4,4 -Dihydroxytruxillic acid

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

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). 

Theoretically, trans-p-coumaric acid can produce 12 isomers depending on whether head-to-tail (4,4 -dihydroxytruxillic acid) or head-to-head (4,4 -dihydroxytruxinic acid) dimerizations occur with syn or anti and with cis or trans ring junctions (37). Mass spectrometric analysis of the tetra-TMSi derivatives showed that head-to-tail dimers split symmetrically on electron impact, whereas head-to-head dimers fragment asymmetrically (Figures 2 and 3) (33,35,38,39). Thus the tetra-TMSi derivative of 4,4 -dihydroxytruxillic acid has a mass spectrum similar to that of the bis-TMSi derivative of p-coumaric acid (33). 

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... 

Figure 2. Symmetrical splitting (mass spectrometry) of the tetra-TMSi derivative of 4,4 -dihydroxytruxillic acid.

There are few examples of lignans/neolignans in the non-woody monocotyledons, except for the formation of dihydroxytruxillic and truxinic acids in certain superorders. These metabolites are apparently formed via photochemical dimerization rather than by enzymatically engendered phenolic coupling i.e., the stereoselective and non-stereoselective coupling enzymes tentatively appear to be absent in the monocotyledons. This decreased reliance upon a full extension of the phenylpropanoid pathway presumably prevents the formation of woody tissue. This, in turn, helps to explain why monocotyledons are generally more readily amenable to biodegradation. 

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