Quinic and Gallic Acids

The remaining shlklmates in Table III also are relatively simple, well known compounds. The phenolic structures of vanillin (125) and gallic acid (127) and the prephenolic structures of the common quinic acid (128) and chlorogenic acid (129) make them candidates for physiological activity. Gallic acid is the monomer for tannins, biological polymers found in the cotton plant (15, 37). 

Simple benzoic acids are synthesized in plants via the shikimate pathway, which is derived from shikimic acid, which is itself derived from quinic acid via 3-dehydroquinic and 3-dehydroshikimic acids (Scheme 68.1). In fact, shikimic, but not quinic, acid has been described in the leaves and stems of this species [14]. The simplest benzoic acids are protocatechuic acid (3,4-dihydroxybenzoic acid) and gallic acid (3,4,5-trihydroxybenzoic acid). The latter proved to be present in the tissues of C. roseus, in addition to vanillic acid (4-hydroxy-3-methoxybenzoic acid) [15]. The quantitative composition of C. roseus in benzoic acids can be seen in Table 68.1, and the structures of these compounds are represented in Fig. 68.1. 

The structural similarities among quinic, shikimic, and gallic acids are striking and their possible relationship in the plant are discussed by Fischer and Dangschat. 

Gallic acid is present in tea leaf and is a known reactant during the complex enzymatic and organochemical reactions that occur when tea components are oxidized.51 The gallic and quinic acids originate via the shikimate/arogenate pathway. The key enzymes in shikimic acid biosyn-... 

Other Phenolic Compounds. There are several phenolic acids important to tea chemistry. Gallic acid (3) and its quinic acid ester, theogallin (4), have been identified in tea (17,18) and have been detected by hplc (19,20). 

Phenolic compounds include a wide range of secondary metabolites that are biosynthesised from carbohydrates through the shikimate pathway [14]. This is the biosynthetic route to the aromatic amino acids, phenylalanine, tyrosine, and tryptophan, and only occurs in microorganisms and plants. In the first step, the glycolytic intermediate phosphoenol pyruvate and the pentose phosphate intermediate erythrose-4-phosphate are condensed to 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP), a step catalysed by DAHP synthase. Intermediates of the shikimate pathway are 3-dehydroquinate, shikimate, and chorismate (Fig. 1). Phenylalanine is biosynthesised from chorismate, and from phenylalanine all the phenylpropanoids. Quinate is produced from 3-dehydroquinate and incorporated into chlorogenic and isochlorogenic acids (caffeoyl quinic acids) by combination with caffeic acid. Gallic acid is produced from shikimate. 

Black walnut cultivars Rawlins and Thomas had considerably lower gallic acid levels of -1% dw, quite close to that of the pistachio cultivar Kerman (0.5% dw), and in vitro aflatoxin production for the walnut Thomas and pistachio Kerman were also similar at 45 and 40 J,g/plate, respectively [17], The hydrolysable tannin in pistachios has a core of quinic acid, rather than glucose (unpublished results), and its stereochemistry is such that dimerization of gallic acid moieties to hexahy-droxydiphenic esters, and consequent formation of ellagic acid, cannot occur. Any aflatoxin inhibitory activity is, therefore, dependent on either the tannin itself or gallic acid. 

Phenolic compounds are widely distributed in plant parts from the roots to the seeds and include phenolic acids, flavo-noids and tannins. The tannins may reduce protein digestibility (Ford and Hewitt, 1979) and perhaps the bioavailability of other nutrients. The flavonoids have been reported to have a number of nutritional and pharmacological activities (Kuhnau, 1976). Phenolic acids include benzoic and cinnamic acid derivatives. The benzoic acid derivatives include p-hydroxy-benzoic, protochate-chuic, vanillic, gallic and syringic acids. The cinnamic acids, p-coumaric, caffeic, ferulic and sinapic are found in most oilseeds used to prepare protein concentrates and frequently occur in the form of esters with quinic acid or sugars. Chlorogenic acid for example is an ester of caffeic acid and quinic acid and is found in several isomeric and derivatized forms. 

Phenolic acids and coumarins Two families of phenolic acids are widely distributed in plants - a range of substituted benzoic (Cg-Ci) acid derivatives and those derived from cinnamic (C -C ) acid. Both types of phenolic acids usually occur in conjugated or esterified form. The simpler types of benzoic acid derivatives include p-hydroxybenzoic, protocatechuic, vannilic, gallic and syringic acids, and the o-hydroxy salicylic and gentisic acids (Fig. 1). The cinnamic acids p-coumaric, caffeic, ferulic, and sinapic, are found in most oilseeds and occur frequently in the form of esters with quinic acid or sugars (Fig. 1). Chlorogenic... 

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