To identify the aroma perceptual quality (PQ) of each olfactory receptor, we formulated isoaromatic mixtures of receptor agonists, i.e. proportioned in such a way that no ingredient dominates the mixture or is drowned out by the other ingredients, and smelled them in order that the note encoded by the target receptor would stand out above the other notes. Doing so has revealed candidate primary odors for OR52D1 (ethereal/painty), OR2J2 (soft, mellow, oxidized lemon), and OR2J3 (sharp, jagged, like cinnamon or black pepper). However, this is inexact as there is no way to isolate the target receptor from the whole aroma, and any combinatorial codes involving the target receptor can distract from the observation.

For each receptor that has known antagonists or inverse agonists, we made the agonist mixture twice and then added antagonists or inverse agonists to one of them, to create an antagonized mixture. This makes it possible to isolate the target receptor, provided the antagonist does not deactivate any unintended receptors activated by the agonist mixture. The two formulae are smelled one after the other, or in an alternating fashion, and any aroma notes present in the "plus" (agonist) mixture and absent in the "minus" (antagonized) mixture are believed to be the result of receptor deactivation, since the agonist mixture contains no odorants which the antagonized mixture lacks.

These are the odorants we used and the results of our experiment. In most cases, we've recorded the amount of each odorant used. In all cases, the +agonist mixture's proportions should be equally intense to the nose of the individual smeller, and the -antagonized mixture should have only enough antagonist or inverse agonist to perceptibly diminish one aroma note enough to describe it. In some cases, an other receptor antagonist (ORA) or inverse agonist has been added to reduce contamination of the result by deactivation of a receptor other than the target. For purposes of this web page, "inverse agonist" and "antagonist" are considered as equivalent.

estragole 5% in EtOH 
citronellal natural 

-OR1A1Antagonists:2-ethyl fenchol 
methyl pyrazine 0.1% in DPG 
ethyl vanillin 20% in EtOH 
methyl salicylate 
geranyl acetate 
terpinyl acetate 

-OR1A1 mixture odor:unpleasant cough drop, humus, stale
+OR1A1 mixture odor:dark green leafy, oregano, cough drops, caraway

Identified OR1A1 PQ:dark green leafy herbal


-OR1D2 mixture odor:aldehydic, waxy, orange, milky
+OR1D2 mixture odor:cleaning products, aldehydic, bright, shining

Identified OR1D2 PQ:clean-bright

Notes:Undecanal was not available, but undecenal should be similar enough in aroma properties.
eugenyl acetate 


-OR1G1 mixture odor:chemical, acetone, waxy, vinyl, swimming pool, olives
+OR1G1 mixture odor:waxy, delicate floral, lactonic, sweet, candy, peach, fruity

Identified OR1G1 PQ:waxy

Notes:We have chosen waxy as the aroma note for OR1G1 because the agonist mixture smells more strongly waxy than the antagonized mixture does, and waxy correlates most strongly with OR1G1 agonism in odor descriptions. The descriptors of lactonic, sweet, candy, and fruity do not correlate as well, and many OR1G1 agonists (e.g. lyral) do not smell at all lactonic, sweet, candy-like, or fruity. It is also worth noting that cyclohexanone, the competitive antagonist, is also a weak agonist.
+OR2A25Agonists:geranyl acetate20

-OR2A25Antagonists:methyl salicylate3

-OR2A25 mixture odor:spearmint, tobacco, smoky
+OR2A25 mixture odor:tobacco, brown, dried leaves, floral, berry

Identified OR2A25 PQ:bold-tabagique
coumarin 10% in EtOH15
coffee difuran 0.1%10
ORAs:propanal 1% in DPG10

-OR2B11Antagonists:ethyl vanillin 20% in EtOH10
terpinyl acetate5

-OR2B11 mixture odor:spicy, cinnamon, coffee, bath tissue, medicinal
+OR2B11 mixture odor:bold, dark brown, coffee, orange, cinnamon

Identified OR2B11 PQ:bold coffee-like
indole 1% in EtOH15
Carriers:isopropyl myristate50


-OR2L3 mixture odor:woody, pine, fruity, floral, berry, burnt
+OR2L3 mixture odor:camphorous, citrus, naphthyl

Identified OR2L3 PQ:camphorous
+OR2W1Agonists:methyl cinnamate 
methyl salicylate 
benzyl acetate 
ethyl vanillin 

-OR2W1Antagonists:2-ethyl fenchol 

-OR2W1 mixture odor:peppermint, humus, cough drop, limnic
+OR2W1 mixture odor:vegetable, minty, onion, limnic. metallic

Identified OR2W1 PQ:vegetable-metallic-alliaceous

-OR2W3Antagonists:isovaleric acid 10% in IPM3

-OR2W3 mixture odor:sweaty, fruity, faint lemon
+OR2W3 mixture odor:lemon zest, cleaning products, cumin, floral

Identified OR2W3 PQ:zesty
+OR2AG1Agonists:bean pyrazine 0.1% in DPG15
phenethyl acetate5
isoamyl acetate 10% in EtOH2
prenyl acetate1

-OR2AG1Antagonists:ethyl pyrazine 10%13

-OR2AG1 mixture odor:peanut butter and banana sandwich
+OR2AG1 mixture odor:green stemmy, sharp, banana, rose

Identified OR2AG1 PQ:stemmy-green
phenylpropyl alcohol5
phenethyl alcohol3
Carriers:isopropyl myristate50

-OR2AG2Antagonists:ethyl pyrazine 10%7

-OR2AG2 mixture odor:floral, rose, lactonic, lemon, cereal, pollen
+OR2AG2 mixture odor:mustard, floral, pollen, stemmy, rose, lemon

Identified OR2AG2 PQ:sharp-pollinic

Notes:There are no known antagonists or inverse agonists for OR2AG2. However, this receptor's high degree of similarity to OR2AG1 may mean it shares the same antagonist. Caution should be used in interpreting these results.

raspberry ketone 

-OR2AT4 mixture odor:woody, creamy, raspberry, sandalwood, rose
+OR2AT4 mixture odor:sharp, sandalwood, sulfurous, pine

Identified OR2AT4 PQ:gritty

Notes:The agonist mixture's odor has a grittiness that resembles that of benzyl salicylate.

-OR3A1Antagonists:bourgeonal 15% in EtOH50
α-methyl cinnamaldehyde 10% in EtOH40

-OR3A1 mixture odor:green, waxy, aldehydic, rotten squash, cinnamon
+OR3A1 mixture odor:green, cyclamen, alcoholic, rhubarb, wet newspaper

Identified OR3A1 PQ:cyclamic
methyl pyrazine 0.1% in DPG20
ORAs:decanal 5% in EtOH20
Carriers:isopropyl myristate50

-OR4M1Antagonists:citronellal natural10

-OR4M1 mixture odor:chemical, cherry, vinyl, chlorine, burnt
+OR4M1 mixture odor:chemical, grassy, peanut, vinyl, malty, chlorine, potato skin, grape

Identified OR4M1 PQ:malty?
+OR5P3Agonists:coumarin 10% in EtOH15
oranger crystals 15% in EtOH7

-OR5P3Antagonists:methyl eugenol20
propanal 1% in DPG15

-OR5P3 mixture odor:bready, caraway, bubblegum, minty
+OR5P3 mixture odor:caraway, hay, acrid, patchouli, burnt plastic, minty

Identified OR5P3 PQ:patchoulic
+OR8D1Agonists:sotolone 0.1%9
heptanoic acid 10% in IPM8
Carriers:isopropyl myristate25


-OR8D1 mixture odor:celery, burnt, sweet, herbal, caramellic
+OR8D1 mixture odor:strong burnt, acrid, caramellic, herbal

Identified OR8D1 PQ:burnt
+OR8H1Agonists:oranger crystals 15% in EtOH3
skatole 1% in EtOH1
Carriers:isopropyl myristate60

-OR8H1Antagonists:strawberry glycidate2

-OR8H1 mixture odor:strawberry, orange, halitosis, burnt sugar
+OR8H1 mixture odor:broccoli, mustard, neroli/orangeflower, naphthyl

Identified OR8H1 PQ:cruciferous
+OR9Q2Agonists:p-cresyl acetate0.35
Carriers:isopropyl myristate35

-OR9Q2Antagonists:bourgeonal 15% in EtOH30
sandranol 10% in EtOH3

-OR9Q2 mixture odor:fresh shorn wool, orange peel, halitosis
+OR9Q2 mixture odor:horse stable, urinous, plasticky

Identified OR9Q2 PQ:urinous
eugenyl acetate3
bourgeonal 15% in EtOH2
Carriers:isopropyl myristate50

-OR10G7Antagonists:decanal 5% in EtOH20
propanal 1% in DPG20

-OR10G7 mixture odor:balsamic, clove, orange, bubble gum, rosin, corn, buttery
+OR10G7 mixture odor:clove, pine, rosin, urinous

Identified OR10G7 PQ:rosinous
+OR11A1Agonists:geosmin 10% in DPG10
2-ethyl fenchol5
Carriers:isopropyl myristate50

-OR11A1Antagonists:allyl phenoxyacetate15

-OR11A1 mixture odor:humus, honey, earthy, pruny
+OR11A1 mixture odor:green, humus, limnic, chalky, earthy, watery

Identified OR11A1 PQ:watery

Notes:Allyl phenylacetate was not available so it is hoped that allyl phenoxyacetate would also be an inverse agonist.
+OR51E1Agonists:methyl eugenol 
butyl butyryl lactate 
eugenyl acetate 
methyl salicylate 


-OR51E1 mixture odor:bubble gum, muguet, waxy, floral, sweet, rose, juicy
+OR51E1 mixture odor:dairy, cheesy, sweaty, minty, clove

Identified OR51E1 PQ:dairy
propionic acid 5% in IPM15
Carriers:isopropyl myristate50

-OR51E2Antagonists:decyl acetate40

-OR51E2 mixture odor:floral, sour, waxy, violet, berry
+OR51E2 mixture odor:vinegar, waxy, plastic

Identified OR51E2 PQ:acetic

Notes:Lee et al 2023 identified C-11 acetate as competitively binding to OR51E2 (https://doi.org/10.3390%2Fbiomedicines11061688), however we were not able to obtain this compound, so we used C-10 acetate instead which should function similarly.
caproic acid0.1
Carriers:isopropyl myristate30


-OR51I2 mixture odor:aldehydic, clean, peach, new rubber balloon, waxy
+OR51I2 mixture odor:peach, old rubber balloon, juicy, waxy

Identified OR51I2 PQ:peach-cheddar juicy

Notes:OR51I2 was unintentionally included in a previous formulation of the OR51E1 differential, and revealed for the two receptors in combination a powdered cheddar note.
+OR51L1Agonists:allyl phenoxyacetate 
caproic acid 
ORAs:ethyl vanillin 
2-ethyl fenchol 


-OR51L1 mixture odor:watery, honey, sweet, waxy, cocoa, moldy
+OR51L1 mixture odor:cloying, molasses, honey, cocoa, sweet

Identified OR51L1 PQ:syrupy

Notes:Allyl phenylacetate was not available so it is hoped that allyl phenoxyacetate would also be a strong agonist. Only a very small amount of caproic acid was used to avoid the mixture smelling sweaty. When made without the OR1A1 inverse agonists (ethyl vanillin and 2-ethyl fenchol), the differential note is strongly pruny.
Carriers:isopropyl myristate 


-TAAR5 mixture odor:burned fish, breaded seafood, ammonia, woody, rotten
+TAAR5 mixture odor:shrimp, prawns, rotting seafood, paper mill

Identified TAAR5 PQ:prawny

Notes:The known TAAR5 agonist trimethylamine (TMA) was unavailable, however TEA has a very similar odor and molecular structure.