The Enigma of Sponge-Derived Terpenoid Isothiocyanate–Thiocyanate Pairs: A Biosynthetic Proposal
The co-occurrence of rare terpenoid thiocyanates (R-SCN), structurally similar to their more common isothiocyanate isomers (R-NCS), poses an enigma: how does the accepted path, terpenyl cation R<sup>+</sup> → R-NC → R-NCS, accommodate R-SCN? The mystery can now be rationalized by the con...
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| Format: | Article |
| Language: | English |
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MDPI AG
2025-05-01
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| Series: | Marine Drugs |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1660-3397/23/5/220 |
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| Summary: | The co-occurrence of rare terpenoid thiocyanates (R-SCN), structurally similar to their more common isothiocyanate isomers (R-NCS), poses an enigma: how does the accepted path, terpenyl cation R<sup>+</sup> → R-NC → R-NCS, accommodate R-SCN? The mystery can now be rationalized by the consideration of three biosynthetic motifs: terpenoid carbocation (R<sup>+</sup>) capture by cyanoformate, NC-COOH (itself in equilibrium with NC<sup>−</sup> and CO<sub>2</sub>); co-localized rhodanese (a dual-function enzyme) that can both convert fugitive inorganic NC<sup>−</sup> to thiocyanate ion, NCS<sup>−</sup>, and alkyl isonitriles to alkyl isothiocyanate (R-NC → R-NCS) and adventitious capture of the NCS<sup>−</sup> by R<sup>+</sup>. The former two scenarios explain the preponderance of isothiocyanates, R-NCS, as products of a linear reaction path—the α-addition of S<sup>0</sup> to R-NC—and the third scenario explains minor, less stable thiocyanates, R-SCN, as products of the adventitious capture of liberated NCS<sup>−</sup> by the penultimate R<sup>+</sup> precursor. DFT calculations support this proposal and eliminate other possibilities, e.g., the isomerization of R-NCS to R-SCN. |
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| ISSN: | 1660-3397 |