Investigating Biogenic Sources of Enantiomers and the Effect of Drought on the Emissions of Chiral Compounds
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Abstract
Chirality is the name given to explain structures that exist in mirror-image forms. In chemistry, it occurs when a compound contains an asymmetric carbon centre, giving rise to two configurations known as enantiomers ((-) and (+)) which are chemically the same but cannot be superimposed onto one another. Traditionally, studies on the analysis of biogenic volatile organic compounds (BVOCs) in the atmosphere have commonly failed to separate enantiomers since enantiomers share many of the same properties, such as boiling point and reaction rates with atmospheric oxidants. Yet biological systems are known to be enantioselective, resulting in imbalances of enantiomers seen throughout plants. At present, the reason why different species of plants have evolved to emit a certain ratio of enantiomers still largely remains a mystery. This thesis addresses the following four topics:
1. Determining the enantiomeric signatures of monoterpene emissions from branches and stems of pine trees (Pinus pinaster Ait.), and the soil in a Mediterranean maritime pine tree plantation. The enantiomeric signatures of a tree were previously thought to be uniform across the whole of the tree’s structure. In this study, branch emissions contained a higher percentage of (+)-pinene enantiomers to the sum of total enantiomers than the emissions from the stem and the soil. Hitherto neglected sources of volatile organic compounds (VOCs), soil and stems, contributed significantly to the overall VOC flux, most notably under high moisture conditions. Hence, these results suggest that the enantiomeric signatures of different sources can be used to identify shifts in the contributions of emission sources.
2. Analysis of the effect of drought on the abundance of chiral monoterpenes and isoprene to an extended drought and rewetting experiment inside a tropical rainforest mesocosm. Tropical rainforests contribute a significant portion of BVOCs to the atmosphere. Thus, it is important to investigate how drought manipulates those emissions since droughts are expected to become more common throughout the 21st century. In this investigation, drought increased the abundance of monoterpenes in the atmosphere, with a larger increase measured for (-)-enantiomers than for (+)-enantiomers). Labelling the atmosphere with 13CO2 revealed that (-)-α-pinene was mainly a de novo emission, whereas (+)-α-pinene was only emitted from storage pools. Severe drought increased the monoterpene emission from storage pools, which has the potential to cause a negative feedback effect on the climate by enhancing cloud formation. These results demonstrate how enantiomers should be considered as separate compounds, instead of the common practice of measuring and modelling them together as a single compound.
3. Analysis of the chiral terpenoid emissions from two different tropical plant species, Clitoria fairchildiana, and Piper sp., during a 9.5 week extended drought and rewetting experiment inside an indoor tropical rainforest mesocosm. Different studies have found that drought increases, decreases or has no effect on monoterpene emissions. Therefore, a complete understanding of how drought affects monoterpene emissions does not yet exist. Many past drought experiments have focused on the effect of drought on Mediterranean and boreal plant species, but few studies exist which focus on tropical plant species. In this study, monoterpene emissions were generally found to increase and decrease at specific points during the drought. Furthermore, the monoterpene enantiomer emissions were found to respond differently from C. fairchildiana, but responded in the same manner from Piper sp.
4. The abundance of chiral monoterpenes emitted from a pine tree, Pinus heldreichii, was measured before and after mechanical wounding using sorbent cartridges in combination with offline gas chromatography-time of flight-mass spectrometry (GC-ToF-MS), and cavity-enhanced chiral polarimetry (CCP). Both instruments showed differences between the dynamics of the total monoterpene concentration and chiral signal of the plant emissions, which became more negative in response to mechanical wounding. This study of the mechanical stress response of chiral monoterpene emissions highlights the importance of real-time in situ measurements of chiral VOCs emitted from vegetation. Additionally, for the first time, CCP was directly connected to GC for the separation and detection of chiral compounds. The first chromatogram to be obtained with a CCP detector is shown here.