Plant phenolics

Plant phenolics represent a very large group of defensive compounds defined here as having a phenol (hydroxybenzene) moiety. In some instances substances having a phenolic precursor (e.g. methoxybenzene derivatives) have conveniently also been included in this category. Phenolics derive biosynthetically from hydroxycinnamoyl coenzyme A (yielding a phenyl-propanoid moiety). 

Antiseptic plant-derived phenols include phenol (Phe-OH, hydroxybenzene, carbolic acid), / -cresol (4-methylphenol), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and pyrogallol (1,2,3-trihydroxybenzene). Other simple phenols with antimicrobial properties include some related to benzoic acid (benzenecarboxylic acid), namely salicylic acid (2-hydroxybenzoic acid), ginkgoic acid (2-hydroxy-6-(pentadec-8-enyl)benzoic acid), gentisic acid (2,5-dihydroxybenzoic acid), pyrocatechuic acid (3,4-dihydroxybenzoic acid) and gallic acid (3,4,5-trihydroxybenzoic acid). Other plant-derived phenol-related compounds include 4-methylcatechol, 1,3-dihydroxy-5-(heptadec-12-enyl)benzene, hydroquinone (1,4-dihy-droxybenzene), 1,4-dihydroxy-2-geranyl (di-isoprenyl)benzene and 4-methoxybenzaldehyde (/ -anisealdehyde). 

Phenols have distinct odours. Notable simple phenol-related odorants/tastants include 4-methoxybenzaldehyde (/i-anisealdehyde), guaiacol (2-methoxyphenol), 4-hydroxyben-zaldehyde, phenethyl alcohol, piperonal (heliotropin, 3,4-methylenedioxybenzoic acid) and Vanilla planifolia (vanilla) (Orchidaceae) pod vanillin (3-methoxy-4-hydroxybenzaldehyde) (Chapter 10). 

Major simple phenylpropanoids include cinnamic acid (Phe-CH=CH-COOH), j6-coumaric. acid (jfr-hydroxycinnamic acid), o-coumaric acid (o-hydroxycinnamic acid), caffeic 

Simple lignans involving a Phe-C -C -Phe structure are illustrated by the antioxidant and Ca2+ channel blocker nordihydroguaiaretic acid (NDGA) (3,4-dihydroxyphenyl-CH2-CH(CH3)-CH(CH3)-CH2-(3, 4 -dihydroxyphenyl)), the bitter-tasting phyllanthin and the cAMP phosphodiesterase inhibitor ar-hinokiresinol. Simple lignans of the C3—Phe—Phe—C3 kind are illustrated by the antibacterials honokiol and the protein kinase inhibitor magnolol. 

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In plants phenol biosynthesis proceeds by building the aromatic ring from carbohydrate precursors that already contain the required hydroxyl group... 

Some kinds of fish and Crustacea contain thiaminases. These enzymes cleave thiamin and thus inactivate the vitamin. Some plant phenols, e.g., chlorogenic acid, may possess antithiaminic properties, too, though their mechanism of action is so far not well understood. 

FRIEDMAN M and jOrgens h s (2000) Effect of pH on the stability of plant phenolic compounds , J Agric Food Chem, 48, 2101-10. 

The chemical formulae for a variety of plant phenols are given in Fig. 16.2, including examples of simpler phenols, such as cinnamic acid derivative, and of tocopherols, flavonoids, flavonoid glycosides and anthocyanidins. The flavonoids include the following subclasses flavanones (taxifolin), flavones (luteolin), flavonols (quercetin) and flavanols (catechin/epicatechin). The... 

Fig. 16.2 Examples of plant phenols with antioxidative potential in foods.

Among the plant phenols, the flavonoids and the anthocyanidins, belonging to the 1,3-diphenylpropans, have been studied in most detail, mainly because of their potential health benefits. With more than 4,000 different flavonoids known, systematic studies of the effects of variation in molecular structure on physico-chemical properties of importance for antioxidative effects have also been possible (Jovanovic et al, 1994 Seeram and Nair, 2002). Flavonoids were originally found not to behave as efficiently as the classic phenolic antioxidants like a-tocopherol and synthetic phenolic antioxidants in donating... 

Table 16.1 Thermodynamic and kinetic parameters for plant phenols of relevance for their antioxidant capacity and antioxidant activity... 

The less specific binding of flavonoids to ferrylmyoglobin is in agreement with establishment of LFERs, linear free energy relationships, i.e. Ink(ferryl) depends linearly on E , for reduction of MbFe(IV)=0 by flavonoids within series of flavonons and ftavonols (Jorgensen and Skibsted, 1998). The relevance of such LFERs, as demonstrated for MbFe(IV)=0 and plant phenols, draws further support from the observation that the same sequence, i.e. chlorogenic caffeic > ferulic > coumaric acid, is seen for reaction of the phenols with... 

MbFe(IV)=0 and with lipid peroxyl radicals (Castellucio et al, 1995). It may accordingly be concluded that the most relevant single parameter for predicting the antioxidative activity of a new plant phenol would be the standard reduction potential, E . 

Porter LJ (1989) Tannins. In Harborne JB (ed) Methods in Plant Biochemistry. Plant Phenolic. Academic Press, San Diego, p 389... 

Inderjit and R. del Moral, Plant phenolics in allelopathy. Bot. Rev. 62 186 (1996). D. L. Jones, Organic acids in the rhizosphere—a critical review. Plant Soil 205 25 (1998). 

EXTRACTION OF PLANTS PHENOLIC ACIDS WITH AMBERLITE IR-45 (OH) RESIN... 

Principle Instead of long-lasting extraction with different organic solvents, acids or bases in the Soxhlet apparatus is unsuitable for the extraction of most flavonoids, the Amberlite IR-45 (OH) ion-exchange resin was used for the extraction of plant phenolics. 

Food and plant phenolics are commonly detected using DAD detectors (Tan and others 2008). Photodiode array detection allows collection of the entire UV spectrum during the elution of a chromatographic peak, which makes it possible to identify a phenolic compound by its spectra. Simple phenols, phenolic acids, flavanones, benzophenones, isoflavones, and flavan-3-ols have maximum absorbance at 280 nm, hydroxycinnamic acids at 320 nm, flavonols, flavones, and dihydroflavonols at 365 nm, and anthocyanins at 520 nm (Ibern-G6mez and others 2002 Merken Hand Beecher 2000). Hydrolyzable tannins show a characteristic shoulder at 300 nm, suitable for identifying them (Arapitsas and others 2007). For stilbenes, maximum absorbance of trans-forms are at 306 nm and at 285 nm for cA-forms (Lamuela-Raventos and others 1995). 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

Proestos C and Komaitis M 2008. Application of microwave-assisted extraction to the fast extraction of plant phenolic compounds. LWT Food Sci Technol 41(4) 652-659. 

J. Summanen, T. Yrjonen, R. Hiltunen and H. Vuorela, Influence of densitometer and video documentation settings in the detection of plant phenolics by TLC. J. Plan. Chromatogr.— Mod. TLC 11 (1998) 421 127. 

Cardiovascular Effects. In a cohort mortality study of workers in a large rubber and tire manufacturing plant, Wilcosky and Tyroler (1983) found a significant increase in mortality from ischemic heart disease in phenol exposed workers. Of the 25 solvents used in the plant, phenol exposure showed the strongest association with mortality from heart disease, greater even than that observed for exposure to carbon disulfide, the only known occupational cause of atherosclerosis. 

Lechner D, Gibbons S, Bucar F. (2008) Plant phenolic compounds as ethid-ium bromide efflux inhibitors in Mycobacterium smegmatis. J Antimicrob Chemother 62 345-348. 

Canada goose Branta canadensis Coastal marsh plants Phenolics Buchsbaum etal., 1984... 

Levin, D. A. (1971). Plant phenolics an ecological perspective. American Naturalist 105, 157-181. 

Plant phenolics as chemical defenses effects of natural phenolics on survival and growth of prairie voles [Microtus ochrogaster). Journal of Chemical Ecology 10,229-244. 

Thomas, D. W., Samson, C., and Bergeron, J. M. (1988). Metabolic costs associated with the ingestion of plant phenolics by Microtus pennsylvaticus. Journal of Mammalogy 69, 512-515. 

D) In many examples of allelopathy, plant phenols are implicated as a component of the phytotoxin and therefore it would be reasonable to expect phenols to be Involved in this situation. However, our studies failed to show an accumulation of soluble phenols in the soil. A substantial elevation in soil phenol levels would be required to inhibit seedling development, and phenolics were not detected in root exudates. 

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