Sensory Efficacy of Alkyl-Branched Pheromone Analogues in Noctuid and Tortricid Lepidoptera

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Abstract
The effect of introducing alkyl substituents (methyl to pentyl) to the chain segment n in long-chain alkenyl acetates, CH3(CH2)nCH = CH(CH2)m-O-COCH3 (I), was investigated in pheromone receptors of Noctuidae, Tortricidae, and Cochylidae species. The receptor types studied were maximally responsive to (Z)-7-or (Z)-9-dodecen-1-yl acetate, (Z)-7-, (Z)-9-, or (Z)-11-tetradecen-1-yl acetate, or (Z)-11-hexadecen-1-yl acetate, representing key compounds with n = 1, n = 3, or n = 5 methylene groups. In terms of the relative stimulus amounts required to elicit equivalent EAG amplitudes, the efficacy of the alkyl-branched derivatives was between 1/1000 to 300 times that of the unbranched chain of the same length and double bond position. The effects of branching were specific to the type of receptor, the length and double bond position of the parent chain, and the kind and position of the alkyl substituent. The most pronounced increase observed occurred with receptors for « = 1 type pheromones, (Z)-9-dodecen-1-yl acetate and (Z) -11-tetradecen-1-yl acetate when an α-methyl group was introduced to elongated derivatives; whereas the greatest decreasing effects were obtained on receptors for the n = 5 type compound, (Z)-7-tetradecen-1-yl acetate. The results show basic differences in structure-response relationships between the Noctuidae vs Tortricidae receptors studied. Various possible effects of the substituent groups during sensory transduction are considered. The data should contribute to further elucidation of interaction mechanisms of unbranched alkenyl acetate pheromones (I) with insect olfactory receptors.

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