Monoterpenes sites

Included in the table are data for L. scoparium obtained from five sites in Australia. The Australian material exhibited a level of a-pinene approaching that of the northern race from New Zealand, but was clearly characterized by the highest level of 1,8-cineole observed in the study, along with the highest value for total monoterpenes and lowest value for total sesquiterpenes. The authors noted the need for further taxonomic study, particularly of the Australian material. 

The third paper in this set Zavarin and Snajberk (1976) described their efforts to detect chemical races within big cone Douglas fir. Analysis of the cortical monoterpenoid fraction of 33 trees revealed that the major component was a-pinene, with P-pinene, 3-carene, and limonene present in lesser amounts. The monoterpene profiles of different populations varied somewhat from each other, but the overall profile of big cone Douglas fir was clearly different from that of Douglas fir. There was no evidence for gene flow between the southernmost population of Douglas fir at Lompoc and the closest population of big cone Douglas fir at Figueroa, sites separated by only 34 km. 

Table 5.5 Variation in selected halogenated monoterpenes in Plocamium cartilagineum coUected from sites in Chile (from San-Martin and Rovirosa, 1986)...

The activity of the FePeCli6-S/tert-butyl hydroperoxide (TBHP) catalytic system was studied under mild reaction conditions for the synthesis of three a,p-unsaturated ketones 2-cyclohexen-l-one, carvone and veibenone by allylic oxidation of cyclohexene, hmonene, and a-pinene, respectively. Substrate conversions were higher than 80% and ketone yields decreased in the following order cyclohexen-1-one (47%), verbenone (22%), and carvone (12%). The large amount of oxidized sites of monoterpenes, especially limonene, may be the reason for the lower ketone yield obtained with this substrate. Additional tests snggested that molecular oxygen can act as co-oxidant and alcohol oxidation is an intermediate step in ketone formation. 

If artificial castoreum scent marks are placed on the banks of a pond, beavers, C. canadensis, are more likely to visit, destroy, and re-mark the sites as the complexity of the artificial odor composition increases (Fig. 6.14). While some single phenolics from castoreum such as 4-ethylphenol trigger marking (Miiller-Schwarze and Houlihan, 1991), the response increases as the mixture grows to 4, 6,10,13, and finally 15 compounds. A mixture of 14 phenolics and 12 neutrals (mostly oxygenated monoterpenes) released responses almost as strong as whole castoreum (Schulte etal., 1995). 

A descriptor for an enzyme active site that permits binding of a family of related compounds (e.g., mimics of the reaction intermediate) that can be derived from the initial binding and conformational changes in the substrate. This concept arose from the observation that a number of monoterpene cyclases were incapable of discriminating between enantiomers of the reaction intermediate, even though the enzyme catalyzes the synthesis of an enantiomerically pure product from an achiral substrate. An example is trichodiene synthase which catalyzes the cyclization of farnesyl diphosphate to trichodiene. 

Mono- and bi-cyclic monoterpenes containing sites of unsaturation tend to be hydroxylated at the allylic position, (more examples of allylic hydroxylation are discussed in Section 1.4.5.2) with regio-isomers occurring if more than one allylic position is accessible. Some illustrative examples of reported hydroxylations are shown in Scheme 10. " The hydroxylation of 1,4-cineole (42) is illustrative of the enantioselectivity that may be achieved in such transformations. Bacillus cereus gives a 1 7 mixture of 2R)-exo- and (25)-e/tdo-monohydroxy-1,4-cineole, both with essentially 100% enantiomeric purity. ... 

In older usage, particularly for monoterpenes, nor denotes loss of all methyl groups attached to a ring system, e.g., norborane, norpinane. The plural form should be bisnor when two carbon atoms are lost from the same site and dinar where they are lost from different sites, bnt in practice the terms are nsed interchangeably. o-. Abbreviation of ortho-. 

Sites of biosynthesis are compartmentalized in the plant cell. While most biosynthetic pathways proceed (as least partially) in the cytoplasm, there is evidence that some alkaloids (such as coniine, quinolizidines and caffeine), furanocoumarins and some terpenes (such as monoterpenes, diterpenes. 

The prenyltransferases that catalyse the s)mtheses of GPP, FPP and GGPP may be important regulatory enz)mies in plant terpenoid bios)mthesis since they are situated at the primary branch points of the pathway, directing flux among the various major classes of terpenoids. The level of prenyltransferase activity is, in fact, closely correlated with the rate of terpenoid formation in many experimental systems (Dudley et at, 1986 Hanley et at, 1992 Hugueney et at, 1996) consistent with the regulatory importance of these catalysts. The localization of specific prenyltransferases in particular types of tissue or subcellular compartments may control the flux and direction of terpenoid synthesis at these sites. For example, the GPP synthase in Salvia officinalis is restricted to the secretory cells of the glandular trichomes, which are the sole site of monoterpene bios)mthesis in this species (Croteau and Purkett, 1989). 

Cyclization of an allylic pyrophosphate is a key step in the biosynthesis of most monoterpenes. Early hypotheses concerning the nature of the acyclic precursor and the cyclization process are first described, and chemical models for the cyclization presented. Following a review of several representative cyclase enzymes and the reactions that they catalyze, a series of stereochemical and mechanistic experiments with partially purified cyclases are reported. The results of these studies have allowed a detailed description of events at the active site and the formulation of a unified stereochemical scheme for the multistep isomerization-cyclization reaction by which the universal precursor geranyl pyrophosphate is transformed to cyclic monoterpenes. 

Another procedure involves enzymatic hydrolysis of monoterpene glycoside mixtures to liberate monoterpenes which can then be analyzed by gas chromatography (26,34). Such methods provide identifications and give good quantitative data for the important aglycon, but not the sugar moieties. When polyols are released by the enzyme, no evidence can be obtained about the site of attachment of the carbohydrates to the terpenols. 

The distribution of monoterpenes among separate fractions of the grape. As a prerequisite to any biosynthetic studies on terpenes in grapes the sites of production and storage within the berry must be known. Such knowledge may have considerable practical consequences by providing valuable guidance in the application of skin contact and press conditions to optimize flavorants in juice. 

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