Structure plant polyphenols

Naturally occurring oxaarenes based on polycyclic pyrans encompass a plethora of structures including the plant polyphenols such as anthocyanins and a-tocopherol (vitamin E). Halogenated dibenzo-p-dioxins and dibenzofurans are formed both as by-products during the manufacture of chlorophenols, and from the incineration of organic matter in the presence of inorganic halides. 

Fukai, T. and Nomura, T., New NMR structure determination methods for prenylated phenols, Basic Life Sciences, 66 (Plant Polyphenols 2), 259, 1999. 

Fulcrand, H. et al.. Electrospray contribution to structural analysis of condensed tannin oligomers and polymers. In Plant Polyphenols 2. Chemistry, Biology, Pharmacology, Ecology (eds G.G. Gross, R.W. Hemingway, T. Yoshida, and S.J. Branham), Kluwer Academic/Plenum Publisher, New York, 1999, p. 223. 

Pietta, P., Minoggio, M., and Bramati, L., Plant polyphenols structure, occurrence and bioactivity, Studies in Natural Products Chemistry, 28 (Bioactive Natural Products (Part I)), Elsevier Science Publishers, Amsterdam, 257, 2003. 

Complex plant polyphenols readily and reversibly associate with proteins and they can precipitate them from dilute solution. This property is however a direct extrapolation of the characteristics of simple phenols themselves. The structural device represented by the plant polyphenols to a great extent obviates the need for a high molal concentration of phenol and it embodies the added feature that cross-linking between different molecular aggregates may be readily achieved. 

The chalcone synthase (CHS) (EC 2.3.1.74) superfamily of type III Polyketide synthases (PKSs) are pivotal enzymes in the biosynthesis of plant polyphenols. They are structurally and mechanistically different from the modular type I and the dissociated type II PKSs of bacterial origin the simple homodimer of 4CM-5 kDa proteins performs a complete series of decarboxylation, condensation, cyclization,... 

Bors W, Michel C, Stettmaier K. (2001) Structure-activity relationships governing antioxidant capacities of plant polyphenols. Methods in Enzym 335 166-180. 

Free radicals are ubiquitous. They are found in living plants and for practical purposes are essential to all life. They also exist in plant material that is dried. These types of free radicals are called persistent free radicals and are normally associated with free radicals present in the structural biomass of the plant (polyphenols, carbohydrates, and lignin). The tobacco precursors of free radicals found in the particulate phase of cigarette mainstream smoke (MSS) are also long-lived, persistent free radicals but arise from the thermolysis of the tobacco biomass to form numerous types of phenolic and quinoidal free radicals [Wooten et al. (27A120)]. Short-lived free radicals are also present in the vapor phase... 

Figure 2 Chemical structures of selected plant polyphenols. Structures include a flavonol (quercetin), isoflavone (daidzein), cinnamic acid (chlorogenic acid), flavan-3-ol (catechin), a lignan microbial metabolite (enterodiol), and a stilbene (resveratrol).

Plant polyphenols, which have the ability to precipitate protein, collectively, are called tannins. These compounds have been used for millenia to convert raw animal hides into leather. In this process, tannin molecules cross-link the protein and make it more resistant to bacterial and fungal attack. Molecules in the molecular weight range of 500-2000 (3000) are most effective, but the ability to bind effectively varies with different tannin structures. Today, however, many substances considered to be tannins by virtue of their structure and bios5mthetic origin have limited, if any, ability to make leather (Hagerman and Butler, 1981). 

The extraction of phenolic compounds from plant materials is considerably influenced by some factors such as the nature of the sample, the chemical structure of polyphenols, the extraction method employed, the extractirMi agent involved, sample particle size, as well as the presence of interfering substances. 

Yoshida, T., Hatano, T., Ito, H. Okuda, T. (2008). Structural diversity and antimicrobial activities of ellagitannins. In Chemistry and Biology of Ellagitannins - An Underestimated Class of Bioactive Plant Polyphenols (ed. S. Quideau), pp. 55 93. World Scientific, Singapore. 

Flavonoids are representative plant polyphenols and commonly recognized as nonnutri-tional dietary components. The diphenylpropane structure with one or more phenolic groups is a characteristic of flavonoids. In 1936, Szent-Gyorgyi first reported that citrus flavonoids (hesperidin and rutin) reduced capillary fragility and permeability in human blood vessels... 

In chemical terms a perhaps unexpected but biosynthetically interesting variant of the hexahydroxydiphenoyl ester group first noted by Schmidt is its dehydro-derivative (22). It was identified by Schmidt as a structural component of brevilagin 1 and 2, polyphenols from the fruit pods of Caesalpinia brevifolia (133, 134). Subsequent work has shown the ester to be present in the structure of other plant polyphenols, notably geraniin (51) in plants of the Geraniaceae (Thble 7.2.9). 

Polyphenols (tannins) form reversible, and often insoluble, complexes with proteins. The principal mechanisms of polyphenol-protein binding are thought to be (i) hydrogen bonding (ii) hydrophobic interactions (iii) ionic interactions. Our current knowledge of the structure of plant polyphenols is sufficient to eliminate (iii). The thrust of recent studies has been to try and define the relative balance between (i) and (ii) for polyphenol complexation with proteins, carbohydrates, and organic bases (the three phenomena being very closely linked). 

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