Hydroxycinnamic acid, structure

Suberin is a composite of polymeric phenylpropanoids and ester-linked long chain fatty acids and alcohols and consists of a hydrophobic layer attached to the cell walls of roots, bark and the vascular system (8,10). The phenylpropanoid portion of suberin purportedly has a lignin-like structure to which both aliphatic domains and hydroxycinnamic acids are esterified. 

Attachment of Hydroxycinnamic Acids to Structural Cell Wall Polymers. Peroxidase mediation may also result in binding the hydroxycinnamic acids to the plant cell wall polymers (66,67). For example, it was reported that peroxidases isolated from the cell walls of Pinus elliottii catalyze the formation of alkali-stable linkages between [2-14C] ferulic acid 1 and pine cell walls (66). Presumably this is a consequence of free-radical coupling of the phenoxy radical species (from ferulic acid 1) with other free-radical moieties on the lignin polymer. There is some additional indirect support for this hypothesis, since we have established that E-ferulic acid 1 is a good substrate for horseradish peroxidase with an apparent Km (77 /tM), which is approximately one fifth of that for E-coniferyl alcohol (400 /iM) (unpublished data). 

In our analysis of the chemical structures which are active tur-inducers (41) it was found that the compounds fell into four groups (1) acetophenones and related structures, (2) monolignols, (3) hydroxycinnamic acids and their esters, and (4) chalcone derivatives (Fig. 1). Each compound had either a guaiacyl or a syringyl nucleus, and with the exception of the monolignols, possessed a carbonyl group. Most were of common occurrence in vascular plants. 

Alkaloids 36-41 were isolated from Lupinus luteus L. seedlings. They are considered to be lupinine esters with 4-hydroxycinnamic acids (94-100). The structures of these new alkaloids were elucidated on the basis of H NMR, MS, and chemical and enzymatic transformations. All these alkaloids were obtained from lupinine and hydroxycinnamic acid by two enzymatic systems (96-97) ligase catalyzed formation of the CoA-thioester, and transferase catalyzed lupinine ester formation from the CoA-thioester. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

Chlorogenic acids occur ubiquitously in plants. They are esters of hydroxycinnamic acids with quinic acid. The structures of chlorogenic... 

Figure 11.2.3 Structures of the hydroxybenzoic acids (A), hydroxyphenylacetic acid (B), and hydroxycinnamic acids (C). Also see UNIT 11.3.

Figure 3-4. The general phenylpropanoid pathway. The enzymes involved in this pathway are (a) phenylalanine ammonia lyase (PAL E.C. 4.3.1.5), (b) cinnamic acid 4-hydroxylase (C4H E.C. 1.14.13.11), and (J) 4-coumaric acid CoA ligase (4CL E.C. 6.2.1.12). (a) depicts tyrosine ammonia lyase activity in PAL of graminaceous species. The grey structures in the box represent an older version of the phenylpropanoid pathway in which the ring substitution reactions were thought to occur at the level of the hydroxycinnamic acids and/or hydroxycinnamoyl esters. The enzymes involved in these conversions are (c) coumarate 3-hydroxylase (C3H E.C. 1.14.14.1), (d) caffeate O-methyltransferase (COMT EC 2.1.1.68), (e) ferulate 5-hydroxylase (F5H EC 1.14.13), and (g) caffeoyl-CoA O-methyltransferase (CCoA-OMT EC 2.1.1.104). These enzymes are discussed in more detail in Section 10.

Ralph, J., and Helm, R. F., 1993, Lignin/hydroxycinnamic acid/ polysaccharide complexes synthetic models for regiochemical characterization, in Forage Cell Wall Structure and Digestibility, H. G. Jung, D. R. Buxton, R. D. Hatfield, and J. Ralph, eds., ASA, CSCA, SSSA, Madison, WI, pp. 201-246. 

Phenolic compounds constitute one of the most numerous and widely distributed groups of phytochemicals in the plant kingdom. More than 8000 phenolic compounds have been described and this list continues to expand.49 Phenolic compounds exist as simple molecules, such as the phenolic acids, to highly polymerized structures, such as the proanthocyanidins. Harbome48 classified phenolics into 10 subclasses based upon their chemical structure these subclasses include the simple phenolics, phenolic acids, hydroxycinnamic acids, and flavonoids, among others. The flavonols represent one of the most commonly distributed classes of flavonoid compounds. 

Demethoxyabresoline (67) was obtained as a noncrystalline solid. Spectroscopic investigation revealed the presence of a phenolic OH, a 1-phenyl-quinolizidine system, and a trans-cinnamyl group. The stereochemistry at C-l, C-3, and C-5 was the same as in abresoline. The molecular formula C25H29N05 was established by mass spectrometry. The presence of fragment ions at m/e 259 (M —164) and 258 was characteristic of p-hydroxy-cinnamyl esters of the phenylquinolizidol (63a). The assigned structure 68 was confirmed by basic hydrolysis to 63a and p-hydroxycinnamic acid as well as by catalytic hydrogenation to a known dihydro derivative (52). 

In a simplistic way, phenolics can be classified according to the number of constitutive carbon atoms in the basic skeleton. With a basic skeleton of Cg, there are simple phenols and benzoquinones with Cg-Ci, there are hydroxybenzoic acids such as gallic acid with Cg-C2, there are the phenylacetic acids and with Cg-Cs, there is a larger class including hydroxycinnamic acids, coumarines, and chromones. Sometimes, these structures... 

The synthesis of novel phosphorane structures continues to be fashionable. When methyl diphenylphosphinite is heated with 2-hydroxycinnamic acid, the spirobicyclic acyloxyphosphorane (79) is formed. (See also Chapter 2). The now well established reaction of the tervalent phosphorus with compounds containing two suitably... 

Castellan et al. [171] have recently suggested that dityrosine protein residues or p-hydroxycinnamic acid are possible candidates for the fluorphores in pure cellulose. Other recent studies [172,173] have shown that typical cellulose processing conditions (e.g., hot alkali) can induce the formation of small amounts of aromatic structures from reducing end groups or hemicelluloses. However, many of the structures identified in these studies are quinones, which are at best weakly fluorescent. 

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