User:KoThalha/Abietane

Introduction
Abietanes, including abietic acid and other resin acids, are a class of diterpenoid organic compounds found in the tissues and resins of certain higher plants, particularly gymnosperms. Although the functions of terpenes are not fully understood, conifers appear to produce abietane diterpenoids as a form of defense against insect and microbial attack. Some abietane diterpenoids, especially aromatic abietenes, are of interest to the pharmacology and natural products communities for their potential biological activities. In the rock record, abietanes are commonly found in amber as well as in fossil wood, sometimes in the form of the mineral fichtelite. Additionally, abietanes are observed in sediments -- both riverine and marine -- and in coals, where they are often interpreted as geochemical biomarkers for terrestrial input from conifers.



Chemical structure and properties
Abietanes are tricyclic 20-carbon diterpenoids characterized by three fused six-membered rings and alkyl functional groups at carbons 4, 10, and 13. In higher plants, abietanes and other diterpenoids are synthesized from four five-carbon isoprene units. Abietanes are generally nonpolar, volatile, and less dense than water. The presence of one or more polar functional groups (typically a carboxylic acid or alcohol) tends to increase the polarity and boiling point of a given abietane relative to its unsubstituted hydrocarbon form.

Biological sources and synthesis
In higher plants, abietanes are synthesized from geranylgeranyl diphosphate (GGPP) via a copalyl diphosphate (CPP) intermediate. First, GGPP is cyclized by a class II diterpene cyclase enzyme to CPP. The conformation of the GGPP molecule dictates the stereochemistry of the CPP intermediate after cyclization. The stereochemistry of the typical abietane skeleton suggests a GGPP precursor in a chair-chair-"normal" conformation. After the initial cyclization to CPP, which forms rings A and B in the abietane skeleton, the C ring is formed with the help of a class I diterpene synthase enzyme. Subsequent methyl migration and dehydrogenation steps yield the abietene isomers.

Preservation and diagenesis
Mention of biological degradation of diterpenoids or terpenoids generally? Diagenetic transformation of biomolecules is not fully understood, but several broad diagenetic patterns are hypothesized to affect the transformation of abietanes as they are heated and pressurized in sediments. The first of these patterns is defunctionalization. In particular, the reducing conditions of diagenesis are believed to cause abietanes to lose oxygen-containing functional groups, including carboxylic acids and alcohols, as well as methyl groups. In addition to defunctionalization, abietanes likely undergo dehydrogenation and aromatization reactions to form more energetically stable systems of conjugated pi bonds in their characteristic three ring structure. The hypothesized diagenetic pathway of abietic acid is illustrative of these general patterns. Abietic acid is dehydrogenated to dehydroabietic acid, which then loses its carboxylic acid functional group to become dehydroabietin. Loss of the 5-Me group and further dehydrogenation form the aromatic 1,2,3,4-tetrahydroretene molecule. Final aromatization produces retene, a common biomarker molecule observed in sedimentary samples.

Measurement Techniques
Abietanes found in modern gymnosperm resins as well as in the rock record are separated and characterized by gas chromatography-mass spectrometry (GC-MS). Because polar functional groups reduce molecular volatility and make separation by gas chromatography difficult, abietane derivatives containing carboxylic acid and alcohol moieties are often derivatized with trimethylsilyl groups by treatment with BSTFA prior to GC-MS analysis. More oxidized abietane derivatives have been studied using thermally assisted methylation using tetramethylammoniumhydroxice (TMAH) followed by GC-MS analysis. MS-MS analysis has been used to elucidate fragmentation mechanisms for mass spectrum peaks of interest. Mass spectra for abietic acid and some other common abietanes are publicly available in the NIST database. The spectrum for abietic acid possesses characteristic peaks at m/z = 256 and 241.

Use as a biomarker
Abietanes preserved in geological settings are typically interpreted to derive from gymnosperms, specifically conifers. Although both modern angiosperms and modern gymnosperms synthesize terpenoids, gymnosperm tissues tend to contain significantly higher terpenoid concentrations than angiosperm tissues. Additionally, the relative abundances of di-, tri-, and penta-cyclic terpenoids varies between gymnosperms and angiosperms. Although some angiosperm families (notably Burseraceae, Euphorbiaceae and Ranunculaceae) are also known to produce abietanes, in general, tricyclic diterpenoids, including abietanes, are much more abundant in gymnosperms. For these reasons, and because conifers produce significant biomass relative to other gymnosperms, abietanes preserved in geological settings are typically interpreted as conifer biomarkers. It is important to note, however, that such interpretations rely on the assumption that terpenoid distributions and abundances in ancient plants were similar to those in modern plants. Loss of more volatile mono- and sesquiterpenoids during diagenetic heating may explain the relative abundance of diterpenoids, including abietanes, in ancient resins and the rock record compared to modern conifer samples.

Examples from archaeology

 * Abietenes from colophony, tar, and pitch have been identified in caulking used on ancient ships.
 * Abietanes have been used to identify conifer resins associated with Egyptian mummies.
 * The ratio of oxidation products of abietanes including dehydroabietic acid and de-7-oxo-dehydroabietic acid and 15-hydroxyl-7-oxo-dehydroabietic acid have been used to estimate the oxidation state of varnish on Vermeer's famous painting, "Girl with a Pearl Earring."

Examples from geology/geochemistry
Isotopic measurements: Paleocene-Eocene carbon isotope excursion observed in conifers and angiosperms using characteristic biomarkers (abietanes for conifers)

Abietanes found in marine sediments have been used as an indicator for terrestrial input to marine environments. (Note: I have this reference but need to locate it and read it more thoroughly)

Abietanes found in oxbow lake sediments

Diterpenoid 13C measurements in modern gymnosperms

Abietanes in fossil wood, amber, coal

Extras
Terpenes are hydrocarbon molecules composed of linked 5-carbon isoprene subunits well known for their role in the characteristic fragrances of plants, including herbs, citrus, conifers, and cannabis. Some terpenes like the diterpene (consisting of 4 fused isoprene subunits for a total of 20 carbons) taxanes have biological activities useful to humans and are therefore of interest to pharmacologists and natural products chemists.

Abietane diterpenoids, including abietic acid, are produced by higher plants, especially gymnosperms. While higher plants are known to produce an incredibly diverse array of terpenoid molecules, the specific functions of most of these organic molecules are not fully understood. Abietane diterpenoids are believed to play a defensive role, deterring attack by herbivores, insects, and microbes. (Note: find microbe ref.).

Abietanes are diterpenoids synthesized from four five-carbon isoprene units. Biosynthesis

In rosemary, abietane-type diterpenes are synthesized from geranylgeranyl diphosphate via a copalyl diphosphate intermediate (Note: rosemary is an angiosperm...). Abietadiene synthase enzymes

Diterpenoid 13C measurements and some information on biosynthesis in modern gymnosperms

abietic acid isomers, abietane, and phyllocladane undergo defunctionalization and aromatization, losing carbon and hydrogen atoms to form phenanthrene derivatives like dehydroabietane, simonellite, and eventually retene.

Diterpenoid abietanes are better preserved in the rock record than their more volatile mono- and sesquiterpenoid counterparts.