Identification and synthesis of volatile compounds from pheromone glands of tropical frogs
Final Report Abstract
Femoral glands, specific organs on the hind legs of males of the species-rich frog subfamily Mantellinae from Madagascar, were studied to decipher the structures of volatile compounds produced by these glands. The unrelated frog family Hyperoliidae, found throughout Africa, uses a trimodal system for communication. This system includes the usual auditory signal, but also vision and chemistry. A yellow gular gland, located at the expanding sound bubble, is exposed during the call, resulting in a yellow signal and the release of a species-specific scent. Again, only males use this form of signaling. These chemical signals are likely to function as a signal in male-female interactions during courtship. The chemical structures of the dominant compounds found in both mantelline and hyperoliid frogs are macrolactones of medium volatility. The macrolactones of the mantellines are structurally diverse and are produced in species-specific mixtures. The structures of the compounds were identified by analysis of gland extracts using gas chromatography coupled to mass spectrometry and direct deposition infrared spectroscopy, derivatizations of extracts, and synthesis. One of these compounds, luteolide from Gephyromantis luteus, is the first natural frog macrocyclic lactone with an ethyl side chain and three stereogenic centers. The absolute configuration was determined by synthesis of the possible stereoisomers and by chiral chromatography. While luteolide is restricted to one species, another newly identified macrolactone, frogolide, occurs in several mantelline and hyperoliid species. It is the first macrocyclic frog lactone of terpene origin. A synthesis was developed and the absolute configuration was determined to be the same in both families. However, the biosynthetic pathways are slightly different in the two families. For example, we found that some mantellines use the terpenoid macrolactone gephyromantolide A, whereas hyperoliids prefer the double bond isomer cinnamomeoventrolide. This regioisomerism is similar to the sex pheromone systems of moths, where species specificity is achieved by mixtures of isomers that differ in the position and configuration of the double bonds. The mantellines must have an isomerase in their biosynthetic repertoire that moves the double bond, which is lacking in hyperoliids. More than 25 different macrolactones have been discovered in the two frog families discussed. Their distribution and synthesis has been summarized in a review. Ring-closing metathesis and opening of chiral epoxides proved to be especially useful in constructing the molecular frameworks. Hyperoliids such as Hyperolius cinnamomeoventris, a species that served as a model in our hands, also release sesquiterpenes from their gular glands. One of these terpenes has been identified as amorph-4-en-10β-ol. This sesquiterpene is rarely found in nature and the absolute configuration was established by synthesis using organo-catalysis. Attempts to isolate a putative sesquiterpene cyclase from frogs using gene expression analysis and heterologous expression led to an active enzyme, although it does not produce the native sesquiterpenes. Several new terpenoid macrolide structures have been identified and are currently being investigated by synthetic methods. Mass spectra of the new compounds can be downloaded in computer readable format from the MACE database, implmented by our group.
Publications
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Frogolide – An Unprecedented Sesquiterpene Macrolactone from Scent Glands of African Frogs. European Journal of Organic Chemistry, 2018(20-21), 2651-2656.
Menke, Markus; Melnik, Kristina; Peram, Pardha S.; Starnberger, Iris; Hödl, Walter; Vences, Miguel & Schulz, Stefan
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Identification and Synthesis of Luteolide, a Highly Branched Macrolide Semiochemical from the Mantellid Frog Gephyromantis luteus. Organic Letters, 21(8), 2851-2854.
Melnik, Kristina; Menke, Markus; Rakotoarison, Andolalao; Vences, Miguel & Schulz, Stefan
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Chemical Diversity of Volatile Macrocylic Lactones from Frogs. Synlett, 32(17), 1683-1701.
Schulz, Stefan; Poth, Dennis; Peram, Pardha Saradhi; Hötling, Susann; Menke, Markus; Melnik, Kristina & Röpke, René
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Cinnamomeoventrolide – Double Bond Regioisomerism in Frog Semiochemicals. Journal of Chemical Ecology, 48(5-6), 531-545.
Kuhn, Johanna & Schulz, Stefan
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Identification and determination of the absolute configuration of amorph-4-en-10β-ol, a cadinol-type sesquiterpene from the scent glands of the African reed frog Hyperolius cinnamomeoventris. Beilstein Journal of Organic Chemistry, 19(2023, 2, 16), 167-175.
Ladwig, Angelique; Kroll, Markus & Schulz, Stefan
