A patent filed on behalf of Takasago International Corp identifies OR2L3 as a receptor for α-terpineol, 4-terpineol, and 4-methyl acetophenone, with additional responses to indole and skatole. Identified as antagonists are cis-3-hexenyl hexanoate, Timberol, and Santaliff. While the agonists as a group may not seem to show an obvious pattern of an aroma note, the differential experiment reveals a terpenic PQ for OR2L3.
Previously, we were unable to identify a single receptor for camphoraceous or coniferous odors. α-Pinene is a strong agonist of OR2A7 and OR2T4, but so is lilial which does not smell camphorous. OR2A1 is sensitive to pinene and farnesol, but again the latter compound does not have a coniferous note.
The Takasago patent on its face claims the use of OR2L3 antagonists as masking agents of fecal odors, specifically skatole and indole. Skatole’s receptor profile has been mostly identified with sufficient detail to explain its scent; it is an agonist of OR5P3 (identified as hot plasticky, acrid, or patchouli-like by differential experiments), OR8H1 (identified as detritic, or decaying-fecal), OR2W1 (identified as vegetable, stale, metallic), and OR5K1 (identified as pyrazinic). Together these notes seem to compose the fecal character of skatole, though its naphthyl and fruity notes at high concentration remain unexplained. Indole’s receptor profile would be expected to resemble that of skatole.
4-methyl acetophenone’s aroma is described as sweet, resembling hawthorn, coumarin, and cherry. Coumarin certainly shares a naphthyl quality with skatole and indole, but otherwise this does not seem to resemble either the piney-terpenic characters of the two terpineols nor the fecal quality of the two nitrogenous bicyclic compounds.
To test the aroma contribution of OR2L3, the following mixture was prepared:
- 15 parts of α-terpineol;
- 8 parts 1% indole in ethanol;
- 3 parts 1% skatole in ethanol;
- 50 parts isopropyl myristate as a dilutant.
- To the antagonized mixture was added 5 parts of Timberol.
Two mixtures were prepared, one containing everything but the Timberol (the “plus mixture”), and the other containing all of the above listed ingredients (the “minus mixture”).
The “plus” mixture exhibits a definite piney, terpenic, somewhat fruity and citrus character with fecal undertones, exactly what would be expected from its constituents. The “minus” mixture, by contrast, smells more subdued, with the fruity and floral aspects of the two nitrogenous compounds brought to the forefront. While the “plus” mixture smells perhaps a little more fecal than the “minus” mixture, there is a striking difference in the presence vs. absence of a pine-citrus terpenic note. The experimental results are consistent with Timberol masking a terpenic aroma quality.
The choice of the term “terpenic” to describe the quality assigned to OR2L3 is not arbitrary, as terpenes represent a variety of camphorous, coniferous, citrus, and herbal aromas. Even mintiness is associated with terpenes, as both menthol (peppermint) and R-carvone (spearmint) are terpenes. So a “terpenic” note would mean one that at least contributes to pine, camphor, and citrus characters, and possibly also to the sensation of mintiness.
The naphthyl aroma note might itself be a mixture of minty with phenolic. If this is true, then to characterize naphthyl aromas would require identifying a minty component and a phenolic component. No single receptor has yet been identified as a minty receptor, though a few are tentatively assigned as such in the PrimaryOdors web app. It may be that one of the ways to produce a minty note is to combine terpenic or OR2L3 with some other aroma note. In this way, we can explain OR2L3’s contribution to the odors of 4-methylacetophenone, skatole, and indole as being part of a combinatorial code for the naphthyl smell.
A follow up experiment will have to be done using cis-3-hexenyl hexanoate and Santaliff as antagonists for the differential method.