Models tannins

Arapitsas P, Menichetti S, Vincieri FF and Romani A. 2007. Hydrolyzable tannins with the hexahydrox-ydiphenoyl unit and the m-depsidic link HPLC-DAD-MS identification and model synthesis. J Agric Food Chem 55(l) 48-55. 

Keywords Mangrove barks, molecular modeling, phase transformation, rast, spectroscopy, tannin, transformation. 

Drawing on this and the reported specificity of tannin-protein interactions ( ) leads to the conclusion that any useful in vitro modelling of the impact of tannins on digestion must consider more than pH and the concentrations of the buffer, enzyme, substrate, and tannin. The actual enzyme-substrate system must be nutritionally realistic to control for specificities of the reaction of tannins with proteins (including enzymes) gastrointestinal mucoproteins should perhaps also be included on the same grounds. Besides all this, misleading results nay still be obtained if bile surfactants are omitted from the equation. 

Salas, E. et al.. Reactions of anthocyanins and tannins in model solutions. J. Agric. Food Chem. 51, 7951, 2003. 

Riou, V. et ak. Aggregation of grape seed tannins in model — effect of wine polysaccharides. Food Hydrocoil 16, 17, 2002. 

Matsuo, T. and Itoo, S., A model experiment for de-astringency of persimmon fruit with high carbon dioxide treatment in vitro gelation of kaki-tannin by reacting with acetaldehyde. Agric. Biol Chem. 46, 683, 1982. 

Hemingway, R.W. et al.. Conformation and complexation of tannins NMR spectra and molecular search modeling of flavan-3-ols, Magn. Reson. Chem., 34, 424, 1996. 

More complex, second order interactions may be imagined, involving more than one natural enemy. For example, consider insects to which tannins are important deterrents and digestion inhibitors. As mentioned above, elevated gut pH appears to be a way of dealing with tannins, since tannin-protein complexes are dissociated or inhibited at alkaline pH (16,32). Indeed, using a model in vitro system in which hemoglobin is employed as a protein substrate, we found that several natural tannins and phenolic extracts do not precipitate this protein when the pH exceeds about... 

Respective Roles of Anthocyanins and Tannins. The mediation of tannins and anthocyanins in the color of red wine, covered in our work (38) has been conducted in model solutions (17). The procedure is as follows. 

Table XV. Anthocyanin and Tannin Coloration of Model Solutions, Analytical Results...

Recent investigations provide new insight on the structural chemistry of dissolved organic matter (DOM) in freshwater environments and the role of these structures in contaminant binding. Molecular models of DOM derived from allochthonous and autochthonous sources show that short-chain, branched, and alicyclic structures are terminated by carboxyl or methyl groups in DOM from both sources. Allochthonous DOM, however, had aromatic structures indicative of tannin and lignin residues, whereas the autochthonous DOM was characterized by aliphatic alicyclic structures indicative of lipid hydrocarbons as the source. DOM isolated from different morphoclimatic regions had minor structural differences. 

Correlation of Structure with Source. Allochthonous-derived DOM (8) was isolated from the Suwannee River at its origin in the Okefe-nokee Swamp in southern Georgia. The fiilvic acid fraction, which is responsible for the black coloration of the water, was extensively characterized (9). Several average molecular models based on quantitative analytical data were presented in that report (10) to denote the mixture characteristics of fiilvic acid. One model, modified to depict biochemical sources and based on quantitative analytical data (10), is presented in Structure 1. Other models of Suwannee River fulvic acid (based on lignins, terpenoids, tannins, and flavonoid sources) were previously proposed (II). 

This does not seem to occur in all cases. Some authors have observed that with an increase in tannin/protein ratio there is the formation of large particles that eventually precipitate. A three-phase model has been proposed to explain this phenomenon [33, 51]. The simultaneous binding of the multidentate tannin to several places in the protein leads to the protein wrapping around the tannins. As the concentration of tannin increases, several tannin molecules bind to the protein surface and crosslink with other proteins, leading to protein association. As more tannin is added, more protein-tannin complexes aggregate, forming larger particles that precipitate. 

The sensation of astringency is felt differently by tasters [86] probably due to differences in individuals saliva in terms of its protein composition [11, 47]. Astringency is also affected by the structure of the phenolic compounds [88], pH [89], the presence of other substances [90-92], and viscosity [11]. In fact it is believed that complex beverages such as wine and beer have subtle sub-qualities of sensorial descriptors related to astringency (soft, grainy, harsh, green, chalky, etc.), that are not perceived in tannin model solutions, which could be due to the presence of other molecules [87]. 

Reactions of Tannin Model Compounds with Methylolphenols... 

Using exponential dilution analysis and an NMR technique, Dufour and Bayonove (1999) confirmed the existence of weak interactions between catequins and aroma compounds in model wine systems and they also agreed that mutual hydrophobicity was the driving force for this interaction. They also showed a different type of interaction depending on the type of polyphenols (catequin or tannin), and on the nature of the aroma compound. 

At present the structures of the wine-derived-tannins are practically unknown and a great part of the structures identified so far have been only demonstrated in wine model studies. Hence, the influence, or contribution, of wine tannins to astringency is far from being ascertained. For that reason, it is crucial to make efforts to clarify those structures and the mechanisms involved in their formation. Moreover, the role of oxygen is not fully understood and it is important to know how to deal with it during winemaking to control oxidation, and therefore improve the wine taste characteristics. 

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