Ferulate dimers

The presence of feruloyl and p-coumaroyl acids linked via L-arabinose residues has been verified in several studies of xylan (6-8). The amounts of these components, however, are rather small. Every 15th arabinose unit in barley straw arabinoxylan is estimated to be esterified with ferulic acid, and every 31st with p-coumaric acid (7). These hydroxycinnamic acids are bound to C-5 of the arabinose residue (7, 8). It has been suggested that oxidative dimerization of ferulic acid residues crosslinks the arabinoxylan chains and renders them insoluble as a result of the diferuloyl bridges (9). 

Dimeric forms of ferulic and p-coumaric acids also occur in bound form within plant cell walls. The most common of these is didehydroferulic acid 23, which has been isolated from some plants of the Gramineae (53,54) and spinach cell cultures (7). While no such p-coumaric acid 2 derived dimer (i.e., C5-C5 linked) has ever been reported, cell walls of Lolium multiflorum contain another intriguing structural variation, namely 4,4 -dihydroxytruxillic acid 24 (5,55). 

It is well established that cinnamic acid and some substituted cinnamic acids (including frans-p-coumaric acid but not ferulic acid) can be dimerized in vitro by sunlight to truxillic and truxinic acids and their derivatives... 

It therefore seems probable that, in the cell walls of the growing plant, dimerization of p-coumaric and ferulic acid units linked to arabinoxylan occurs under the influence of sunlight. Earlier work (34) showed that trans-p-coumaric acid converts readily in sunlight to 4,4 -dihydroxytruxillic acid. It has recently been shown (40) that FA-FA and PCA-FA type cyclodimers can be obtained in low yield (less than 10%) by irradiating mixtures of the... 

The only stem materials examined were from the Setaria anceps and Digitaria decumbens grasses. The major substituted cyclobutane dimers in these walls appeared, from the mass spectral data, to be mixed dimers of ferulic acid and coniferyl alcohol (FA-ConAlc type). Further examination of the aromatics of stems is required but if such dimers are present then they may well be involved in the biosynthesis of lignin (41,42). 

In the case of ferulic acid, dimers are known one of these has a difuran structure and another a benzofuran structure. 

The phenolics ( + )catechin and (— )epicatechin are common flavanols in several fruits (128). Apples and pears contain other phenolic compounds such as quinic, shikimic, chlorogenic, and caffeic acids (39). Durkee and Poapst (162) reported that the two major phenolic constituents of core tissues and seeds of McIntosh apples were chlorogenic acid and phloridzin. After hydrolysis of extracts from core tissues, the identified phenolics were phloretin, caffeic acid, p-coumaric acid, phloretic acid, and trace amounts of ferulic acid. Studies have shown that apple leucoanthocyanins yield catechin, epicatechin, cyanidin, and pelargonidin after hydrolysis (163, 164). Van Buren et al. (164) also reported that a purified leucoanthocyanin from apples was either a dimer or oligomer containing ( —) epicatechin, and 5,7,3, 4 -flavin-3,4-diol. 

The ubiquitous plant compound chlorogenic acid (isolated from green coffee beans) is formed by transesterification with the glycoside cinnamoyl-glucose.187 Coumaroyl-CoA is converted into monomeric and dimeric amides with agmatine, which provides barley plants with resistance to mildew.188 Similar compounds with various polyamines and derived from p-coumaric, caffeic, ferulic, or sinapic acid appear to... 

Thioacidolysis allows the distinction between products derived from lignin and products derived from />coumaric and ferulic acids, and the distinction between products derived from cinnamaldehydes and cinnamyl alcohols. Recent improvements have made it possible to estimate the fraction of free phenolic groups in uncondensed lignin (see Section 1.3.1), and to depolymerize the dimers, so that they can be included in the analysis of the lignin composition. 

Ferulic acid has often been reported as a model compound for the investigation of bioavailability of phenolic acids, as free or esterified forms (Table 2.3), since it is abundantly present in food. Absorption via the gastrointestinal tract has also been found for dimers of ferulic acid, or diferulic acids, released from cereal brans [Andreasen et al., 2001b]. 

Figure 2.8 Summary of behavior of phenolic acid esters and free forms in the GIT and the liver. 5-O-CQA, 5-O-caffeoylquinic acid 5-O-FAAF, 5-O-feruloyl-L-arabinofura-nose C, conjugate (sulfate, glucuronide or methyl) CA, caffeic acid FA, ferulic acid FA-FA, dimer of ferulic acid FA-oligosac., ferulic acid esterified to an oligosaccharide FA-Xy, ferulic acid esterified to xylan R, alkyl group.

Title Ferulic Acid Dimers and Their Pharmaceutically Acceptable... 

Stereoselective oxidative dimerization of cinnamyl derivatives bearing an Oppolzer sultam as chiral auxiliary has been performed both enzymatically (HRP/H2O2) and by chemical means with Ag20. This method provides an enantioselective access to dehydrodiconiferyl ferulate (Equation 148) <2001T371>. 

The electrooxidations of allyl- and propenyl-phenols may follow even more different pathways leading to a rich diversity of products. At a carbon anode in methanol-lithium perchlorate, ferulic acid formed the tricyclic diacid (306 c/. Section 2.9.5), in a concentration-dependent process probably involving [4 + 2] dimerization of the primary oxidation product, the dienone acetal (307). 

Dimers 39 and 40, issued from grass samples, reveal the linkage type of p-cou-maric and ferulic esters and show that some ester bonds survive thioacidolysis. Dimer 39 confirms thatp-coumaric acid is ester-linked at Cy of S lignin units [60,61]. Compound 40, tentatively assigned from its mass spectral fragmentation pattern, may originate from ferulic units ester-linked to pentose units [85]. 

Lignans are dimers of phenylpropanoid units linked by the central carbons of their side chains while naturally occurring dimers that exhibit linkages other than this type of bond are known as neolignans. Much attentions has been devoted to the oxidative, acidic or alkaline dimerisations of arylpropenoid molecules and particularly 4-hydroxycinnamic acids such as ferulic and caffeic acids [24-34]. 

Ferulic acid is dimerized in the plant cell wall by peroxidases to several neolignans. In fact, class III plant peroxidases, such as that purified from barley, are able to oxidize ferulic acid with fa values of 2.9 pM-1 s 1 at pH 3. 96 [34], The main products of ferulic acid dimerization by peroxidase are 5,8 -diferulic acid (XX), 5,5 -diferulic acid (XXI), 8-0-4 -diferulic acid (XXII) and a dihydrobenzofuran dimer /rans-5-[( T)-2-carboxyvinyl]-2-(4-hydroxy-3-methoxyphenyl)-7-methoxy-2,3-dihydrobenzofuran-3-carboxy-lic acid (XXIII) derived from a 5-8 coupling mechanism (Scheme X) [100],... 

Although it is generally accepted that ferulic acid dimers (XX-XXHI) are generally less effective than ferulic acid (XIX) as antioxidants, when this property is evaluated through their free radical scavenging properties [100], it should be mentioned that the n-buthyl ester of the dihydrobenzofuran dimer (XXIV) obtained by oxidative dimerization of ferulic acid showed more potent cytotoxicity against several tumor cell lines that ferulic acid itself [101],... 

The fatty acid spectrum of rice bran oil is 22-25% palmitic acid, 37-41% oleic acid and 37-41% linoleic acid. More recently, interest in rice oil escalated with its identification as a healthy oil that reduces serum cholesterol. Rice bran is a good source of antioxidants including vitamin E and oryzanol (ferulic esters of sterols and triterpene alcohols), cholesterol-lowering waxes and antitumor compounds like rice bran saccharide.Besides applications in nutrition and in phyto-chemicals, rice bran oil has traditionally been used for industrial applications, such as dimer acid manufacturing, depending on pricing for alternative vegetable oils. 

Phenylalanine and p-coumaric acid, but not 3,4-dihydrox-ycinnamic (caffeic) acid or acetate, were incorporated into podophyllotoxin (51). In Podophyllum hexandrwn (Berberi-daceae), both cinnamic and ferulic acids are incorporated into both halves of the molecule. The carbon of the 0-methyl group of ferulic acid is distributed into both rings A and C. Sinapic and 3,4,5-trimethoxycinnamic acids are not incorporated. This suggests that other oxidation and methylation occurs after the initial dimerization reaction (Jackson and Dewick, 1984 Pelter, 1986 Poulton, 1981). 

Cross-linking between polysaccharide-polysaccharide units can occur in different forms such as ester-linked hydroxycinnamate dimers (Ralph et al., 2004), ferulic acid and/orp-coumaric acid monomers as cyclic dimers (Grabber et al., 2004), and direct linkage between polysaccharides and xyloglucans (Harris and Stone, 2009). Some important interactions between lignin and polysaccharides in the cell walls are the following ... 

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