Most 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 most 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 multicell organisms on Earth are made of L-(or levo-)amino acids: that is, most of the molecule's atoms are on the left side. 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. Referencing this fact is a fairly common way for science-fiction writers to Show Their Work. 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. Expect some mention of the fact that ethanol is one of the few biologically interesting molecules 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.
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 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, 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.
- 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 even number of wormholes) and one for the ones who are "left-handed" (having passed through an odd 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 Callahans Crosstime Saloon has a "Mirror Earth" with right-handed proteins in the place of our left-handed ones. The protagonists eventually discover that this inverts the perceived quality of liquor: our swill is their Wonderbooze and vice versa.
- 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! by James Blish, the first Star Trek Expanded Universe novel, 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 are incompatible with human biology. It is also mentioned that the Hive is unique among all known species in that it is able to digest proteins of either chirality.
- The Isaac Asimov short story "Left to Right" has a scientist create 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.
- An important plot point in 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.)
- An important plot point in Marti Steussy's Dreams of Dawn. Juvenile (and not yet sentient) 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.
- In 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.
- In the Arthur C. Clarke short story "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.
- An episode of Black Hole High was actually called "Chirality," and introduced the concept in a chemistry class—then used it as a justification for a Personality Swap episode.
- In GURPS Time Travel, this is one of the entries on the "something went wrong with our dimension-traveling device" chart.
- 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 can (at best) pass through one's system without providing any nutrients and (at worst) lead to dangerous allergic reactions.
- 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.
- From the files of the SCP Foundation comes SCP-739, 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.
- 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. 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).
- This is not to be confused with the molecule's optical activity: that is, if we shine a light through its solution, l-isomer will rotate its polarization plane (a plane the EM-wave is oscillating) to the left, that is, counter-clockwise, and d-isomer, or dextrorotatory one, will do it to the right, or clockwise. Many L-isomers are in fact l- (or (-), as chemists now prefer to designate them) isomers, and vice versa, but it's not true for all of them.