Bohman Bjorn, Karton Amir, Flematti Gavin R, Scaffidi Adrian, Peakall Rod
School of Molecular Sciences, The University of Western Australia, Crawley, WA, 6009, Australia.
Research School of Biology, The Australian National University, Acton, ACT 2601, Australia.
J Chem Ecol. 2018 May;44(5):436-443. doi: 10.1007/s10886-018-0946-0. Epub 2018 Mar 17.
Sexually deceptive orchids attract specific pollinators by mimicking insect sex pheromones. Normally this mimicry is very specific and identical compounds have been identified from orchids and matching females of the pollinators. In this study, we conduct a detailed structure-activity investigation on isomers of the semiochemicals involved in the sexual attraction of the male pollinator of the spider orchid Caladenia plicata. This orchid employs an unusual blend of two biosynthetically unrelated compounds, (S)-β-citronellol and 2-hydroxy-6-methylacetophenone, to lure its Zeleboria sp. thynnine wasp pollinator. We show that the blend is barely attractive when (S)-β-citronellol is substituted with its enantiomer, (R)-β-citronellol. Furthermore, none of the nine-possible alternative hydroxy-methylacetophenone regioisomers of the natural semiochemical are active when substituted for the natural 2-hydroxy-6-methylacetophenone. Our results were surprising given the structural similarity between the active compound and some of the analogues tested, and results from previous studies in other sexually deceptive orchid/wasp systems where substitution with analogues was possible. Interestingly, high-level ab initio and density functional theory calculations of the hydroxy-methylacetophenones revealed that the active natural isomer, 2-hydroxy-6-methylacetophenone, has the strongest intramolecular hydrogen bond of all regioisomers, which at least in part may explain the specific activity.
性欺骗兰花通过模仿昆虫性信息素吸引特定传粉者。通常这种模仿非常特异，已从兰花和与之匹配的传粉者雌性个体中鉴定出相同的化合物。在本研究中，我们对参与蜘蛛兰（Caladenia plicata）雄性传粉者性吸引的信息化学物质的异构体进行了详细的构效关系研究。这种兰花采用两种生物合成不相关的化合物（S）-β-香茅醇和2-羟基-6-甲基苯乙酮的不同寻常混合物来吸引其泽勒蜂属（Zeleboria sp.）细腹蜂传粉者。我们发现，当（S）-β-香茅醇被其对映体（R）-β-香茅醇取代时，该混合物几乎没有吸引力。此外，当天然信息化学物质的九种可能的羟基甲基苯乙酮区域异构体取代天然的2-羟基-6-甲基苯乙酮时，均无活性。鉴于活性化合物与一些测试类似物之间的结构相似性，以及之前在其他性欺骗兰花/黄蜂系统中用类似物替代可行的研究结果，我们的结果令人惊讶。有趣的是，对羟基甲基苯乙酮的高水平从头算和密度泛函理论计算表明，活性天然异构体2-羟基-6-甲基苯乙酮在所有区域异构体中具有最强的分子内氢键，这至少在一定程度上可以解释其特异性活性。
