Secondary Metabolites

It has been suggested that phenolic compounds have been chosen by natural selection,20 and may play several important ecological roles including defense and protection from environmental stress. Harbome21 suggested that phenolic compounds represent one method of defense utilized by plants against herbivores. Rhodes13 stated that the diversity of the secondary metabolites and their uneven distribution between and within plant families indicates that a single physiological role cannot be attributed to these compounds. Zobel19 reported that a compound that is an allelochemical in one species does not have to be an allelochemical in another. Therefore, defining the physiological and biochemical role(s) of a secondary metabolite can be very complex; however, Harbome22 suggested that compounds of the same group share a number of similar properties.

3.1 Chemistry, Distribution and Function of Coumarins in Plants

The biosynthesis of coumarins is complex and involves several different enzymes (Murray et al., 1982).8 Hydroxycinnamic acids and coumarins are phenylpropanoids22' 23 as they contain at least one phenylpropane (C6C3) structure. All of these compounds are derived from the aromatic amino acid phenylalanine. Phenylalanine is synthesized by the shikimic acid pathway and is converted by PAL to cinnamic acid, a key intermediate in phenylpropanoid biosynthesis.24' 25 From cinnamic acid, derivatives are synthesized by the substitution of hydroxyl and methoxy groups to the aromatic ring. One such derivative synthesized is o-coumaric acid.

Lactonization of o-coumaric acid produces coumarin.23> 25 The simplest coumarin (Figure 1) has no hydroxyl groups (i.e. all R groups are hydrogen) thus, coumarin itself does not absorb UV radiation nor does it possess an anti-oxidant potential. However, several hydroxylated derivatives of coumarin exist (Figure 2) and these compounds can absorb UV radiation,1 scavenge free radicals,27 and transduce UV radiation into visible light.1

Figure 1 The typical coumarin ring. Coumarin (CfljOJ has a melting point of 70 °C, a boiling point of 297°C, and contributes to the scent of many plants.26 It is a common volatile plant constituent and, depending upon the specific coumarin, the R groups may be H, OH, OCHy or O-Glucose. Coumarin is freely soluble in water, alcohol, and ether.26

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