Structure

Taraxerol is an oleanan-3-ol with an alpha-methyl substituent at position 13, a missing methyl group at position 14, and a double bond between 14 and 15. The dominant biological stereoisomer in plant leaves and in sediments has the taraxer-14-en-3β-ol configuration. Taraxerol is a double-bond isomer of β-amyrin, another important naturally-occurring triterpenoid in higher plants. It is a colorless solid under room temperature with an estimated melting point of 283.50 °C and boiling point of 490.70 °C. It is practically insoluble in water and has a solubility of 9.552 × 10⁻⁵ mg/L estimated from octanol-water partition coefficient.^[4]

Taraxerol carbon numbering.

Synthesis

While syntheses of pentacyclic triterpenoids in general have been proven challenging, partial synthesis of 11,12-α-oxidotaraxerol, an epoxide taraxerene derivative, has been reported by Ursprung et al. from α- and β-amyrin. Exposing an ethanolic solution of α- and β-amyrin in summer sunlight for 12 weeks yields a colorless precipitate, and saponification of the precipitate gives 11,12-α-oxidotaraxerol. Alternatively, the process could be accelerated by exposing ethanolic β-amyrin solution under ultraviolet light. In this case, the precipitate can be collected in less than 3 weeks.^[5]

Transformation in sediment

During early diagenesis, taraxerol loses its hydroxyl group and gets transformed to taraxer-14-ene. Taraxer-14-ene can undergo rapid isomerization to form 18β-olean-12-ene, in which the double bond can migrate and form a mixture of olean-12-ene, olean-13(18)-ene, and olean-18-ene. The oleanene isomers form rapidly from taraxerol rearrangements during diagenesis even under cool geothermal conditions.^[6] Further reduction during catagenesis of the three compounds gives predominantly 18α-oleanane and its counterpart 18β-oleanane as a minor product. The direct reduction product of taraxerol, taraxerane, is hardly present in natural sediments. Oleanane seems to be the dominant product as a result of the transformation process.^[7]

Transformation of taraxerol during diagenesis and catagenesis. Adapted from Killops & Killops (2013).^[7]

Mangrove biomarker case study

Rhizophora racemosa represents the dominant mangrove species in equatorial and sub-equatorial west Africa. Versteegh et al. analyzed the leaf lipids of R. racemosa as well as surface sediments and sediment cores from Angola Basin and Cape Basin (southeast Atlantic) to assess the suitability of using taraxerol as a proxy for mangrove input in marine sediments. The hypothesis is that there should be a "base-level" for taraxerol in general sediments and elevated levels at places where Rhizophora has significant contribution.

Analysis suggests that taraxerol dominates the inside and the total composition of R. racemosa leaves (7.7 mg/g leaf). As a result, increase in taraxerol level relative to other higher plant biomarkers in sediments should indicate when and where Rhizophora contributes substantially. In the most part of SE Atlantic, taraxerol/normal C₂₉ alkanes (n-C₂₉) ratio in surface sediments is low. High ratios are observed in a zone along the continental slope, in which maxima always occur near present-day on shore mangrove trees. This pattern strongly corroborates the link between high levels of taraxerol and input from mangrove ecosystems. This link is also supported by a similar, though less prominent, trend in Rhizophora pollens.

Examination of the sediment cores reveals further connections between mangrove population, taraxerol levels, and climate conditions. One important climate condition is glaciation/deglaciation. During deglaciations when rates of sea level rise exceeded 12 cm/100 yr, mangrove populations could not persist due to lack of sediment supply.^[10] After this rate slowed down, mangrove populations can expand again in the freshly developed estuaries and deltas.^[11][12] Periods of mangrove development and rise in taraxerol levels in the basin, however, sometimes do not coincide with each other. In times of fast sea-level rise, coastal mangrove deposits can be transported to the basin, resulting in an increase in taraxerol input, while mangrove development would actually happen afterward. In some other cases where fluctuation in taraxerol levels was not related to sea-level changes, it can also be attributed to local climate variations in temperature and humidity.^[1]

GC/MS

There are several treatment procedures before running leaf or sediment samples containing taraxerol through GC/MS analysis. Dried and grinded samples are saponified with strong base (e.g. potassium hydroxide), extracted in polar solvent (e.g. dichloromethane), separated into fractions by column chromatography, and finally derivatized. Common choices for derivatization include N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) and mixture of pyridine and bis(trimethylsilyl)trifluoroacetamide (BSTFA), both of which aim to convert the free hydroxyl groups to trimethylsilyl ethers, making the molecules more non-polar and thus more suitable for GC/MS analysis.^[1][9] In GC/MS, taraxerol has a signature peak with a mass-to-charge ratio (m/z) of 204.^[1]

Total ion current traces of R. racemosa saponified leaf extracts, showing taraxerol-OTMS (6) with β-amyrin methyl-ether (7) and germanicol-OTMS (8). Adapted from Versteegh et al. (2004).^[1]

HPTLC

Alternatively, determination and quantification of taraxerol can also be achieved with good reliability and reproducibility using HPTLC. In this case, linear ascending development is performed (e.g. using hexane and ethyl acetate (8:2 v/v) as mobile phase) in a twin trough glass chamber on TLC aluminum plates. Quantification can be achieved by spectrodensitometric scanning at a wavelength of 420 nm.^[13]