What Makes Sulfur Smell So Bad?

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If you’ve ever caught a whiff of natural gas, you know the stench is similar to that of bad eggs or rotting cabbage. But the unsavory smell also saves lives: Gas leaks put our homes at risk for fires and explosions, and can even cause asphyxiation and death. Since pure natural gas (a.k.a. methane) has no smell, utility companies add smelly, sulfur-containing odorants called mercaptans, or thiols, to warn us if anything is amiss with our pipelines.

In fact, scientists say that our ability to discern even miniscule levels of volatile sulfur compounds is key to human survival in general, as it helps us detect rotten food, atmospheres with low oxygen levels, and even the urine of potential predators. That’s why chemists and neurobiologists from the University at Albany set out to find which of the nose’s olfactory receptors are responsible for their stench, along with factors that affect our sensitivity to it. Their results were recently published in the Journal of the American Chemical Society.

Olfactory receptors are present in specialized sensory cells called olfactory sensory neurons, which line our noses. They’re responsible for detecting molecules in our surroundings and sending messages to our brain so that we can recognize and label a smell. Chemistry professor Eric Block and other researchers from the Shanghai Institutes for Biological Sciences, Duke University, Yale University, and the National Institute of Standards and Technology located the receptor—known as OR2T11—that’s most responsive to thiols.

The researchers also found that the presence of copper ions in our nose’s mucus greatly amplifies our sensitivity to them. “Using molecular dynamics simulations, the team found two binding sites which contained copper,” Chemistry World explained. “The importance of both sites in detecting thiols was confirmed by site-directed mutagenesis: genetically engineered receptors lacking the amino acids responsible for copper binding lost all functionality.”

“Obviously it is essential for everyone to be able to detect gas leaks by recognizing the smell of the sulfur odorant,” Block said in a release. “Unfortunately, some people have a diminished sense of smell, or the absence of smell all together. Understanding how we smell sulfur could help doctors treat those who are not responsive it.”

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