Proanthocyanidin intermediate with

The second mechanism is based on observations by Haslam regarding breakdown of interflavonoid bonds under acidic conditions. As a result of this, a carbocationic proanthocyanidin intermediate could react with an anthocyanin at the phloroglucinol ring figure 2). 

Proanthocyanidins (PAs), also known as condensed tannins, are oligomeric and polymeric flavan-3-ols. Procyanidins are the main PAs in foods however, prodelphinidins and propelargonidins have also been identified (Gu and others 2004). The main food sources of total PAs are cinnamon, 8084 mg/100 g FW, and sorghum, 3937 mg/100 g FW. Other important sources of PAs are beans, red wine, nuts, and chocolate, their content ranging between 180 and 300 mg/100 g FW. In fruits, berries and plums are the major sources, with 213.6 and 199.9 mg/100 g FW, respectively. Apples and grapes are intermediate sources of PAs (60 to 90 mg/100 g FW), and the content of PAs in other fruits is less than 40 mg/100 g FW. In the majority of vegetables PAs are not detected, but they can be found in small concentrations in Indian squash (14.8 mg/ 100 g FW) (Gu and others, 2004 US Department of Agriculture, 2004). 

In the initial stages of this analytical method the proanthocyanidins are cleaved by acid catalysis into their constitutive subunits. Proanthocyanidin extension subunits, upon cleavage, form unstable electrophilic intermediates. Phloroglucinol (1,3,5-trihydroxy-benzene) is added to the reaction mixture as a nucleophile, where it combines with extension subunit intermediates to form analyzable adducts. 

Flavanol oligomers and polymers are also called condensed tannins or proan-thocyanidins. The term tannin refers to their capacity to interact or react with proteins and precipitate them out. When heated under acidic conditions, these molecules release red anthocyanidin pigments, hence the term proanthocyanidins. The term leucoanthocyanidin, also referring to this particular property, is sometimes encountered in the literature. However, this should be restricted to another group of compounds, flavan 3,4-diols, which are intermediates in the biosynthetic pathway leading to flavanols and anthocyanins (Stafford and Lester 1984 Nakajima et al. 2001 Abrahams et al. 2003) but have never been isolated from grapes, presumably due to their instability. 

The reaction starts with acid catalysed cleavage of a proanthocyanidin interfla-vanic linkage (Figs. 9B.3 and 9B.5(1)). The intermediate carbocation thus generated then undergoes nucleophilic addition. When the nucleophile is another flavanol (Fig. 9B.5(2a)), the product is a new proanthocyanidin molecule. As a result of this... 

Key intermediates in the biosynthesis of proanthocyanidin polymers seem to be flav-3-en-3-ol derivatives or biological equivalents. These intermediates allow plants to synthesize either cis- or trans-stereochemistry with equal facility. 

An explanation for the formation of 2jR,3jR-(2,3-c/5)-proanthocyanidins from the 2i ,3S-(2,3-/ra/Z5 )-flavan-3,4-diols could lie in a tautomeric rearrangement of quinone methide intermediates to flav-3-en-3-ols, which could then be stereospecifically converted back to either 2,3-trans or 2,3-cis quinone methides (145). Chemical evidence supporting this thesis has been obtained by the formation of diarylprqpanone derivatives from the reaction of polymeric procyanidins with phenylmethanethiol under alkaline conditions (223). Enzymes controlling the quinone methide to flav-3-en-3-ol rearrangements rather than C-3 inversion of dihydroflavonols may be involved. In either case, evidence continues to mount that the flavan-3,4-diols are indeed central intermediates in the biogenesis of proanthocyanidins and that this conversion is under enzymic (genetic) control (219, 341, 342). 

Other groups of flavonoids are chalcones, intermediates in the biosynthesis of all other flavonoids, as well as dihydrochalcones, aurones, or dihydroflavonols, together with biflavonoids, which are flavonoid dimers different from proanthocyanidins, since they are based on flavones and flavanones linked through a carbon-carbon bond or less frequently by an ether bond. " In general, all these flavonoids are less abundant in foods than the compounds mentioned above, although in some cases they may have important applications in the food industry as for the sweetener neohesperidin dfliydrochalcone (NHDC). 

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