Chemistry makes me cry

A recent discussion on Twitter about lachrymators (from lacrima meaning tear in Latin) has got me to wondering about chemicals that we use in the lab and the effect they can have on our “emotional” state.  Clearly, lachrymators don’t change our emotional state but when they are released they do make us look like immensely sad.

The most common culprit in our undergraduate labs is benzyl bromide. On contact with the eye, the chemical stimulates sensory neurons creating a stinging, painful sensation which causes tears to be released from the tear glands to dilute and flush out the irritant. When this gets released in a lab, everyone knows about it, especially when there are dozens of undergraduates using it.

Benzyl bromide

More commonly found lachrymators are onions, which release syn-propanethial S-oxide on slicing. The release is due to the breaking open of the onion cells and their releasing enzymes called alliinases, which then break down amino acid sulfoxides, generating sulfenic acids. A specific sulfenic acid, 1-propenesulfenic acid, is rapidly rearranged by a second enzyme, called the lachrymatory factor synthase (LFS), giving syn-propanethial S-oxide.  

Syn-propanethial S-oxide

1-propenesulfenic acid

Lachrymators were commonly used in World War I as “tear gas”.  Extremely low concentrations of lachrymators caused an intense irritant action on the eyes. This caused tears and pain which then resulted in reduced vision which meant that the soldier became impaired. Benzyl bromide, bromoacetone, dibrommethylethylketone (which could prove fatal), ethyl iodoacetate and xylyl bromide were the common culprits.

There are also many other lachrymators that are used today for crowd control purposes. In the past, phenacyl chloride was used but this has mostly been replaced by 2-chlorobenzalmalononitrile (CS gas), dibenzoxazepine (CR gas) and pepper spray (OC gas) due to the toxicity of phenacyl chloride. Pepper spray contains capsaicin which is a capsaicinoid which is produced as a secondary metabolite by chili peppers. It is why you may ‘cry’ when you are chopping up chillies…you may notice it more acutely when you accidentally rub your eyes after cooking with chillis.

I have explained that chemicals can make us ‘cry’ (probably not as much as our PhDs did) but chemicals can also have the opposite effect and make us giddy with laughter. I have never experienced this but I am told it is quite pleasant!

Nitrous oxide, or laughing gas, was discovered in 1799 by British chemist Humphry Davy, is an example of a chemical that makes us feel happy. Inhalation of nitrous oxide for recreational use, with the purpose of causing euphoria and possibly slight hallucinations, began in 1799 and was commonly used by the upper class at ‘laughing gas parties’. Nitrous oxide abuse has even been documented (Emergency Medicine Australasia (2010) 22, 88–90). It is also used as an analgesic, particularly in the dental profession although my dentist always chooses to use the unpleasant Lidocaine.

Nitrous oxide

Nitrous oxide quickly enters the bloodstream through the alveoli in the lungs and is distributed quickly through the whole body, including to synapses in the brain. Nitrous oxide is an uncompetitive NMDA channel blocker which blocks the ion channel by binding to a site within it. The NMDA receptor is a receptor which allows for the transfer of electrical signals between neurons in the brain and in the spinal column. For electrical signals to pass, the NMDA receptor must be open but when nitrous oxide is used, it is blocked.

Obviously, the laughing gas is causing a change in the chemistry of our brains, unlike lachrymators, which just make us look like we are upset but I just wanted to show that chemistry can make us cry, but it can also make us happy.**

Interesting reads:

J. Carson, Food Reviews International, 1987, 3, 71-103
Nitrous Oxide: No Laughing Matter – D. Wuebbles, Science, 2009, 326, 56-57

** I am not condoning the use of laughing gas to make you happy. Eat some chocolate instead.

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