Daniele Magnolato

Polyphenol complexation—some thoughts and observations

Abstract Current views and opinions on polyphenol (tannin)-protein complexation are outlined. ‘Structure-activity’ relationships are delineated as are experimental approaches which seek to define the modes and sites of binding of ligand (polyphenol) to the receptor (protein). A model for polyphenol-protein complexation is proposed.

The metabolism of gallic acid and hexahydroxydiphenic acid in plants: Biogenetic and molecular taxonomic considerations

Abstract The distinctive and unique features of gallic acid metabolism in plants are discussed and recent observations on new metabolites are presented. The potential application of these results to taxonomic questions is outlined.

Glucosylated flavonoids and other phenolic compounds from sorghum

Abstract The principal tannin constituents of sorghum are proanthocyanidins or condensed tannins. Analysis of the methanolic extract of a Hungarian sorghum (szegedi toerpe) containing 6% catechin equivalents of tannins resulted in the separation and purification of 4 procyanidins having the basic formula epicatechin-(epicatechin)n-catechin and one procyanidin trimer corresponding to epicatechin-catechin-epicatechin. Apart from these procyanidins, the monomeric flavonoids eriodictyol 5-glucoside and (+)-taxifolm 7-glucoside together with their aglycones eriodictyol and taxifolin were found. Glucosylated dimeric and trimeric flavanoids with eriodictyol or eriodictyol 5-glucoside as the lower unit were also identified with the help of negative ion FABMS. Polymeric flavonoids formed between a chalcone and a flavonoid, as yet not identified, are also present in the grain.

The metabolism of gallic acid and hexahydroxydiphenic acid in plants. Part 1. Introduction. Naturally occurring galloyl esters

The present state of knowledge of the metabolism of gallic acid in higher plants is reviewed and its unique features are noted. Methods for the structure determination of natural galloyl-D-glucose esters are outlined and applied to the elucidation of the structure of several new galloyl-D-glucose derivatives isolated from the leaves of higher plants. Finally the structure of the ‘gallotannins’ based on β-penta-O-galloyl-D-glucose is reassessed on the basis of h.p.l.c. analysis and 13C n.m.r, spectroscopic measurements.

The caffeine—potassium chlorogenate molecular complex

Abstract The crystal structure of the caffeine—potassium chlorogenate 1:1 molecular complex derived from coffee beans is described. The relationship of this structure to the general question of the mechanisms of polyphenol association and precipitation is commented upon.

The electrogenic, Na+-dependent I− transport system in plasma membrane vesicles from thyroid glands

Using vesicles from the plasma membrane of hog thyroid, we have characterized its Na+-dependent I- transport system. We have found it to be totally Na+ dependent; K+ cannot substitute and Li+ can partially substitute for Na+; the Na+:I- flux ratio is larger than one; the system is electrogenic, being stimulated by a delta psi negative inside the vesicles. A number of large, lipophilic anions are fully-competitive inhibitors of Na+-dependent I- uptake; the closer their atomic radii are to that of iodine, the smaller their Ki values.

Polyphenol–caffeine complexation

Crystal structures of complexes of caffeine (1) with methyl gallate (2), m-nitrobenzoic acid (3), and potassium chlorogenate (4) are described; the results are used to comment on possible modes of association of natural polyphenols with substrates such as proteins.

Polyphenol interactions. Part 4. Model studies with caffeine and cyclodextrins

Studies of the reversible complexation of a range of phenols and natural polyphenols in aqueous media with caffeine and related heterocycles, and with α- and β-cyclodextrin, using a range of physical methods (1H and 13C NMR spectroscopy, microcalorimetry, and X-ray crystallographic analysis) are reported. Values of the assciation constants (K) for the formation of 1:1 complexes between caffeine and a range of natural polyphenols hav been determined (K 15–138 dm3 mol–1). Amongst galloyl esters there is a dependence for strong binding on molecular size, conformational flexibility, and the ‘free’ galloyl ester group content of the polyphenol. With phenolic flavan-3-ols, association is enhanced by galloylation at C-3. The extent of precipitation of polyphenols by caffeine is related to the assciation constants (K), the molecular size of the polyphenol, and the initial concentration of both substrates. Polysaccharide cavity sequestration of polyphenols has been studied by means of the model substrates α- and β-cyclodextrin. Compared with that of the natural galloyl esters (K 76–340 dm3 mol–1) the binding of phenolic flavan-3-ols to β-cycloextrin is strong (K 210–6232 dm3 mol–1). Models are proposed for encapsulation within the cyclodextrin cavity. The results of these model studies are discussed in terms of the relative significance of hydrophobic effects and hydrogen bonding in polyphenol complexation. They provide a basis for the interpretation of the behaviour of polyphenols in their association with proteins, polysaccharides and other macromolecules.

The association of polyphenols with caffeine and α- and β-cyclodextrin in aqueous media

The association of polyphenols in aqueous media with caffeine and with α- and β-cyclodextrin has been studied in binary and ternary systems using 1H n.m.r. spectroscopy and microcalorimetry; the results confirm that complexation of polyphenols with caffeine is considerably modified in the presence of certain types of saccharides.