Hydroxycinnamic conjugates

Phenolic acids are rarely present as free forms, except in processed food, but occur more frequently as soluble or insoluble esters. These esters are formed with polysaccharides or simple sugars, with quinic acid or other carboxylic acids such as tartaric or shikimic acids [Herrmann, 1989], with other phenolic acids, with lipids [Clifford, 2000], with sterols or glycerol [Clifford, 1999], or with amino acids [Clifford and Knight, 2004], To quinic acid, they can be conjugated as mono-, di-, tri-, and tetra-esters [Clifford, 2000]. The multiple esters can contain the same or different hydroxycinnamic acids. Among the hydroxycinnamic conjugates, caffeoylquinic and di-caffeoylquinic acids... 

General phenylpropanoid pathway and formation of hydroxycinnamate conjugates... 

Of course, classes/groups of metabolites which occur (almost) ubiquitously or at least frequently in the plant kingdom ( general secondary metabolites ) are also constituents of both large Solanales families (i) phenolics such as simple cinnamic acid derivatives (Sect. 6.4), hydroxycoumarins (Sect. 6.6), hydroxycinnamate conjugates (Sect. 6.7), flavonoids (Sect. 6.8), lignans (Sect. 6.9), (ii) sterols (Sect. 7.6), (iii) carotenoids (Sect. 7.12), (iv) fats/oils and fatty acids (Sect. 8.1), (v) carbohydrates (Sect. 8.2) etc. 

Hydroxycinnamate conjugates are defined as condensation products of hydroxycin-namates (preserving their structure) with - predominantly - alkanols (Sect. 6.6.1) or alcoholic hydroxyl groups of other molecules e.g., quinic acid (Sect. 

In certain fruits the disappearance of phenolic acids may occur in relation to the biosynthesis of other phenolic compounds, for example, the decrease in hydroxycinnamic conjugates during growth and maturation of Vitis vinifera berries and the rapid increase in anthocyanin levels that occurs at the same time in the red cultivars [2]. In fruits of chili pepper (Capsicum frutescens) the onset of capsaicinoid accumulation and ligninlike material parallels the disappear-... 

Hydroxycinnamic acids and their derivatives (free, soluble conjugated, cell wall bound)... 

Keywords phenylpropanoid metabolism hydroxycinnamic acid conjugates lignans coumarins furanocoumarins gallotannins eUagitarmins metabolic channelling... 

Important hydroxycinnamic acid conjugates from plants are hydroxycin-namic acid esters and hydroxycinnamic acid amides (see Petersen et al, 1999). 

Moreover, hydroxycinnamate-CoAs can conjugate with organic acids. In the biosynthesis of sugar derivatives of hydroxycinnamic acids, the transfer of glucose from uridine diphosphoglucose to hydroxyciimamic acid is catalyzed by glucosyl transferase [8, 16]. 

Phytosterols (PS) are plant sterols or stands found in plants. Plant sterols belong to the triterpene family and differ from cholesterol by having a methyl or ethyl group in C24. Plant stanols, on the other hand, are the saturated form of the plant sterols (Fig. 113.1). PS are present in free or conjugated form as fatty-acyl esters, hydroxycinnamate steryl esters, steryl glycosides, or acylated steryl glycosides. The main function of plant sterols/stanols is to stabilize plant membranes and serve as precursors in the synthesis of steroidal saponins, alkaloids, and other steroids [1]. 

For ipobscurines, hydroxycinnamic acid conjugates of serotonin (5-hydroxytryp-tamine), see Sect. 6.6.4 for withanamides, fatty acid amides of serotonin, see Sect. 8.I.2.I. 

Amides of the polyamine spermidine bearing aliphatic or hydroxycinnamic acyl residues are widely distributed in the plant kingdom (Bienz et al. 2002). In most cases the acyl moieties represent the latter residnes. For snch conjugates see Sect 6.6.4.3. A novel metabolite with an aUphatic acyl residue, A/ A/ -ditigloylspermi-dine, has been isolated from the seeds of Ipomoea nil (L.) Choisy collected on Zanzibar/Tanzania (Schimming et al. 2005). It turned out to dominate to an unusual extent in the alkaloid fraction beside seven minor congeners. Most of them were isomers of the main metabolite. All these metabolites were also present in unripe fruits and stems. However, they were not detectable in snch parts from another provenance (Ecnador) of this species (seeds not checked). 

Hydroxycinnamates as well as their conjugates may act as powerful antioxidants. In edible plants they have received much attention as protecting agents against oxidative deterioration of food. Antioxidant mechanism studies on ferulic acid and its coupling products with linoleate on the molecular level have been conducted recently. A radical scavenging reaction occurred at the 3 -position of the ferulate radical with four types of peroxyl radicals of ethyl linoleate. The produced peroxides subsequently underwent intramolecular rearrangement to afford stable tricyclic peroxides (Masuda et al. 2006). 

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