Phenolic compounds, classification

An excellent classification of this type of enzymes was that of Burton (2003). Because of their importance in polyphenol degradation, we have studied four enzymes closely related with the oxidation of phenolic compounds polyphenoloxidase, peroxidase, laccase, and lipoxygenase. 

Phenolic compounds are of interest due to their potential contribution to the taste (astrin-gency, bitterness, and sourness) and formation of off-flavor in foods, including tea, coffee, and various fruit juices, during storage. Their influence on the appearance of food products, such as haze formation and discoloration associated with browning in apple and grape products, is also significant. Furthermore, analysis of these phenolic compounds can permit taxonomic classification of the source of foods. The importance of each phenolic compound and its association with the quality of various foods is described further in Sec. IV, on food applications. 

For phenolics in fruit by-products such as apple seed, peel, cortex, and pomace, an HPLC method was also utilized. Apple waste is considered a potential source of specialty chemicals (58,62), and its quantitative polyphenol profile may be useful in apple cultivars for classification and identification. Chlorogenic acid and coumaroylquinic acids and phloridzin are known to be major phenolics in apple juice (53). However, in contrast to apple polyphenolics, HPLC with a 70% aqueous acetone extract of apple seeds showed that phloridzin alone accounts for ca. 75% of the total apple seed polyphenolics (62). Besides phloridzin, 13 other phenolics were identified by gradient HPLC/PDA on LiChrospher 100 RP-18 from apple seed (62). The HPLC technique was also able to provide polyphenol profiles in the peel and cortex of the apple to be used to characterize apple cultivars by multivariate statistical techniques (63). Phenolic compounds in the epidermis zone, parenchyma zone, core zone, and seeds of French cider apple varieties are also determined by HPLC (56). Three successive solvent extractions (hexane, methanol, aqueous acetone), binary HPLC gradient using (a) aqueous acetic acid, 2.5%, v/v, and (b) acetonitrile fol-... 

There is growing evidence from human feeding studies that the absorption and bioavailability and thus bioactivity of phenolic compounds and flavonoids are very much dependent on the nature of their chemical structure. Their chemical classification and dietary occurrence is briefly discussed in the following section. 

Tanacetum species contain mainly sesquiterpenoids and flavonoids, whereas the other terpenoids and phenolic compounds are rarely found. Sesquiterpenoids which are the main constituents of the genus, supposed to be bioactive principles of the plants. Flavonoids and essential oils are also pointed out as active substances in some species. On the other hand, there is a confusion on the systematic position and classification of several species of Asteraceae, therefore chemotaxonomy of the species will help the systematic studies. 

Beltran et al. (50) succeeded in classifying 172 Chilean wines according to the type of grapes (cabernet sauvignon, merlot, and carmenere). First, phenolic compound chromatograms were developed with FIPLC-DAD. Second, features were extracted from the chromatographic data with different feature extraction techniques, like discrete Fourier transform and Wavelet transform. Finally, next to other different classification techniques, LDA and QDA were applied. From CV, both methods were found to result in acceptable correct classification rates without statistically significant difference between both rates. 

Beltran et al. (50) also tested kNN to classify the Chilean wines according to their grape type. Again different feature extraction techniques were tested to reduce the dimensionality of the chromatographic data, describing the phenolic compounds. In most cases, kNN resulted in a slightly lower average correct classification rate than LDA and QDA. 

The phenolic compounds present in these three commodities fall into two general classifications, cinnamic acid derivatives and flavonoids. Included in the former are chlorogenic acid and its isomers, free cinnamic acids such as caffeic and p-coumaric acid and various esters of those two acids. Included among the flavonoids are the following ... 

Although this formal division into five aspects is convenient from the classification point of view, aromatic chemistry is still an area of both striking as well as more conventional developments and phenolic compounds and aromatic ethers (Chapter 4) have been in the forefront of recent novel synthetic methodology. 

The phenolic compounds are amongst the most important grape and wine constituents. They are responsible for all the differences in color and taste between red and white wines. They represent a diverse group of compounds for which a number of different classification systems have been applied. 

A straightforward classification attempts to divide the broad category of phenolics into simple phenols and polyphenols, based exclusively on the number of phenol subunits present, but many plant phenolic compounds are polymerized into larger molecules. Thus, the term plant phenolics encompasses simple phenols, phenolic acids, coumarins, flavonoids, stilbenes, hydrolyzable and condensed tannins, lignans, and hgnins [4]. 

Vermerris W, Nicholson RL (2006) Phenolic compound biochemistry. Springer, Dordrecht Vincken JP, Heng L, de Groot A, Gruppen H (2007) Saponins, classification and occurrence in the plant kingdom. Phytochemistry 68 275-297... 

One of the most recurrent case studies for the demonstration of the operation and possibilities offered by BioET is that of the determination of phenolic compounds, by phenol-degrading enzymes tyrosinase and laccase. Departing from these measurements, there can be developed different applications (1) to perform the iden-tification/classification of types of samples, (2) to estimate general indexes and (3) to resolve the presence of specific phenolic compounds. And many different specific applications can be developed to estimate polyphenolic compounds in wine, beer, juices, teas, coffee, fruits, etc., or to estimate polluting load in wastes originated in this type of industries. 

The imides, primaiy and secondary nitro compounds, oximes and sulphon amides of Solubility Group III are weakly acidic nitrogen compounds they cannot be titrated satisfactorily with a standard alkaU nor do they exhibit the reactions characteristic of phenols. The neutral nitrogen compounds of Solubility Group VII include tertiary nitro compounds amides (simple and substituted) derivatives of aldehydes and ketones (hydrazones, semlcarb-azones, ete.) nitriles nitroso, azo, hydrazo and other Intermediate reduction products of aromatic nitro compounds. All the above nitrogen compounds, and also the sulphonamides of Solubility Group VII, respond, with few exceptions, to the same classification reactions (reduction and hydrolysis) and hence will be considered together. 

The most characteristic reaction of butadiene catalyzed by palladium catalysts is the dimerization with incorporation of various nucleophiles [Eq. (11)]. The main product of this telomerization reaction is the 8-substituted 1,6-octadiene, 17. Also, 3-substituted 1,7-octadiene, 18, is formed as a minor product. So far, the following nucleophiles are known to react with butadiene to form corresponding telomers water, carboxylic acids, primary and secondary alcohols, phenols, ammonia, primary and secondary amines, enamines, active methylene compounds activated by two electron-attracting groups, and nitroalkanes. Some of these nucleophiles are known to react oxidatively with simple olefins in the presence of Pd2+ salts. Carbon monoxide and hydrosilanes also take part in the telomerization. The telomerization reactions are surveyed based on the classification by the nucleophiles. 

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