Aroma, phenylpropanoid-derived

The smell and taste of plants rely on aroma and fragrance compounds, many of which (besides fhe terpenoids) are derived from phenylpropanoid metabolism. In food and cosmetic industry, such fragrance and aroma compounds play an important economical role. Simple phenolic fragrance compounds are, e.g., eugenol, isoeugenol or (methyl)chavicol (Fig. 4.2), the biosynthesis of which has been clarified recently more complex compounds are phenolic esters. Evolutionary aspects of the bios)mthesis of flavours and scents have been reviewed by Gang (2005). 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

The volatile fractions of many plants are isolated by steam distillation, distillation, or by solvent extraction (Banthorpe, 1991). The resulting volatile compounds that make up the essence or aroma of plants are called essential oils. Altiiough essential oils are comprised of many types of compounds, monoterpenes are major among them. These oils may also contain metabolically modified fatty acids, aldehydes, hydrocarbons, esters, phenylpropanoid compounds, acetylenic compounds, volatile alcohols, volatile alkaloids, phenylpro-panoids, and other shikimic acid derivatives. 

Essential oils are volatile compounds responsible for the aromas commonly associated with many plants (see essay "Terpenes and Phenylpropanoids")- The chief constituent of the essential oil from cloves is aromatic and volatile with steam. In this experiment, you will isolate the main component derived from this spice by steam distillation. Steam distillation provides a means of isolating natural products, such as essential oils, without the risk of decomposing them thermally. Identification and characterization of this essential oil will be accomplished by infrared spectroscopy. 

Polyketides with one or two phenylpropa-noid moieties and only one carbon derived from malonyl-Co A are the raspberry aroma/>-hydroxy-phenylbutan-2-one, the curcuminoids of turmeric Curcuma), and phenylphenalenones from Musa dmd Anigozanthos species (Fig. 23). Curcuminoids and phenylphenalenones contain two phenylpropanoid moieties on both sides of the acetate-derived carbon and are therefore classified as diaryIheptanoids. These two groups of natural products are pigments due to their conjugated system of 7i-electrons. Curcuminoids are widely used as spices and also have antiinflammatory, antioxidative, and anti-cancer properties (Joe et al., 2004). 

Phenylpropanoids and benzenoids constitute the second most ubiquitous class of plant volatile compounds [9] and are derived from the amino acid L-phenylalanine (Phe) (Fig. 2). Despite their abrmdance, diversity, and importance in the scent and aroma of countless plant species, the precise biochemical pathways leading to the formation of volatile phenylpropanoids and their derivatives are still mostly unknown. The first committed step through the... 

Flavor and aroma of fruit and scent of flowers is represented by a mixture of dozens of volatile components [160, 161]. Most of these volatiles are derived from three chemical groups, namely isoprenoids, phenylpropanoids and aliphatics. Several volatiles in the mixture will have a dominant effect on the overall fruit aroma or flower scent that is independent from their relative levels. Namely, trace levels of a certain component might have a substantial effect on the typical aroma and scent associated with a particular fruit or flower. Due to promiscuity of enzymes associated with volatile compounds metabolism, a relatively large number of volatile components could be altered by modifying a single gene. For instance, overexpression of the ( )-j3-caryophyllene synthase from rice in Arabidopsis results in emission of a bouquet of terpenoid volatiles, including (EyP-caryophyllene, j8-elemene and a-humulene... 

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