Turian shopkeeper: You don't mix your spice chirality! What cooking school did you say you went to?
Many of the molecules required for life have the property known as chirality or "handedness". That is, they are not the same as their mirror image, like a left shoe which will not fit properly on your right foot no matter how you rotate it. That wouldn't have been that much of a problem, if not for the one insignificant fact that a lot of the stuff our bodies are made of, mainly the amino acids, which are the building blocks for the proteins we consist of, are chiral. And all multicellular organisms on Earth are made of L-(or levo-)amino acids.note Dextro-amino acids (except cysteine) are exceedingly rare in Earth organisms and are used only by some bacteria and in a few very specialized cases by larger creatures. Science-fiction writers have commonly Shown Their Work by referencing this fact. It generally comes up in one of two situations:
- Aliens can have their entire biochemistry built on molecules with the opposite chirality from that used on Earth, making it simultaneously identical to Earth chemistry (and therefore clearly possible) and incompatible with it.
- Some piece of Applied Phlebotinum may be capable of mirror-reversing people or objects.
In either case, this usually manifests itself as an inability to eat the same food in the same way. Either food tastes different depending on your handedness, or wrong-handed food has no nutritional value or is actively dangerous. There may be some mention of the fact that ethanol is a biologically interesting molecule that doesn't have this property — so, regardless of chirality, everyone can always get drunk together, and you can make moonshine from organic stuff that's otherwise useless for you because it's of the wrong chirality (assuming you have the right, or rather the left, handedness of microbes).
This is often a form of Artistic License Chemistry, especially when authors try to extrapolate to what happens with molecules that are too complex to be synthesized chemically. Contrast No Biochemical Barriers.
- In the massively crossover fanfic Bring Me the Head of Harry Potter, it's revealed that Voldemort used a spell that turned people inside out. Examining a battlefield long after the fact, Willow Rosenberg feels the earth's pain from wrong-chirality organic molecule poisoning.
- One Stargate SG-1 fanfic mentions a right-handed world being used as a prison planet. Plenty of oxygen, but if you escape the prison camp there's absolutely nothing to eat. Unless you dismember the guards.
- In Through the Looking Glass, Alice speculates that looking-glass milk might not be good to drink. This is probably the Ur-Example; in fact, it predates the coining of the word "chirality" by several years. In The Annotated Alice, Martin Gardner discusses the chemical reasons why this would be true, before moving on to point out that looking-glass milk would likely be made of antimatter,note making it really bad to drink.
- Doorways in the Sand by Roger Zelazny has an "n-axial inversion device", which mirror-reverses anyone or anything that passes through it. The protagonist goes through it about halfway through the book. It turns out that quite a few things taste better when you're mirror-reversed. Especially bourbon. This is because although alcohol is symmetric, the various flavorings are not.
- Of course, one has to be careful when using the device. One unfortunate individual accidentally goes through the wrong way and gets turned inside out!
- In There and Back Again by Pat Murphy, anyone who passes through a wormhole gets mirror-reversed. Any gathering of people who have done a lot of wormhole travel needs two cooks: one for the travelers who are currently "right-handed" (having passed through an odd number of wormholes) and one for the ones who are "left-handed" (having passed through an even number).
- In The Boy Who Reversed Himself by William Sleator, the protagonists gain the ability to enter the fourth dimension and therefore end up reversed. They learn that mirror-ketchup is an extremely addictive drug.
- One of the stories in Callahan's Crosstime Saloon has a "Mirror Earth" with right-handed proteins in the place of our left-handed ones. A con artist discovers a means to travel between the two, but finds that there's little he can do with it; Equivalent Exchange is in effect - if he does anything that benefits one world at the expense of the other, the gate will fail, possibly trapping him in a world where he can't digest the food. He finally hits on ethanol; it's not a chiral molecule as said above, but the esters (which determine taste and texture) are. Result: one world's really bad moonshine whiskey is the other's Wonderbooze.
- Destiny's Road by Larry Niven is set on a planet whose indigenous life uses right-handed proteins. This is initially problematic as they need to entirely sterilize an area of the planet so as to have somewhere to grow edible food, but it proves to have some advantages; it means they're immune to native diseases, and they discover that the planet's sea life is the perfect diet food as their bodies are incapable of metabolizing it into fat.
- Spock Must Die! has a mirror-reversed copy of Spock created in a transporter accident. He's unable to eat anything, and so he finds himself slowly starving while he works with a chemistry set in order to create mirrored food he can eat and survive.
- Tour of the Merrimack: The crew aren't worried about picking up any diseases in the Myriad because life on those worlds uses opposite-handed proteins, which are incompatible with human biology. It's likened to attempting to attach a four-post wheel to a five-post axle with opposite-thread bolts. It is also mentioned that the Hive is unique among all known species in that it is able to digest proteins of either chirality.
- Isaac Asimov's "Left To Right": The main character, a scientist, creates a device that will apply some sort of directional interchange on whatever passes through; the scientist intends to use himself as a test subject. There is a discussion about potential issues if he ends up with his heart on the wrong side or unable to eat because of wrong-handed proteins, but he plans on reversing himself again before that becomes an issue. It turns out not to matter; the device actually just changes the scientist's name from Robert L. Forward to Robert L. Backward.
- David J Lake's The Right Hand Of Dextra and The Wildlings Of Westron: Dextran food tastes vile and has no food value for humans; the same applies to Dextran natives and terrestrial produce. Of course, the Dextran Mothers can alter a creature's biochemistry. (Which has the side-effect of creating a connection to the planetary over-consciousness.)
- Marti Steussy's Dreams Of Dawn: Juvenile (and not yet sapient) Kargan natives like the taste of terrestrial food, but it can cause their metamorphosis process to go fatally awry thanks to some meddling by Abusive Precursors. The side effects of Kargan food on terrestrial species likewise range from indigestion to fatalities. Solved at the end by altering the colonists' biochemistry to match that of the native species, after much trial and error.
- The Pentagon War: Centaurian metabolism is based around levulorotary glucose (instead of the dextrorotary glucose we Earth people use). Centaurian food has no nutritional value to humans, and can sometimes be downright poisonous to them — and vice-versa.
- Arthur C. Clarke's "Technical Error": An experimental superconducting power station accidentally inverts Richard Nelson so that all his body chemistry is opposite-handed. He needs to have special "left-handed" food synthesized for him, and since no one is sure if we've missed any essential micronutrients that would also need to be inverted for him, he figures his best option for long-term survival is to repeat the accident and try to re-invert himself.
- Star Carrier series:
- Osiris, a planet orbiting 70 Ophiuchi, is a rare garden world where humans can survive unprotected. However, they have to grow their own food crops since the native life has the opposite chirality from Earth life and is therefore inedible.
- Vulcan, a planet orbiting Epsilon Eridani A, has local life with the exact same chirality as terrestrial life. Unfortunately, the same is true for the new aliens discovered in the fifth book, and these aliens are looking for some new prey to hunt and eat.
- In The Documents in the Case by Dorothy L. Sayers, stereochemistry is proof that the death was a murder, not an accidental poisoning — the poison is optically inactive (and hence produced by synthesis in a laboratory) rather than optically active (as it would have been if it came from a poisonous toadstool eaten by mistake).
- In GURPS Time Travel, this is one of the entries on the "something went wrong with our dimension-traveling device" chart.
- Many colony worlds in the 2300 AD universe have dextro-based life. To get around this, human colonists sterilize large patches of ground and use "pay dirt" from Earth to set up a localized biosphere suitable for growing crops. On the other hand, many native creatures from dextro-worlds (most infamously Aurore) will take a bite out of humans or their livestock without worrying whether it will make them sick.
- In Mass Effect, the turians and quarians have roughly humanoid biochemistry, but with opposite-chirality proteins, while several other species have the same chirality of protein as humans do. Eating food intended for an opposite-chirality species will at best pass through one's system without providing any nutrients and at worst lead to dangerous allergic reactions. Discussed several times, particularly in Mass Effect 2:
- The page quote comes from an overheard interaction where a turian grocery store operator has to deal with a not-too-bright human customer.
- In a "deleted scene", a turian is amazed by human alcohol.
- In her bar on Ilium, Matriarch Aethyta warns Shepard against eating the beer nuts in the red bowls because they're for turians and quarians. She also speaks of a Noodle Incident where a krogan drank liquefied turian on a dare. Krogans are infamous for Super Toughness to the point where it's a Running Gag, but according to Aethyta "nobody came out of that one looking pretty."
- Krogan in general can eat just about anything. The worst they can get from eating dextro food is diarrhea.
- In the Dark Star Lounge on the Citadel, when Shepard asks for the strongest drink in the bar, the bartender serves up a green liquor and remarks: "[It's] guaranteed to knock you on your ass, unless you're dextro-DNA like me. If you are, it'll kill ya."
- This comes up in the second game if you want to romance your dextro-based squadmates. If a female Shepard romances her turian teammate Garrus, Dr. Mordin Solus warns her against swallowing turian semen. If a male Shepard romances his quarian teammate Tali, Dr. Mordin Solus warns him against unprotected oral sex (for Tali's sake, Shepard's sake or both).
- One of the "Cerberus Daily News" briefs mentions a new treatment that temporarily allows dextro- or levo-based species to safely eat the other type of food. Despite the fact that the eater gets no nutritional benefit from the mirror food and they have to "purge" within 24 hours, it becomes a fad among gourmands.
- In case you're wondering, Mass Effect pulls a major case of Artistic License Biology with this. Turns out the amino acid differences aren't dangerous to those that use the other ones. If anything they'd taste like mint or some kind of sugar.
- Freefall. In the 'garden-world' of Pfouts, life developed with opposite chirality as compared to humans, making it more inhospitable to human colonization than a bare ball of rock could ever be. Thus, some clever humans are speculating about uplifting a local species to sentience, thus gaining a colony-by-extension for humanity. One of the main characters is an uplifted wolf, made as a proof-of-concept for that endeavour.
- In LeavingTheCradle insectoid species are D-chiral, which makes them incompatible with other species biochemically, much like turians from Mass Effect. This sometimes causes some political problems, since their preferred method of colonizing other life-supporting worlds is to terraform them by blazing large patches of the continent to cinder and reseed it with samples of their own ecosystem.
- SCP Foundation:
- SCP-739 is a small closet with mirrored interiors. Closing a person in the booth once will flip all their amino-chains. Closing them in twice will switch them back, but repeating the experiment too many times with the same subject leads to something else emerging. As a fun side benefit, locking sugar packets in the closet will flip them as well. The scientist specifically notes that they would make excellent diet sweeteners.
- One of the items vended by SCP-261, an anomalous vending machine, is a bar of Hershey's with all text mirrored. Scientific analysis of the chocolate reveals the chirality of the amino acids and sugars that it contains is the opposite to those of Earth-life organic compounds.
- SCP-5300 is theorized to be a pocket of space possessing topology akin to that of a Klein bottle. Traversing the entire manifold and returning to where you started causes you to invert chirality. This caused lots of bad stuff to happen to the crew that got sucked into the anomaly.
- A recurring research project carried out by organic chemists has been to develop a sugar with at least part of the molecule having a reversed chirality: the goal is to have a sugar that will trigger the taste buds for sweetness the same but be unable to be taken up by the body (and as a bonus, the bacteria that cause tooth decay), thus being calorie-free.
- Receptors for proteins, hormones, and other organic chemical compounds are made to fit certain molecular structures of a certain chirality. If a molecule does not have the correct chirality, it doesn't fit in the receptor, or worse, can actively harm the organism. As if that's not enough, the human body can sometimes metabolize a useful structure into its harmful mirror image. This was what happened with Thalidomide - a certain chiral structure (also called an enantiomer) of the compound was useful for the effects of morning sickness in pregnant women, but the other enantiomer of it caused terrible birth defects. The early testing of the compound was performed on just the desired enatiomer, but the mass-production synthesis method produced a racemic form (Note that Science Marches On ... this story is probably still told to first year organic chemistry students as a cautionary tale about the importance of chirality, but it turns out that thalidomide racemizes in vivo and even if the testing was done with one chirally pure form, some of it would have converted to the other enantiomer as soon as it was taken). This is also the case with the cancer-fighting drug Platinol, which only causes sickness because scientists can't separate the helpful enantiomer of the drug from the toxic one, so it's sold as a mixture of the two (any mixture of both enantiomers of a compound is called a racemic mixture).
- Also played by big pharmaceutical companies. A company will frequently develop and market a medication consisting of some compound in a racemic mixture, sell that until its patent exclusivity expires, and then release a "new" medication consisting of only the biologically active enantiomer of the original compound, so as to gain a whole new patent life for essentially the same compound. A good clue to this tactic is a generic drug name that looks like the original with a S or D added. Examples include Prilosec (omeprazole) vs. Nexium (esomeprazole), Celexa (citalopram) vs. Lexapro (escitalopram), Claritin (loratadine) vs. Clarinex (desloratadine), and so on.
- To be fair, separating the enantiomers is bloomin' hard and expensive. And pharmaceutical companies are all about making profit.
- The artificial sweetener Aspartame (which you might see under trade names NutraSweet (tm) or Equal (tm).) One enantiomer tastes sweet (sweeter than sugar, in fact), and the other tastes bitter. (This is why diet sodas can taste "off" to some people.) Also worth noting that citric acid reverses the bitter enantiomer into the sweet one.
- The pungently orange-scented solvent popular under various trade names as a household cleaner is the D-enantiomer of the hydrocarbon limonene. The R-enantiomer is also a solvent that can be used as a cleaner, but smells of pine. Upon learning this, some people suddenly realize the true but often-overlooked fact that pine and citrus are very similar scents. Well, often overlooked by everyone except chefs and bartenders—why do you think (piney) rosemary goes so well with lemon on chicken and fish, or why citrus is so often paired with gin (whether it be lime juice in a gimlet, a lime wedge in a G&T, or a lemon twist in a martini)?