Relationship between chemical function groups on Eucalyptus secondary metabolites and their effectiveness as marsupial antifeedants
Lawler, IR, Eschler, BM, Schliebs, DM & Foley, WJ 1999, Journal of Chemical Ecology, vol. 25, no. 11, pp. 2561-2573.
Significant variation in the type and concentration of diformylphloroglucinol compounds (DFPCs) is observed within species of Eucalyptus, which contributes to their resistance against foliage consumption by marsupials. Jensenone was used as a model DFPC and compared to structural variants to determine the functional group causing the deterrent activity, with the aldehyde on the aromatic ring determined to reduce ringtail possum food intake.
To determine the functional group responsible for the deterrent activity, compounds with a similar basic structure but varying phenol and aldehyde groups were added into common ringtail possums’ diets and their preference for the food was recorded. Jensenone, a compound found in Eucalyptus leaves that is known to be highly deterrent, has both phenol and aldehyde groups, whereas torquatone, another naturally occurring Eucalyptus compound, has neither of these groups. Acetyl-jensenone was created from jensenone and only has the aldehyde group, and dimethyl-torquatone was derived from torquatone with only the phenol group. Jensenone was the most effective deterrent, followed by acetyl-jensenone, with torquatone and dimethyl-torquatone showing considerably less activity, suggesting that the aldehyde group contributes significantly to the deterrent activity of the molecule.
Total phenolics assays are a common technique in the field and indicate the presence of a specific functional group while overlooking any other functional groups present and their locations within the molecule. The results of this experiment demonstrate that even small variations in structure significantly vary deterrent activity, suggesting that screening with such low specificity may be insufficient for explanations of marsupial feeding. In addition, the relative inactivity of torquatone could possibly be due to lacking a free phenol group in an ortho position relative to an isoprene, a trend found in general chemical groups previously studied. The authors of the study also expected that their modified compounds would have activities intermediate to the natural compounds due to having intermediate structures. While this was the case for acetyl-jensenone, this result was likely due to the compound reverting back to jensenone in the acidic conditions of the stomach as the acetyl group is known to be prone to acidic cleavage. Demethyl-torquatone, however, had lower activity than torquatone, probably due to being more polar and therefore less likely to absorb across lipid membranes, which may be necessary to have a deterrent effect.
A greater understanding of the structure of key components of Eucalyptus foliage informs the importance of functional groups to marsupial deterrent activity, which facilitates a better understanding of the ecological interactions between koalas and Eucalyptus.
Summarised by Laura Wait
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