Virtually all plants contain a wide variety of natural products which include an aromatic ring that generally contains one or more hydroxyl substituents. The vivid colors that light up the plants around us are derived generally from three sources: the tetrapyrroles, principally chlorophyll; the terpene-based carotenes, that we have already seen; and the aromatics, also referred to as the acetogenins. Several thousand aromatics are known and new structures are continously being discovered. In some cases their functions are well known. For example, the polyphenolic lignins serve as structural components of the cell wall. In other cases, including the flavonols, a variety of functions have been hypothesized depending on the particular compound being investigated. Aromatic compounds are formed by several biosynthetic routes, including the polyketide and shiki-mate pathways, as well as from terpenoid origins. Due to the acidity of the phenol functionality (pKa of 8 to 11 depending on substituents), phenolic substances tend to be water soluble and frequently form ether linkages with carbohydrate residues. Several individual groups exist that will be considered separately.
FIGURE 1.16 Some non-phenolic aromatic plant natural products. 1.3.1 Non-Phenolic Aromatics
The vast majority of aromatic compounds of plant origin contain one or more hydroxyl functionalities. Several important natural products, however, do not contain this functionality and many will be treated in separate sections. These include the amino acids, tryptophan and phenylalanine, the indole alkaloids, and auxin (indole-3-acetic acid), an important plant hormone (Figure 1.16).
The chlorophylls are perhaps the most well-known plant constituents. As the primary catalysts of photosynthesis, they occur in several similar cyclic tetrapy-role forms and are located in the chloroplasts of virtually all photosynthetic plant tissues (Figure 1.17).
Other porphyrin pigments occur in plants in much smaller amounts. The cytochromes, for example, are critical components in the respiratory chain of both plants and animals. Finally, the linear tetrapyrroles, including phytochrome, phy-coerythrin, and phycocyanin, are believed to be critical components for plant morphogenesis, the process by which numerous important plant developmental processes are initiated.
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