Phenolic acids found

Table 2.1 Summary of Phenolic Acids Found in Vivo and in Vitro from Different Food Sources and Havonoids ... 

It should be emphasized that a complex of substances is generally involved when allelopathic interferences occur, often with each below a threshold level for impact. This is illustrated by the combinations of phenolic acids found in decomposing crop residues (25-27) and from soils (28-34). In allelopathic situations which implicate phenolic acids, soil quantities of ferulic, p-coumaric, and caffeic acids have ranged from below 10 to above 1,000 ppm for each compound (11,35). The lower end of this spectrum is below a concentration required for an effect in current bioassays. However, additive and synergistic effects have been documented for combinations of cinnamic acids (35), benzoic acids (36), benzoic and cinnamic acids (37), and p-hydroxybenzaldehyde with coumarin (38). Each of the allelochemicals in these tests was not equally toxic, but they contributed incrementally to inhibition of germination and growth. Whereas combinations of many allelochemicals have not been determined, it appears that both additive and synergistic interactions are extremely important under field conditions. 

Table 6. Composition of phenolic acids found in natural humus beneath bilberry Vaccinium myrtillus) plants in clearings of Norway spruce (Picea abies) in eastern France. Data ifom Souto et al., 2000.

Among the phenolic acids found regularly in normal human urine, Armstrong et al. (A14) identified p-hydroxyhippuric, p-hydroxyphenyl-... 

The phenolic acids found in natural sources are present in variable quantities and those with therapeutic properties have a higher added value in the market. An overview of the general therapeutic and chemical properties of phenolic acids has already been given, and the most important compounds are now described in more detail, including their present applications. 

Fig. 3.10 Effects of a 7-phenolic acid solution modeled after phenolic acids found in wheat stub-ble/soybean (no-till) soil extracts (pH 5) on radicle and hypocotyl lengths of crimson clover as modified by solute potential of PEG (polyethylene glycol a r = 0.61) and Hoagland s solution (b r = 0.37) based on freezing point depression (mOsm, mUliosmoles) of solutions. The 7-phenohc acid mixture was composed of 10% caffeic acid, 9% ferulic acid, 35% p-coumaric acid, 15% p-hydroxybenzoic acid, 4% sinapic acid, 10% syringic acid, and 17% vanillic acid. Figures based on regressions from Blum et al. (1992). Plenum Publishing Corporation, regressions used with permission of Springer Science and Business Media...

Table 1.1 Chemical structures of some phenolic acids found naturally in plants and foods (Robbins 2003 Vermerris and Nicholson 2006 Cueva et eil. 2010)...

Under the same conditions the even more reactive compounds 1,6-dimethylnaphthalene, phenol, and wt-cresol were nitrated very rapidly by an autocatalytic process [nitrous acid being generated in the way already discussed ( 4.3.3)]. However, by adding urea to the solutions the autocatalytic reaction could be suppressed, and 1,6-dimethyl-naphthalene and phenol were found to be nitrated about 700 times faster than benzene. Again, the barrier of the encounter rate of reaction with nitronium ions was broken, and the occurrence of nitration by the special mechanism, via nitrosation, demonstrated. 

Many applications of novolacs are found in the electronics industry. Examples include microchip module packaging, circuit board adhesives, and photoresists for microchip etching. These applications are very sensitive to trace metal contamination. Therefore the applicable novolacs have stringent metal-content specifications, often in the low ppb range. Low level restrictions may also be applied to free phenol, acid, moisture, and other monomers. There is often a strong interaction between the monomers and catalysts chosen and attainment of low metals levels. These requirements, in combination with the high temperature requirements mentioned above, often dictate special materials be used for reactor vessel construction. Whereas many resoles can be processed in mild steel reactors, novolacs require special alloys (e.g. Inconel ), titanium, or glass for contact surfaces. These materials are very expensive and most have associated maintenance problems as well. 

Arabinoxylans with a substitution of the P-(l—>4)-D-xylopyranose backbone at position 2 or 3 with ArbF can be esterified partly with phenolic acids. This type is frequently found in the starchy endosperm and the outer layers of cereal grains. 

Some phenolic acids such as ellagic acid can be used as floral markers of heather honey (Cherchi et al., 1994 Ferreres et al., 1996a,b), and the hydroxyciimamates (caffeic, p-coumaric, and ferulic acids) as floral markers of chestnut honey (Cherchi et al., 1994). Pinocembrin, pinobanksin, and chrysin are the characteristic flavonoids of propolis, and these flavo-noid compounds have been found in most European honey samples (Tomas-Barberan et al., 2001). However, for lavender and acacia honeys, no specific phenolic compoimds could be used as suitable floral markers (Tomas-Barberan et al., 2001). Other potential phytochemical markers like abscisic acid may become floral markers in heather honey (Cherchi et al., 1994). Abscisic acid was also detected in rapeseed, lime, and acacia honey samples (Tomas-Barberan et al., 2001). Snow and Manley-Harris (2004) studied antimicrobial activity of phenolics. 

Several general characteristics of the results compiled in Table I are worthy of mention. Compared to the variety of chemicals postulated to be involved in allelopathy (1), few specific compounds have been tested for inhibition of mineral absorption. The most extensively studied compounds are the phenolic acids, probably because of their being ubiquitously found in nature (1). Also, several flavonoids are inhibitory to mineral absorption (Table I). Both of these groups of compounds are often cited as being responsible for allelopathic interactions between plants. 

Two additional characteristics of the inhibition of mineral absorption by phenolic acids were observed. The inhibition of both P0 absorption (27) and K+ absorption (31, 32) was reversed when the phenolic acid was removed from the absorption solution. Harper Balke (32) found this reversibility to be dependent upon pH the lower the pH, the less the reversal. Also, kinetic plots of the inhibition of mineral absorption showed that the phenolic acids did not competitively inhibit either P0 (26, 28) or K+ (31) absorption. Rather, ferulic acid inhibited PO -absorption in a noncompetitive (26) or uncompetitive (28) manner and jr-hydroxybenzoic acid inhibited K+ absorption in an uncompetitive manner (31). 

A further point of preparative significance still requires explanation, however. Highly reactive aromatic compounds, such as phenol, are found to undergo ready nitration even in dilute nitric acid, and at a far more rapid rate than can be explained on the basis of the concentration of N02 that is present in the mixture. This has been shown to be due to the presence of nitrous acid in the system which nitrosates the reactive nucleus via the nitrosonium ion, NO (or other species capable of effecting nitrosation, cf. p. 120) ... 

How the aliphatic monomers are incorporated into the suberin polymer is not known. Presumably, activated co-hydroxy acids and dicarboxylic acids are ester-ified to the hydroxyl groups as found in cutin biosynthesis. The long chain fatty alcohols might be incorporated into suberin via esterification with phenylpro-panoic acids such as ferulic acid, followed by peroxidase-catalyzed polymerization of the phenolic derivative. This suggestion is based on the finding that ferulic acid esters of very long chain fatty alcohols are frequently found in sub-erin-associated waxes. The recently cloned hydroxycinnamoyl-CoA tyramine N-(hydroxycinnamoyl) transferase [77] may produce a tyramide derivative of the phenolic compound that may then be incorporated into the polymer by a peroxidase. The glycerol triester composed of a fatty acid, caffeic acid and a>-hydroxy acid found in the suberin associated wax [40] may also be incorporated into the polymer by a peroxidase. 

The Folin-Ciocalteu assay is the most widely used method to determine the total content of food phenolics (Fleck and others 2008). Folin-Ciocalteu reagent is not specific and detects all phenolic groups found in extracts, including those found in extractable proteins. A disadvantage of this assay is the interference of reducing substances, such as ascorbic acid (Singleton and others 1999). The content of phenolics is expressed as gallic acid or catechin equivalents. 

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