrogen and ruminal ammonia and reduced urinary N excretion. Feeding the BFT mixture reduced concentrations of milk true protein and milk urea nitrogen and depressed apparent nutrient digestibility. Among diets containing the individual BFT silages, linear reductions in DMI and yield of milk, fat, true protein, lactose, and SNF were observed with increasing CT concentration. By contrast, a previous trial with the same BFT populations showed that substituting BFTH silage containing 1.6% CT for AS in rations containing 60% silage dry matter had no effect on intake, increased yield of milk, energy-corrected milk and milk components, elevated protein use-efficiency, but with a more modest reduction in milk urea nitrogen and urinary N excretion. Silage analyses suggested that the inconsistent responses among trials were related to growth environment or ensiling effects that altered tannin-protein interactions in BFT silage. Differences in diet formulation among trials may have also influenced responses. Results from the current and previous trials indicate further work is needed to identify optimum tannin levels in forages.

Identification of Structural Features of Condensed Tannins That Affect Protein Aggregation.

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.

Protein-Tannin Interactions of Tryptic Digests of α-Lactalbumin and Procyanidins.

Protein-tannin interactions on a molecular level were investigated by using a model system containing peptides of α-lactalbumin and berry tannins (procyanidins). Oxidation of isolated tryptic peptide LDQWLCEK (m/z 1034) with procyanidin B2 or procyanidin fraction (PF) isolated from aronia juice was monitored by LC-ESI-MS. Procyanidin B2 and PF showed radical scavenging activities toward oxidation of the peptide with the peptide also preventing procyanidin B2 from degradation. Oxidation enhanced the cleavage of peptide between tryptophan and glutamine. Interaction products arising from WLCEK or WLCE residue and degradation product of procyanidin B2 were also identified using both size exclusion chromatography and LC-MS. Tryptophan and lysine were the amino acids most prone to interact with procyanidin B2. The study shows that protein-tannin interaction takes place during oxidation leading to both degradation of the parent compounds and formation of interaction products. This may in turn affect the quality of protein and tannin containing food.

Astringency reduction in red wine by whey proteins

Whey is a by-product of cheese manufacturing and therefore investigating new applications of whey proteins will contribute towards the valorisation of whey and hence waste reduction. This study shows for the first time a detailed comparison of the effectiveness of gelatin and β-lactoglobulin (β-LG) as fining agents. Gelatin was more reactive than whey proteins to tannic acid as shown by both the astringency method (with ovalbumin as a precipitant) and the tannins determination method (with methylcellulose as a precipitant). The two proteins showed similar selectivity for polyphenols but β-LG did not remove as much catechin. The fining agent was removed completely or to a trace level after centrifugation followed by filtration which minimises its potential allergenicity. In addition, improved understanding of protein–tannin interactions was obtained by fluorescence, size measurement and isothermal titration calorimetry (ITC). Overall this study demonstrates that whey proteins have the potential of reducing astringency in red wine and can find a place in enology.

Do goats have a salivary constitutive response to tannins?

ABSTRACTThe effect of tannin-rich fodder (TRF) consumption on the saliva response of Criollo goats without previous browsing experience was determined. Eighteen kids were allocated into three treatments (n = 6 each): control group (CG), short-term tannin stimulus (TS) and long-term tannin stimulus (TL). Three experimental periods were used: adaptation (two weeks) in which the three treatment groups were fed Pennisetum purpureum grass and supplemented with a balanced feed. In period 1 (five weeks), TS and TL treatments were fed TRF (Lysiloma latisiliquum) with 55.5 g/kg DM condensed tannins (CT), grass and balanced feed. In period 2 (three weeks), L. latisiliquum fodder was withdrawn from the TS group. Thus, both the TS and the CG kids were only offered grass and balanced feed, while TL kids continued to receive TRF. In each period, saliva samples were collected to measure the tannin–protein interaction, salivary protein and protein turbidity index (PTI). The salivary protein was similar in all treatments and periods (P > .10). The goats' saliva reacted similarly when mixed with either tannic acid or L. latisiliquum water-acetone extract. Although the PTI tended to increase in the TL group compared to CG and TS with time, such difference was not significant. Thus, TRF intake failed to further increase the salivary response or PTI. Therefore, the saliva of goats from Yucatan, without previous browsing experience do have a constitutive response as it can block tannins irrespective of TRF stimulus.

The complexity of condensed tannin binding to bovine serum albumin – An isothermal titration calorimetry study

Isothermal titration calorimetry was applied to study the binding of purified proanthocyanidin oligomers to bovine serum albumin (BSA). The molecular weight of the proanthocyanidin oligomer had a major impact on its binding to BSA. The calculated change in enthalpy (ΔH) and association constant (Ka) became greater as the oligomer size increased then plateaued at the heptameric oligomer. These results support a model for precipitation of proteins by proanthocyanidin where increased oligomer size enhanced the opportunity for cross linkages between proteins ultimately forming sediment-able complexes. The authors suggest tannin binding to proteins is opportunistic and involves multiple sites, each with a different Ka and ΔH of binding. The ΔH of binding comprises both an endothermic hydrophobic interaction and exothermic hydrogen bond component. This suggests the calculated entropy value (ΔS) for tannin–protein interactions is subject to a systematic error and should be interpreted with caution.

Ethanol Concentration Influences the Mechanisms of Wine Tannin Interactions with Poly(L-proline) in Model Wine.

Changes in ethanol concentration influence red wine astringency, and yet the effect of ethanol on wine tannin-salivary protein interactions is not well understood. Isothermal titration calorimetry (ITC) was used to measure the binding strength between the model salivary protein, poly(L-proline) (PLP) and a range of wine tannins (tannin fractions from a 3- and a 7-year old Cabernet Sauvignon wine) across different ethanol concentrations (5, 10, 15, and 40% v/v). Tannin-PLP interactions were stronger at 5% ethanol than at 40% ethanol. The mechanism of interaction changed for most tannin samples across the wine-like ethanol range (10-15%) from a combination of hydrophobic and hydrogen binding at 10% ethanol to only hydrogen binding at 15% ethanol. These results indicate that ethanol concentration can influence the mechanisms of wine tannin-protein interactions and that the previously reported decrease in wine astringency with increasing alcohol may, in part, relate to a decrease tannin-protein interaction strength.

Comparison of microcalorimetry and haze formation to quantify the association of B-type procyanidins to poly-l-proline and bovine serum albumin

Though many different methods have been applied to protein-tannin interactions, divergent results are often reported. To better understand the origin of these differences, we compare here haze/aggregates formation and thermodynamic parameters occurring for protein-procyanidin interactions. Proteins well referenced for interaction with polyphenols, namely a polypeptide of extended structure that resembles salivary proteins (poly-l-proline (PLP) and a standard globular protein (bovine serum albumin (BSA)) were used. Flavan-3-ols of three degrees of polymerization were tested, (-)-epicatechin (EPI), a dimer (DP2) and a procyanidin (DP8). The interactions were tested in identical conditions for both methods. The association constants determined by isothermal titration calorimetry (expressed as constitutive unit) varied with the following scale: PLP-DP8 >> BSA-DP8 ≈ BSA-DP2 ≈ PLP-DP2 > BSA-EPI (no affinity detected for PLP-EPI). However aggregates formed more readily for highly polymerized procyanidins DP8 with BSA than with PLP, and the scale (with 10 mmol/L of constitutive unit of polyphenol and 0.07 mmol/L of proteins) was: BSA-DP8 > BSA-DP2 >> PLP-DP8 > PLP-DP2 ≈ PLP-EPI, with no haze formation for BSA-EPI. However the impact of polyphenol concentration on haze formation and precipitation was different for the two proteins.