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Why do sulfur compounds smell?

Why do sulfur compounds smell?



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Many of the compounds of sulfur have a strong odor. Hydrogen sulfide from rotten eggs, the mercaptans of a skunk, the odor compounds in onions and garlic, the bitter taste of brassicas (cabbage, Brussels sprouts, collard greens, and similar plants) are all sulfur compounds. What is it about sulfur that makes this so common?


Nice question! It would, obviously, be a long answer if I talk about all types of sulfur compounds here. So I will take up just 2 examples to explain this.

  1. Thiols: when we talk about sulfur compounds with foul smell, thiols (mostly) come to the top in the list. Thiols (R-SH) are a class of compounds famous for their smell. Some examples of thiols include:

    • compounds in onion and garlic (see this answer for more details).

    • a mix of compounds responsible for the smell of skunk's spray (Anderson et al, 1975)

    • (R)/(S)-3-methyl-3-sulfanylhexan-1-ol found in human sweat

    • (Methylthio)methanethiol found in male mouse urine

Some biologically important examples would include coenzyme-A, glutathione and cysteine. However, not all thiols have fowl smell. For example, furan-2-ylmethanethiol provides the aroma of roasted coffee while grapefruit mercaptan gives grapefruit its characteristic scent. You can see a complete list here.

After seeing some examples, lets come to the main point i.e. why thiols have such foul smell. Thiols are decomposition products of proteins. When amino acids, like cysteine and methionine, are decomposed, thiols are one of the last things to form. This would explain why it becomes important to smell and avoid them (you wouldn't want to eat a dead and decaying mammoth, and a blind hunter might not be able to tell if he has found a decaying mammoth if he can't smell such compounds). As a side note, this also explains why organic acids have sour taste and smell. Compounds like glucuronic acid, citric acid, oxalic acid, lactic acid, butyric acid, acetic acid, etc. are all decomposition products (if you know how vinegar is made traditionally). This is why your body tries to get rid of them (I saw that in a video, if you eat things with foul smell and sour taste (it was a Japanese dish probably), you'll suffer nausea and vomiting since your body will do all it can to prevent its harmful effects). See this Wikipedia page for more info.

  1. Hydrogen Sulfide: H2S, or hydrogen sulfide, is another compounds famous for its smell. It smells like rotten eggs, and indeed it is because H2S is what rotten eggs release. The basic reason why H2S smells so bad is again the same. Hydrogen sulfide is one of the final products of proteolysis during decomposition (see same Wikipedia article). But there is another cause why H2S has such foul smell (and why I didn't just stop at thiols): H2S is itself toxic.

H2S has many biological signalling functions, similar to NO and CO. These include:

  • H2S serves as endotheium-derived relaxing factor (EDRF) and endothelium-derived hyperpolarizing factor (EDHF). In short, it acts as smooth muscle relaxant and vasodilator. It also increases the response of NMDA receptor and facilitates long term potentiation in the brain.

  • it is converted to sulfite by thiosulfate reductase, and further to thiosulfate and sulfate by sulfite reductase, in the mitochondria. Sulfate is excreted in the urine.

  • it acts on ATP-sensitive potassium channels in smooth muscles, and does the blood vessel-relaxing work in smaller blood vessels.

  • it blunts, reverses and promotes healing of diverse inflammatory reactions. A full list is available on this Wikipedia page.

  • the most pronounced effect of H2S is much similar to that of CO. It also binds to the iron in the mitochondrial cytochrome c oxidase and other mitochondrial cytochrome enzymes, thus preventing cellular respiration and causing death.

This would explain why external source of H2S would be so harmful since, being biologically such an active molecule, its amount needs to be controlled strictly. Now, if you want to know why decaying bodies would be dangerous, this Wikipedia article would be a good starting point.

EDIT: The bitter taste of some vegetables is because of a type of compounds called glucosinolates found in some plants. These compounds dissociate upon eating to form isothiocyanate and are thought to be a part of the plants' defense system (see this answer for more details). Interestingly, a similar type of compound, cucurbitacin, found in Cucurbitacae plants, does not contain sulfur, yet it is responsible for the bitter taste of these plants.


What's behind smelly wine?

Aging often improves the flavor of wine, but sometimes the beverage emerges from storage with an unpleasant smell. One of the prime culprits is hydrogen sulfide (H2S), which can give the affected wine an aroma of sewage or rotten eggs. In a report in ACS' Journal of Agricultural and Food Chemistry, researchers have now identified some potential sources of this stinky compound.

H2S is a volatile sulfur compound that's produced naturally during fermentation. Most of it disappears or is removed in subsequent winemaking steps, but it can reemerge after bottling. Ironically, it might derive from polysulfanes and other sulfur byproducts created during H2S removal. Marlize Bekker and colleagues wanted to check if that theory was correct.

The researchers created a model wine containing a mixture of polysulfanes and then treated it with antioxidants such as sulfur dioxide and ascorbic acid, which are often added to wine as preservatives during bottling. The scientists then identified and measured the concentration of a variety of sulfur compounds in the wine during six months of storage. They found that polysulfanes containing four or more linked sulfur atoms per molecule tended to decompose during wine storage, correlating with a rise in H2S. In addition, the polysulfane decomposition and H2S release occurred more frequently in the wine treated with sulfur dioxide than in untreated wine or wine treated with ascorbic acid. The findings provide strong evidence that polysulfanes were the source of re-emergent H2S, though this conclusion will need to be confirmed in real wines, the researchers say. Confirming such a role for polysulfanes could help identify practical ways to manage the re-emergence of stinky sulfur compounds, one of the major faults in bottled commercial wine.


1.1 Introduction

The chemical biology of living organisms is typically cast in terms of the properties of the four major elements, (tetravalent) carbon, (monovalent) hydrogen, (trivalent) nitrogen and (divalent) oxygen and the variety of their compounds and embedded functional groups therein that comprise the metabolites of cells. Metabolomics, the science of metabolic transformations across cells, tissues or the whole organism, follows the rules and reactivity of the organic chemistries open to those functionalized molecular scaffolds.

The fifth element that is essential for life is sulfur, a third-row element. Directly below oxygen in the periodic table, sulfur consequently has similarities to oxygen chemistry, but dramatic differences, arising in substantial part from its electronic configuration with 3p electrons (1s 2 2s 2 2p 6 3s 2 3p 4 ) and empty 3d orbitals (Figure 1.1A). Sulfur is believed to form in the hot cores of stars by fusion of silicon and helium atoms. Unlike atomic oxygen (“O”) which is not a stable compound under physiological conditions, elemental sulfur, S 0 , is a solid, with various allotropes. 1 Indeed, elemental sulfur is an active metabolite in bacterial and archaeal reduction–oxidation (redox) pathways.

Among the most remarkable distinctions between sulfur and oxygen is the range of accessible oxidation states available for sulfur biology (chemed.chem.purdue.edu/chemed/bodnergroup/index.html). While oxygen is restricted to divalent chemistry [e.g. water (H2O) and dioxygen (O2)], sulfur is protean by comparison in its compounds. Although H2S has analogies to H2O in the formal oxidation state of −2, as a divalent hydride, H2S is a gas at room temperature and pressure in contrast to liquid H2O because of the lessened hydrogen bonding tendency in sulfur vs. oxygen. 2 The tendency of sulfur to form two bonds to partner atoms (e.g. H2S) is consistent with the two partially filled 3p orbitals (Figure 1.1A).

Most notably, though, sulfur makes covalent bonds to partner atoms at oxidation states of −2, +2, +4 and +6. Most abundant in the earth's crust is the dianion of sulfuric acid 3 (H2SO4) in which S has an oxidation state of +6 (Figure 1.1B and 1.2) Sulfur monoxide has sulfur in the formal +2 oxidation state while sulfur dioxide has sulfur in the +4 oxidation state. Figure 1.1B also shows the S–S bonding in three of the more common forms of inorganic oxygenated sulfur ions, dithionate, trithionate and thiosulfate.

One comparison of the biological roles of phosphorus and sulfur can be made between phosphate and sulfate dianions. Both are water soluble forms of the P and S atoms, in their most oxidized biologic states. Phosphate groups, of course, are pervasive in biology, from small molecules, such as glucose-6-phosphate, to phosphoenzyme intermediates in signaling pathways to the covalent phosphodiester backbone linkages in RNA and DNA. In contrast, sulfate ions are much more restricted in their biological distribution. Yet, phosphorus chemical biology is restricted largely to phosphorus in the +5 oxidation state. It is not readily reduced to low-valent phosphorus metabolites or reoxidized back up to phosphate. 4 It would not be suitable for evolution of electron transport machinery in organisms.

The availability of sulfur to access so many distinct oxidation states between −2 and +6 allows it to occupy a quite distinct and complementary set of metabolic niches from phosphate.

On the organic side of Figure 1.1B, thiols, sulfenic acids and thioethers are examples of sulfur in the −2 oxidation state. 5,6 We shall note in subsequent chapters that thiols and thioethers have nucleophilic sulfur atoms while in sulfenic acids, the S atom is electrophilic. The sulfur atoms in disulfides are electrophilic while the formally comparable persulfide can react either as a sulfur nucleophile or electrophile. 7 Methionine with its thioether side chain offers the possibility of alkylation to form the trivalent sulfonium cation in the metabolite S-adenosylmethionine, thereby activating the three substituents at that trigonal sulfur for transfer to cosubstrate nucleophiles.

The flexibility of sulfur to populate such a broad range of oxidation states is a function of rehybridization of atomic valence orbitals to hybrid molecular orbitals involving 3s, 3p and 3d atomic orbitals (https://mysite.du.edu/∼jcalvert/phys/sulphur.htm, 2003). For example, the abiotic compound SF6 (Figure 1.2) uses a set of equivalent sp 3 d 2 molecular orbitals to generate the bonding pattern for hexavalent sulfur. The dramatic polyvalency of sulfur makes it much more versatile than oxygen in both its inorganic and organic chemistry in the molecules of life in distinct metabolic niches.

Sulfur makes up approximately 3% of the earth's mass, most of it in the core (an estimated 0.07% of the earth's crust), 8 but both elemental sulfur (S 0 , the yellow solid that can take on several allotropic forms) and gaseous H2S are emitted by volcanoes and undersea smoker vents (Figure 1.3). Solid sulfur as yellow chunks is still harvested from the immediate environs of Indonesian volcanoes. (sulfur liquifies at 159 °C). In the bible, sulfur and its attendant hydrogen sulfide with its characteristic odor (detectable at 0.02 ppm, 8,9 often characterized as the smell of “rotten eggs”) was labeled “brimstone” as in the phrase “fire and brimstone” (brimstone is literally “burning stone”: solid yellow sulfur burns in air to release malodorous sulfur dioxide – the odor of brimstone). Purportedly, the large deposits of sulfur in the Italian region of Campania that includes Mount Vesuvius, gave rise to the Latin name sulphur, now preferably sulfur, in many regions of the world.

During the two billion or so years when the planetary atmosphere was largely oxygen-free, sulfur in those low oxidation states was abundant. The sulfide at −2 oxidation state was subject to complexation with ferrous iron (also stable under anoxic conditions). A particularly abundant iron–sulfur compound is pyrite, FeS2 (“fools gold”). 10 The bonding in pyrite is approximated by Fe 2+ S2 2− , such that pyrite is more an iron persulfide than a ferrous sulfide. Other ionic forms of low-valent sulfur of historical import in ores are cinnabar (HgS), the source of mercury and of historical use in gold mining, lead sulfide (PbS) known as galena, as well as zinc sulfide (ZnS).

As oxygen levels rose (from the global metabolic production of O2 by photosynthesizing cyanobacteria), the weathering of rocks and flow of dissolved sulfur compounds, first into rivers, and then into global oceans was accompanied by biological and abiological oxidation of low-valent sulfur molecules. Among the various oxygenated forms of sulfur, inorganic sulfates (Figure 1.3) are the most oxidized and predominant forms. Some 10 22 g of sulfur, largely as sulfate are estimated to be in the earth's crust. 9 These days extensive mining of sulfur results in approximately 10 12 grams (approximately 4–5 million tons!) being mined each year, largely for use as sulfuric acid. 3 The sulfuric acid is used for many purposes, the largest of which are generation of sulfate and phosphate fertilizers, gunpowder, “vulcanization” of rubber tires and pesticides. 11

The average concentration of sulfate in global oceans is about 28 mM, corresponding to a total of 1.3吆 21 grams of dissolved dianion 3 [“With an estimated flux of 1.0吆 14 g per year into (and out of) the ocean, the oceanic sulfate reservoir has a long response/turnover time” (approximately 13 Myr)]. The dissolved divalent calcium cation (Ca ++ ) is contemporaneously around 10 mM. During the multiple geological periods of warming and evaporation of seas, calcium sulfate as the dihydrate or pentahydrate (CaSO4ܨH2O or CaSO4ܫH2O) precipitated out first (before the more abundant NaCl), as the material known as gypsum, accounting for huge inventories of gypsum beds and salt domes in sedimentary rock.


Understanding sulfur's rotten smell

Eric Block, distinguished professor of chemistry. Credit: Mark Schmidt

Though odorless in its normal state, utility companies add sulfur-containing odorants - called mercaptans or thiols - to the gas so it's easy to detect a leak. The foul smell is reminiscent of rotting cabbage or spoiled eggs.

In a new study, co-authored by University at Albany distinguished professor of chemistry Eric Block, researchers have not only determined which of our olfactory receptors (ORs) is responsible for this nasty smell, but also the effects metals such as copper have on our sensitivity to the stench.

"Obviously it is essential for everyone to be able to detect gas leaks by recognizing the smell of the sulfur odorant," Block said. "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."

The research team identified OR2T11 as the receptor that responds most strongly to natural gas odorants. More importantly, they discovered the presence of copper and/or silver amplifies our sensitivity by one hundred to one thousand fold. In fact, without the metals, OR2T11 only weakly responds to the gas odorants.

Their results were recently published in the Journal of the American Chemical Society. Collaborating with Block were neuroscientists Hanyi Zhuang (Shanghai Institutes for Biological Sciences) and Hiroaki Matsumoto (Duke University), computational chemist Victor Batista (Yale University), spectroscopist Jessica Burger (the National Institute of Standards and Technology), and their respective colleagues.

"Our collaborative research shows nature has developed a way to make sure we have maximum possible sensitivity to sulfur, since often the presence of sulfur compounds signals a hazard," Block said. "The metal effect makes sense, as metal has a strong attraction to sulfur, for example silverware rapidly turns dark when eggs are cooked. However, there was very limited evidence for the role of metals in olfaction prior to our work."

A renowned organosulfur chemist, Block has spent the majority of his 50-year career studying sulfur compounds.

His latest research connecting smells with individual olfactory receptors began several years ago. Humans have more than 350 unique receptors, with which an almost infinite number of odors and odor mixtures can be identified. Ultimately, Block and his team hope their findings can help scientists develop an artificial nose that can detect sulfur compounds from various sources.

"A receptor is considered to be an 'orphan' until paired with the odorants it is most responsive to," Block said. "The process of 'deorphanizing' all of our human receptors is difficult and time consuming. It has only been possible in recent years with the development of specialized techniques in molecular biology. My colleagues and I are working at the frontier of this field."


How to get Rid of Sulfur/Rotten Egg Smelling Farts in 4 Steps

If you have a sulfur/rotten egg smell to your fart, then you need to figure out what’s wrong or not right in your gut. Here are some ways that you could use to get rid of the smell. Some are related to lifestyle changes while others involve actual management and treatment of a condition.

Step#1: Investigating underlying conditions

it is important to visit a doctor in order to ensure that you are not infected. The doctor will be able to prescribe some drugs for you in case it is. If the smell is accompanied by other symptoms such as impaction, then further investigation will be made.

Step#2: Dietary changes

You could make a few changes in your diet. Some of them include:

Plenty of water

Drink lots of water. Perhaps even more than just 8 glasses in a day. Make sure that you spread out the glasses or ounces of water throughout your day, concentrated more after meals or during a time of increased physical activity.

Water is good for proper digestion and proper absorption of food. Hence, there will be little hydrogen sulfide or other compounds in the gut. It also flushes out the excessive waste and helps relieve constipation.

Green tea

This is a herbal that is known to improve digestion and detoxification process of your body. It leads to better general health inclusive of the gastrointestinal system.

Honey

There is one particular type of honey known as Manuka honey related to the tea tree and indicated for the treatment of infectious bacteria such as E. coli, Salmonella typhimurium and Staphylococcus aureus. Apart from this, it also helps in maintaining the patency of the gut walls hence aiding in the management of Irritable Bowel Syndrome (IBS)

Reduce on the intake of alcohol

Alcohol irritates the gut wall and may cause injury to them. This leads to malabsorption syndrome. It disrupts digestion in the stomach where proteins are supposed to be degraded. Hence, improper digestion leads to formation of hydrogen sulfide and then the rotten egg smell of fart and burps.

Reduce on trigger foods

The foods that lead to formation of hydrogen sulfide such as those with sulfur in them should be reduced. If garlic causes this reaction when you have it, then you need to either reduce on it or avoid it completely.

You need to identify the food that mostly leads to the smell and get an alternative. Refer to the previous sections of this material for foods that predispose you to hydrogen sulfide in the gut.

Step#3: Treatment of prevailing conditions

pylori needs to be treated for its adverse effects on the stomach. This bacterium is also responsible for the formation of hydrogen sulfide and dimethyl sulfide which make your breathe or burp really bad. This bacterium is just one of the many Halitogenic bacteria that exist in our gut.

Therefore, your doctor may prescribe some medication to curb release of excess acid due to this bacterium that makes you get the ulcers but will also make sure that you get an antibiotic drug. This will help eliminate the bacterium.

If your fart smell occurs alongside diarrhea or other stomach problem, then you might be infected by other bacterium such as Salmonella spp or some E. coli. These two are commonly associated with systemic symptoms such as a fever. There are tests to identify their presence and institution of appropriate treatment made.

Step#4: Physical exercise

This piece will never underscore the importance of involving yourself in some physical exercises each day. It is known to improve your digestion.

[1] Watanabe M. Osada J. Aratani Y. Kluckman K. Reddick R. Malinow MR. Maeda N. Mice deficient in cystathionine beta-synthase: animal models for mild and severe homocyst (e) inemia. Proc Natl Acad Sci U S A. 1995 92:1585–1589.

[2] Dimethylsulphidemia: the significance of dimethyl sulphide in extra-oral, blood borne halitosis. Harvey-Woodworth CN, Br Dent J. 2013 Apr 214(7):E20.


San Francisco's Rotten Egg Smell Mystery Is All About The Sulfur Chemistry

Officials are still working to pinpoint what caused a rotten-egg stench reported across the San Francisco Bay area last week.

San Francisco Bay as seen from above in San Francisco, California on October 9, 2015. (Credit: Josh . [+] Edelson/AFP/Getty Images)

No injuries have been reported, but air quality and health officials in the region said in multiple statements that they’re continuing to monitor a petroleum refinery on San Francisco Bay, a county composting facility, landfills, sewage treatment facilities, and shipping traffic.

So what is rotten egg smell and why is it so offensive to our noses?

Blame the element sulfur. Sulfur's in some of the most gut-wrenching stenches in the book. Hydrogen sulfide, a compound of one sulfur atom and two hydrogen atoms (H2S), is the culprit when something reeks of “rotten egg”. Skunk spray is a cocktail of sulfur compounds. Putrid food? Burning rubber? Bad breath? Yup, sulfur compounds. (More detail can be had from infographic guru Andy Brunning at Compound Interest, as well as this handy guide to sulfurous stenches from pharmaceutical chemist and pioneering science blogger Derek Lowe, if you’re into that sort of thing.)

It doesn’t take many of these molecules to raise a stink. Human noses can detect them at extremely low concentrations - more dilute than a single drop of dye inside of a 16-gallon water tank. While not every sulfur-containing molecule has a disgusting odor, experts think our sensitivity to them could have evolved as a biological warning system for predators (body odor) or poisons (hydrogen sulfide is highly toxic, and some sulfur compounds emanate from rotting food). Primates smell sulfur compounds as easily as people can or more so. Our sensitivity to this type of stench can be a helpful feature in some cases-- natural gas companies add a stinky sulfur compound to their product so that people can detect a gas leak at home.

Humans perceive foul odors of these sulfur-containing compounds at low concentrations. The motif . [+] circled in green is called a "thiol" group and is common in malodorous sulfur-containing molecules. (Credit: Carmen Drahl)

You might now be wondering how sulfur compounds could end up at the Bay Area sites under investigation.

Let’s start with the refinery. Hydrogen sulfide, the rotten-egg-smell compound, is a natural component of crude petroleum and natural gas. Bay Area authorities are looking at two instances of “flaring” at the refinery as possible causes of the stench. Flaring is when a refinery burns off excess gases, either as a safety measure during incidents like power glitches and pressure buildups, or because the gases cannot be processed and sold. Burning hydrogen sulfide in a flare produces sulfur dioxide, a compound with the smell of a just-lit kitchen match. It’s possible to minimize sulfur release through other kinds of processing.

Chevron’s own investigation shows no connection between the flaring and the odors, a spokeswoman told KQED News. However, the air-quality agency’s data suggest sulfur was present.

What about the waste locations, like wastewater treatment plants, landfills and composting facilities? With proper aeration, sulfur in waste can convert to compounds called sulfates. Sulfates don’t have the pungent aroma of rotten eggs or fetid feet. Without proper aeration, however, foul odors can crop up. That’s why folks who compost are advised to regularly turn and fluff up their compost pile.

As for shipping traffic, petroleum products would again be the most likely source of a stench coming from there. Vapors containing hydrogen sulfide can leak when a product is being transferred from ship to shore, as was suspected in a 2012 incident in southern California.

Finally, it’s worth noting that shoreline cities are no strangers to stomach-turning rotten-egg stench, and that it sometimes has natural causes, like algae blooms and volcanoes. All that's left to do is for officials to keep probing the possibilities.

This post may give sulfur a bad rap, but the element is also in many pleasant-smelling aroma compounds, such as the ones in truffles, fresh-brewed coffee, and a multitude of foods.

At Scientific American, Ash Jogalekar posted about stenches involving elements below sulfur on the periodic table. Two words— tellurium breath.

UPDATE 1/23: Image caption updated, because it does not depict Golden Gate Bridge as previously stated. It depicts San Francisco Bay and the San Francisco-Oakland Bay Bridge.


Why do boiled eggs smell but scrambled eggs don't?

This question was posed by a friend and I don't know the answer. She's curious about the smell when you finish cooking eggs by the different methods. I think she may not be able to smell scrambled eggs because of how often she eats them. Please correct me!

Boiled eggs smell because sulphur compounds in the proteins making up the egg white release hydrogen sulphide when heated to 100 C. When making scrambled eggs (1) the temperature is usually less than 100 C and (2) compounds in the egg yolk (which is kept separate when boiling) react with any hydrogen sulphide thus neutralising any smell.

My guess would be that when you boil an egg the gasses stay concentrated but more importantly it is an anaerobic process. No oxygen is present during the moment where the egg is exposed to heat. The smellyness arises from sulfur compounds probably degradation products of proteins containing the cystein amino acid. Oxygen likes to react with sulfur, creating sulfoxides. Sulfur compounds lose their smellyness when they are converted into sulfoxides (it also makes them way less volatile). So the absence of oxygen during the heating makes the smelly sulfur survive untill you cool it down and open the gas bomb. When you scramble the egg the proteins are exposed to both oxygen and higher temperatures, leading to conversion of sulfoxides and thus getting rid of the smell.


How We Smell Those Delightful Little Sulfur Compounds

We humans have a huge number of different smell receptors, but some of the most famous are the ones that are sensitive to thiols. We don’t miss out on many low oxidation state sulfur compounds: S-alkyl and SH groups reek to the skies as far as our noses are concerned (as do the corresponding selenium compounds). One classic example is ethanol versus ethanethiol, SH for OH on a two-carbon chain. Ethanol smells quite pleasant, even for those of us who don’t drink it, but ethanethiol is something else again. It’s the prototype Sulfur Smell, sort of like what you’d get if someone tried to extinguish a smoldering tractor tire by dumping dead skunks on it. A lot of the lower-molecular-weight thiols fall somewhere on that burning rubber/angry skunk axis, but other delightful notes creep in as well (Major Spill at the Garlic Factory. . .Ah, That’s Why People Don’t Make Hard-Boiled Eggs By Grilling Them In Their Shells. . .Who Left All This Shredded Cabbage Under This Tarp. . .I’ve Eaten Nothing But Raw Broccoli and Green Onions For Six Days Now. . .that sort of thing). Once in a while this can also show up as a drug side effect and there’s not much to be done.

We’ve developed our insane sensitivity to these compounds – the human nose can detect ethanethiol at levels at least one hundred million times lower than we can ethanol, and this effect is used deliberately so that we can smell natural gas leaks. Sulfur compounds like these are the smell of things that will kill us – rotten food, dangerous vapors, probably carnivore excretion/body odors as well. Evolutionarily, there’s clearly been a very strong selection pressure away from such substances, and we’re descended from a long, long time of creatures that put time and effort into avoiding them. But how does that sensitivity work?

This new paper has an interesting answer: copper atoms. Robert Crabtree had proposed this mechanism years ago, and it appears that he’s right. The OR2T11 receptor, known to be sensitive to short-chain thiols, seems to have a very pronounced dependence on copper for it to work at all. A close look at its structure shows two very likely metal binding sites near its receptor pocket as well, and NMR experimental results are consistent with the mechanism. Not all the thiol receptors show this copper effect (human odor sensing is very complex), but this one certainly does.


  • Sulfate reduction is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments.
  • Sulfate reducers may be organotrophic, using carbon compounds, such as lactate and pyruvate as electron donors, or lithotrophic, and use hydrogen gas (H2) as an electron donor.
  • Before sulfate can be used as an electron acceptor, it must be activated by ATP -sulfurylase, which uses ATP and sulfate to create adenosine 5&prime-phosphosulfate (APS).
  • Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth.
  • Toxic hydrogen sulfide is one waste product of sulfate-reducing bactera, and is the source of the rotten egg odor.
  • Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils.
  • lithotrophic: Obtains electrons for respiration from inorganic substrates.
  • organotrophic: Obtains electrons for respiration from organic substrates.

Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain. Compared to aerobic respiration, sulfate reduction is a relatively energetically poor process, though it is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments.

Many sulfate reducers are organotrophic, using carbon compounds, such as lactate and pyruvate (among many others) as electron donors, while others are lithotrophic, and use hydrogen gas (H2) as an electron donor. Some unusual autotrophic sulfate-reducing bacteria (e.g., Desulfotignum phosphitoxidans) can use phosphite (HPO 3- ) as an electron donor, whereas others (e.g., Desulfovibrio sulfodismutans, Desulfocapsa thiozymogenes, and Desulfocapsa sulfoexigens) are capable of sulfur disproportionation (splitting one compound into two different compounds, in this case an electron donor and an electron acceptor) using elemental sulfur (S0), sulfite (SO3 2&minus ), and thiosulfate (S2O3 2&minus ) to produce both hydrogen sulfide (H2S) and sulfate (SO4 2&minus ).

Before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine 5&prime-phosphosulfate (APS). APS is subsequently reduced to sulfite and AMP. Sulfite is then further reduced to sulfide, while AMP is turned into ADP using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction.

All sulfate-reducing organisms are strict anaerobes. Because sulfate is energetically stable, it must be activated by adenylation to form APS (adenosine 5&prime-phosphosulfate) to form APS before it can be metabolized, thereby consuming ATP. The APS is then reduced by the enzyme APS reductase to form sulfite (SO3 2&minus ) and AMP. In organisms that use carbon compounds as electron donors, the ATP consumed is accounted for by fermentation of the carbon substrate. The hydrogen produced during fermentation is actually what drives respiration during sulfate reduction.

Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon after life emerged on Earth. Sulfate-reducing bacteria are common in anaerobic environments (such as seawater, sediment, and water rich in decaying organic material) where they aid in the degradation of organic materials. In these anaerobic environments, fermenting bacteria extract energy from large organic molecules the resulting smaller compounds (such as organic acids and alcohols) are further oxidized by acetogens, methanogens, and the competing sulfate-reducing bacteria.

Many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell components this is known as assimilatory sulfate reduction. By contrast, sulfate-reducing bacteria reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste this is known as &ldquodissimilatory sulfate reduction. &rdquo Most sulfate-reducing bacteria can also reduce other oxidized inorganic sulfur compounds, such as sulfite, thiosulfate, or elemental sulfur (which is reduced to sulfide as hydrogen sulfide).

Toxic hydrogen sulfide is one waste product of sulfate-reducing bacteria its rotten egg odor is often a marker for the presence of sulfate-reducing bacteria in nature. Sulfate-reducing bacteria are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge. Thus, the black color of sludge on a pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

Figure: Black sludge: The black color of this pond is due to metal sulfides that result from the action of sulfate-reducing bacteria.

Some sulfate-reducing bacteria play a role in the anaerobic oxidation of methane (CH4+ SO4 2- &rarr HCO3&ndash + HS&ndash + H2O). An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing bacteria in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments.This process is also considered a major sink for sulfate in marine sediments. In hydrofracturing fluids used to frack shale formations to recover methane (shale gas), biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing bacteria in order to avoid anaerobic methane oxidation and to minimize potential production loss.

Sulfate-reducing bacteria often create problems when metal structures are exposed to sulfate-containing water. The interaction of water and metal creates a layer of molecular hydrogen on the metal surface. Sulfate-reducing bacteria oxidize this hydrogen, creating hydrogen sulfide, which contributes to corrosion. Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the biogenic sulfide corrosion of concrete, and sours crude oil.

Sulfate-reducing bacteria may be utilized for cleaning up contaminated soils some species are able to reduce hydrocarbons, such as benzene, toluene, ethylbenzene, and xylene. Sulfate-reducing bacteria may also be a way to deal with acid mine waters.


The primary candidates for volatilization of odorous sulfur gases are $ce$ and $ce$. Unless the elemental sulfur has been physically ground and dispersed into the air it is not likely the direct source of the smell. According to the International Volcanic Health Hazard Network, who of course study gaseous volcanic emissions with respect to safety issues: "Sulfur dioxide ($ce$) is a colourless gas with a characteristic and irritating smell. This odour is perceptible at different levels depending on the individual's sensitivity, but is generally perceived between 0.3-1.4 ppm and is easily noticeable at 3 ppm." Regarding $ce$, according to the US Occupational Safety and Health Administration (OSHA), $ce$ has a perceptible smell at between 0.01 and 1.5 ppm. The two gases have different smells, $ce$ being that of rotten eggs and that of $ce$ being described as "pungent and irritating". So that would be one way to differentiate which gas you are smelling. The most likely candidate is $ce$ as our air is generally oxidizing, and to me elemental sulfur smells more like $ce$ than rotten eggs. Request Professional Help To Sanitize Your Kitchen Sink

Of course, you can always rely on a high quality professional to come out and the service your kitchen sink if you feel incapable of doing so yourself.

Often, this is the best option because you can avoid getting face to face with the smell, especially if you find it to be overpowering. Please make sure any companies you work with are well-reviewed and have excellent reputations before you make any commitments otherwise, this is probably the way to go.