Interaction between Wine Phenolic Acids and Salivary Proteins by Saturation-Transfer Difference Nuclear Magnetic Resonance Spectroscopy (STD-NMR) and Molecular Dynamics Simulations

A look upon the adsorption of different astringent agents to oral models: Understanding the contribution of alternative mechanisms in astringency

Salivary proteins precipitation by interaction with polyphenols is the major mechanism for astringency. However, alternative mechanisms seem involved in the perception of different subqualities of astringency. In this study, adsorption of four astringent agents to in vitro oral models and their sensory properties were assessed. Overall, green tea infusion and tannic acid have shown a higher adsorption potential for models with oral cells and absence of saliva. Alum and grape seed extract presented higher adsorption in models with presence of oral cells and saliva. Multiple factor analysis suggested that adsorption may represent important mechanisms to elicit the astringency of alum. Models including saliva, were closely associated with overall astringency and aggressive subquality. Models with cells and absent saliva were closely associated with greenness, suggesting a taste receptor mechanism involvement in the perception. For the first time a correlation between an oral-cell based assay and astringency sensory perception was shown.

Supramolecular Study of the Interactions between Malvidin-3-O-Glucoside and Wine Phenolic Compounds: Influence on Color

Supramolecular study of the interactions between the major wine anthocyanin, malvidin-3-O-glucoside (Mv3G) and different wine phenolic compounds (quercetin 3-O-β-glucopyranoside (QG), caffeic acid, (-)-epicatechin, (+)-catechin, and gallic acid) has been performed at two different molar ratios (1:1 and 1:2) in acidic medium where flavylium cation predominates (pH ≤ 2). Color variations have been evaluated by differential colorimetry using CIELAB color space. These studies have been complemented with isothermal titration calorimetry assays and molecular dynamics simulations. The color of Mv3G flavylium cation is modified by the interaction with QG toward more bluish and intense colors. Interaction constants between the anthocyanin and the different phenolic compounds were obtained, ranging from 9.72 × 108 M-1 for QG to 1.50 × 102 M-1 for catechin. Hydrophobic interactions and H-bonds are the main driving forces in the pigment/copigment aggregation, except for the interactions where caffeic acid is involved, in which hydrophobic interactions acquire greater preponderance.

Review on NMR as a tool to analyse natural products extract directly: Molecular structure elucidation and biological activity analysis

INTRODUCTION

Natural products, the small organic molecules produced by plants, microbes and invertebrates, often present in the form of a mixture, this leads to the structural characterisation of natural extracts often requiring time-consuming multistep purification procedures. Nuclear magnetic resonance (NMR) technology is traditionally utilised as a tool for the structural elucidation of pure compounds. Contemporarily, an up-to-date trend in the application of NMR in natural product research is shifting to the direct NMR analysis of crude mixtures, to obtain molecular structure and biological activity information without performing cumbersome separation.

OBJECTIVE

To review works of literature on the evolution, principle and progress of NMR technologies for analysing mixtures, we focus on the successful application of NMR technologies in direct analysis of natural product extracts.

METHODOLOGY

Based on our research experience, academic tracking and extensive literature search, which involved, but not limited to, the use of various databases, like Web of Knowledge and PubMed. The keywords used, in various combinations, to retrieve recent literature on the successful application of NMR technologies to sheer natural product extracts, and excluded artificially natural product mixture and biofluid.

RESULTS

NMR technologies for direct natural extracts analysis, including two-dimensional J-resolved spectroscopy (2D-JRES), pure shift NMR, diffusion-ordered NMR spectroscopy (DOSY), statistical correlation spectroscopy (STOCSY), concentration-ordered NMR spectroscopy (CORDY), saturation transfer difference (STD) and water-ligand observed via gradient spectroscopy (WaterLOGSY) were illustrated.

CONCLUSIONS

By these methods, molecular structure and biological activity information will be directly obtained from NMR analysis of natural products extract, aiming to save experimental time and expenses.

Investigating the Role of Odorant-Polymer Interactions in the Aroma Perception of Red Wine: A Density Functional Theory-Based Approach

The aroma of red wine results from the intricate interplay between aroma compounds (odorants) and complex polymers generated during fermentation. This study combines density functional theory (DFT), human sensory experiments, and nuclear magnetic resonance to investigate the impact of odorant-polymer interactions on wine aroma. Molecular aggregation patterns of odorants with polymer segments are identified, indicating the crucial role of intermolecular noncovalent interactions, such as hydrogen bonds and van der Waals interactions, in stabilizing odorant-polymer conformations. Certain odorants, including 3-isobutyl-2-methoxypyrazine and cis-whisky lactone, exhibit high binding affinity to specific polymer segments, such as (+)-catechin and p-coumaric acid, resulting in substantial changes in the perceived aroma. Their strong binding affinities correlate with changes in sensory experiments for binary mixtures. The results provide insights into the molecular mechanisms of odorant-polymer interactions in red wine with the potential of DFT calculations as a tool for predicting and tailoring red wine aroma.

Paper chromatography approach for the assessment of interaction between red wine and whole saliva

In-mouth interaction of red wine compounds with salivary proteins is a primary event allegedly responsible for eliciting the mouth-feel sensation of astringency. Those interactions have been currently associated with precipitation of salivary protein/polyphenol complexes. However, such single physicochemical evidence for interaction does not account for the complexity of astringency. This study aimed to develop a paper chromatography method to assess interactions between red wine and the salivary protein fraction using stepwise series of red wine/saliva binary mixtures from 100% wine to 100% saliva ("Alpha and Omega series"). Aliquots of each one of the mixtures were spotted on a cellulose membrane to scrutinize independently the distribution areas of wine components (naturally pink-colored) and salivary protein (stained blue in Coomassie Brilliant R-250). This double target detection revealed interactions between saliva and red wine components along most of the quantitative Alpha and Omega series, a point of equivalence corresponding to maximum interactivity for both complex reactants and a non-diffusible sub-fraction of saliva displaying the highest interactivity. The results indicate a novel way to assess quantitatively physicochemical interactions between red wines and human saliva but also provide new lights to approach the identification of molecular salivary structures involved in triggering astringency.

The perception and influencing factors of astringency, and health-promoting effects associated with phytochemicals: A comprehensive review

Astringency as the complex sensory of drying or shrinking can be perceived from natural foods, including abundant phenolic compounds. Up to now, there have been two possible astringency perception mechanisms of phenolic compounds. The first possible mechanism involved chemosensors and mechanosensors and took salivary binding proteins as the premise. Although piecemeal reports about chemosensors, friction mechanosensor's perception mechanisms were absent. There might be another perception way because a part of astringent phenolic compounds also triggered astringency although they could not bind with salivary proteins, however, the specific mechanism was unclear. Structures caused the differences in astringency perception mechanisms and intensities. Except for structures, other influencing factors also changed astringency perception intensity and aimed to decrease it, which probably ignored the health-promoting effects of phenolic compounds. Therefore, we roundly summarized the chemosensor's perception processes of the first mechanism. Meanwhile, we speculated that friction mechanosensor's probably activated Piezo2 ion channel on cell membranes. Phenolic compounds directly binds with oral epithelial cells, activating Piezo2 ion channel probably the another astringency perception mechanism. Except for structure, the increase of pH values, ethanol concentrations, and viscosity not only lowered astringency perception but were beneficial to improve the bioaccessibility and bioavailability of astringent phenolic compounds, which contributed to stronger antioxidant, anti-inflammatory, antiaging and anticancer effects.

Mouthfeel subqualities in wines: A current insight on sensory descriptors and physical-chemical markers

Astringency and more generally mouthfeel perception are relevant to the overall quality of the wine. However, their origin and description are still uncertain and are constantly updating. Additionally, the terminology related to mouthfeel properties is expansive and extremely diversified, characterized by common traditional terms as well as novel recently adopted descriptors. In this context, this review evaluated the mention frequency of astringent subqualities and other mouthfeel attributes in the scientific literature of the last decades (2000-August 17, 2022). One hundred and twenty-five scientific publications have been selected and classified based on wine typology, aim, and instrumental-sensorial methods adopted. Dry resulted as the most frequent astringent subquality (10% for red wines, 8.6% for white wines), while body-and related terms-is a common mouthfeel sensation for different wine types, although its concept is still vague. Alongside, promising analytical and instrumental techniques investigating and simulating the in-mouth properties are discussed in detail, such as rheology for the viscosity and tribology for the lubrication loss, as well as the different approaches for the quantitative and qualitative evaluation of the interaction between salivary proteins and astringency markers. A focus on the phenolic compounds involved in the tactile perception was conducted, with tannins being the compounds conventionally found responsible for astringency. Nevertheless, other non-tannic polyphenolic classes (i.e., flavonols, phenolic acids, anthocyanins, anthocyanin-derivative pigments) as well as chemical-physical factors and the wine matrix (i.e., polysaccharides, mannoproteins, ethanol, glycerol, and pH) can also contribute to the wine in-mouth sensory profile. An overview of mouthfeel perception, factors involved, and its vocabulary is useful for enologists and consumers.

Molecular Recognition Patterns between Vitamin B12 and Proteins Explored through STD-NMR and In Silico Studies

Ligand-receptor molecular recognition is the basis of biological processes. The Saturation Transfer Difference-NMR (STD-NMR) technique has been recently used to gain qualitative and quantitative information about physiological interactions at an atomic resolution. The molecular recognition patterns between the cyanocobalamin (CNBL)/aqua cobalamin (OHBL) and different plant and animal proteins were investigated via STD-NMR supplemented by molecular docking. This study demonstrates that myoglobin has the highest binding affinity and that gluten has the lowest affinity. Casein also shows a higher binding affinity for cyanocobalamin when compared with that of plant-based proteins. STD-NMR results showed the moderate binding capability of casein with both CNBL and OHBL. Computer simulation confirmed the recognition mode in theory and was compared with the experiments. This work is beneficial for understanding the binding affinity and biological action of cyanocobalamin and will attract researchers to use NMR technology to link the chemical and physiological properties of nutrients.

Chemical aspects of polyphenol-protein interactions and their antibacterial activity

The hunt for novel antibiotics has become a global public health imperative due to the rise in multidrug-resistant microorganisms, untreatable infection cases, overuse, and inefficacy of modern antibiotics. Polyphenols are getting much attention in research due to their multiple biological effects; their use as antimicrobial agents is attributed to their activity and that microbes have a hard time developing resistance to these natural compounds. Polyphenols are secondary metabolites produced in higher plants. They are known to possess various functional properties in the human body. Polyphenols also exhibit antibacterial activities against foodborne pathogens. Their antibacterial mechanism is based on inhibiting bacterial biofilm formation or inactivating enzymes. This review focused on polyphenol-protein interactions and the creation of this complex as a possible antibacterial agent. Also, different phenolic interactions on bacterial proteins, efflux pump, cell membrane, bacterial adhesion, toxins, and other bacterial proteins will be explored; these interactions can work in a synergic combination with antibiotics or act alone to assure bacterial inhibition. Additionally, our review will focus on polyphenol-protein interaction as a possible strategy to eradicate bacteria because polyphenols have shown a robust enzyme-inhibitory characteristic and a high tendency to complex with proteins, a response that neutralizes any bactericidal potential.

Molecular basis of the formation and removal of fruit astringency

Astringency is a dry puckering mouthfeel mainly generated by the binding of tannins with proteins in the mouth. Tannins confer benefits such as resistance to biotic stresses and have antioxidant activity, and moderate concentrations of tannins can improve the flavor of fruits or their products. However, fruits with high contents of tannins have excessive astringency, which is undesirable. Thus, the balance of astringency formation and removal is extremely important for human consumption of fruit and fruit-based products. In recent years, the understanding of fruit astringency has moved beyond the biochemical aspects to focus on the genetic characterization of key structural genes and their transcriptional regulators that cause astringency. This article provides an overview of astringency formation and evaluation. We summarize the methods of astringency regulation and strategies and mechanisms for astringency removal, and discuss perspectives for future exploration and modulation of astringency for fruit quality improvement.

Interaction between salivary proteins and cork phenolic compounds able to migrate to wine model solutions

This work reports the study of the interaction of human salivary proteins (SP) with phenolic compounds that migrate from cork stoppers to wine. This study yields valuable data to understand the influence that these compounds may have on the sensory perception of wine from an astringency perspective. For that, three cork fractions containing the phenolic compounds that migrate in greater amounts from cork to model wine solutions were selected. Fraction M1 contains gallic acid, protocatechuic acid, vanillin and protocatechuic aldehyde; fraction M2 comprises essentially gallic acid and ellagic acid, as well as castalagin and dehydrocastalagin; and fraction M3 contains the two isomeric ellagitannins castalagin and vescalagin. The reactivity of each fraction towards SP was M3 > M2 > M1. Within M3 fraction, castalagin showed a higher ability to precipitate SP (mainly aPRPs, statherin and P-B peptide) comparatively to vescalagin. In M1 fraction, caffeic and sinapic acids were the compounds with the highest interaction with SP, mainly cystatins. In addition, there also seems to be a matrix effect (presence of other compounds) that could be affecting these interactions.

Sensorial Perception of Astringency: Oral Mechanisms and Current Analysis Methods

Understanding consumers' food choices and the psychological processes involved in their preferences is crucial to promote more mindful eating regulation and guide food design. Fortifying foods minimizing the oral dryness, rough, and puckering associated with many functional ingredients has been attracting interest in understanding oral astringency over the years. A variety of studies have explored the sensorial mechanisms and the food properties determining astringency perception. The present review provides a deeper understanding of astringency, a general view of the oral mechanisms involved, and the exciting variety of the latest methods used to direct and indirectly quantify and simulate the astringency perception and the specific mechanisms involved.

Chemical and sensorial investigation of in-mouth sensory properties of grape anthocyanins

Anthocyanins are water-soluble pigments found in the cell vacuoles of fruits and flowers, performing several roles from insects attraction to stress protection. Their antioxidant activity contributes to human health, and consuming plant-derived products provides their higher source in the diet. Although their colour and nutritional features, their contribution to sensory properties of foods has not been widely investigated. In wine, preferences are connected with sensory attributes of colour, aroma, taste, and mouthfeel. In this study, grape anthocyanin extracts (TAE) were fractionated using centrifugal partition chromatography (CPC) and preparative HPLC in three fractions, i.e glucoside (GF), acetylated (AF) and cinnamoylated (CF) anthocyanins. Sensory properties were investigated by chemical analysis, as reactivity towards bovin serum albumin (BSA) and salivary proteins, and in tasting sessions to assess anthocyanins best estimated thresholds (BET) in wine-like solution. Anthocyanins reacted with both BSA and salivary proteins, but to different extents, because higher interaction between salivary proteins and anthocyanins were found. Cinnamoylated anthocyanins are the most reactive to salivary proteins. Tasting sessions suggested an involvement of anthocyanins as in-mouth contributors in wine, since their BETs were 255, 297, 68, and 58 mg/L for TAE, GF, AF, and CF, respectively, and the descriptors reported were astringency and bitterness.

Interaction between cyanine dye IR-783 and polystyrene nanoparticles in solution

The interactions between small molecule drugs or dyes and nanoparticles are important to the use of nanoparticles in medicine. Noncovalent adsorption of dyes on nanoparticle surfaces is also important to the development of nanoparticle dual-use imaging contrast agents. In this work, solution-state NMR is used to examine the noncovalent interaction between a near-infrared cyanine dye and the surface of polystyrene nanoparticles in solution. Using 1D proton NMR, we can approximate the number of dye molecules that associate with each nanoparticle for different sized nanoparticles. Saturation-Transfer Difference NMR was also used to show that protons near the positively charged nitrogen in the dye are more strongly associated with the negatively charged nanoparticle surface than protons near the negatively charged sulfate groups of the dye. The methods described here can be used to study similar drug or dye molecules interacting with the surface of organic nanoparticles.

Interactions between wine phenolic compounds and human saliva in astringency perception

Astringency is a complex perceptual phenomenon involving several sensations that are perceived simultaneously. The mechanism leading to these sensations has been thoroughly and controversially discussed in the literature and it is still not well understood since there are many contributing factors. Although we are still far from elucidating the mechanisms whereby astringency develops, the interaction between phenolic compounds and proteins (from saliva, oral mucosa or cells) seems to be most important. This review summarizes the recent trends in the protein-phenol interaction, focusing on the effect of the structure of the phenolic compound on the interaction with salivary proteins and on methodologies based on these interactions to determine astringency.

Molecular Approach to the Synergistic Effect on Astringency Elicited by Mixtures of Flavanols

The interactions between salivary proteins and wine flavanols (catechin, epicatechin, and mixtures thereof) have been studied by HPLC-DAD, isothermal titration microcalorimetry, and molecular dynamics simulations. Chromatographic results suggest that the presence of these flavanol mixtures could facilitate the formation of precipitates to the detriment of soluble aggregates. Comparison between the thermodynamic parameters obtained showed remarkably higher negative values of ΔG in the system containing the mixture of both flavanols in comparison to the systems containing individual flavanols, indicating a more favorable scenario in the mixing system. Also, the apparent binding constants were higher in this system. Furthermore, molecular dynamics simulations suggested a faster and greater cooperative binding of catechin and epicatechin to IB7₁₄ peptides when both types of flavanols are present simultaneously in solution.