Mirror life

Many of the essential molecules for life on Earth can exist in two mirror-image forms, often called "left-handed" and "right-handed", where handedness refers to the direction in which polarized light skews when beamed through a pure solution of the molecule, but living organisms do not use both.{{Cite magazine |title=Building a parallel universe |language=en-GB |magazine=Wired UK |url=https://www.wired.co.uk/article/building-a-parallel-universe |access-date=2023-10-27 |issn=1357-0978}} RNA and DNA contain only right-handed sugars; proteins made by the ribosome{{efn|many bacteria and fungi are able to synthesise non-ribosomal peptides containing right-handed amino acids, as the example of peptidoglycan synthesis shows.}} are exclusively composed of left-handed amino acids. This phenomenon is known as homochirality.{{cite book |last1=Plaxco |first1=Kevin W. |last2=Michael |first2=Michael |title= Astrobiology: A Brief Introduction |date=2011 |publisher=Johns Hopkins University Press |isbn=978-1-4214-0194-2 |pages=140–141}} It is not known whether homochirality emerged before or after life, whether the building blocks of life must have this particular chirality, or indeed whether life needs to be homochiral.{{cite news |last1=Sedbrook |first1= Danielle |title=Must the Molecules of Life Always be Left-Handed or Right-Handed? |url= https://www.smithsonianmag.com/space/must-all-molecules-life-be-left-handed-or-right-handed-180959956/ |accessdate=8 May 2018 |work= Smithsonian.com |date=28 July 2016 |language= en}} Protein chains built from amino acids of mixed chirality tend not to fold or function well, but mirror-image proteins have been constructed that have identical function but on substrates of opposite handedness.

Advances in synthetic biology, like synthesizing viruses since 2002, partially synthetic bacteria in 2010, and synthetic ribosomes in 2013, may lead to the possibility of fully synthesizing a living cell from small molecules, which could enable synthesizing mirror cells from mirrored versions (enantiomers) of life's building-block molecules. Some proteins have been synthesized in mirror-image versions, including polymerase in 2016.{{cite journal |last1=Wang |first1=Zimou |last2=Xu |first2=Weiliang |last3=Liu |first3=Lei |last4=Zhu |first4=Ting F. |title=A synthetic molecular system capable of mirror-image genetic replication and transcription |journal=Nature Chemistry |volume=8 |issue=7 |year=2016 |pages=698–704 |issn=1755-4330 |doi=10.1038/nchem.2517 |pmid=27325097 |bibcode=2016NatCh...8..698W}}{{cite journal |last1=Xu |first1=Yuan |last2=Zhu |first2=Ting F. |title=Mirror-image T7 transcription of chirally inverted ribosomal and functional RNAs |journal=Science |publisher=American Association for the Advancement of Science (AAAS) |volume=378 |issue=6618 |date=2022-10-28 |issn=0036-8075 |doi=10.1126/science.abm0646 |pages=405–412 |pmid=36302022 |bibcode=2022Sci...378..405X |s2cid=253183402}}

Reconstructing regular lifeforms in mirror-image form, using the mirror-image (chiral) reflection of their cellular components, could be achieved by substituting left-handed amino acids with right-handed ones, in order to create mirror reflections of proteins, and likewise substituting right-handed with left-handed nucleic acids.{{Cite magazine |last=Bohannon |first=John |date=2010 |title=Mirror-image cells could transform science - or kill us all |url=https://www.wired.com/2010/11/ff_mirrorlife/ |archive-url=https://web.archive.org/web/20200809194019/https://www.wired.com/2010/11/ff_mirrorlife/ |archive-date=2020-08-09 |magazine=Wired |volume=18 |issue=12}} Because the phospholipids of cell membranes are also chiral, American geneticist George Church proposed using an achiral fatty acid instead of mirror-image phospholipids for the membrane.{{efn|An achiral version of phospholipids is not strictly required, as both chiralities of phospholipids are already used in the cell membrane of existing life forms: eukaryotes and bacteria use one chirality (G3P) while archaea use the other (G1P). The two have even been mixed using genetic engineering, producing viable modified E. coli.{{cite journal |last1=Yokoi |first1=Takeru |last2=Isobe |first2=Keisuke |last3=Yoshimura |first3=Tohru |last4=Hemmi |first4=Hisashi |title=Archaeal Phospholipid Biosynthetic Pathway Reconstructed in Escherichia coli |journal=Archaea |date=2012 |volume=2012 |pages=1–9 |doi=10.1155/2012/438931|doi-access=free |pmid=22645416 |pmc=3357500 }} Genetic evidence for a natural mixed-membrane system have also been found, pending definitive proof by chemical analysis.{{cite journal |last1=Villanueva |first1=Laura |last2=Bastiaan von Meijenfeldt |first2=F A |last3=Westbye |first3=Alexander B |last4=Yadav |first4=Subhash |last5=Hopmans |first5=Ellen C |last6=Dutilh |first6=Bas E |last7=Sinninghe Damsté |first7=Jaap S |title=Bridging the membrane lipid divide: bacteria of the FCB group superphylum have the potential to synthesize archaeal ether lipids |journal=The ISME Journal |date=1 January 2021 |volume=15 |issue=1 |pages=168–182 |doi=10.1038/s41396-020-00772-2 |doi-access=free|pmid=32929208 |pmc=7852524 |bibcode=2021ISMEJ..15..168V }}}}

Electromagnetic force (chemistry) is unchanged under such molecular reflection transformation (P-symmetry). There is a small alteration of weak interactions under reflection, which can produce very small corrections that theoretically favor the natural enantiomers of amino acids and sugars,{{cite journal |last1=Tranter |first1=G.E. |title=Parity violation and the origins of biomolecular handedness |journal=Biosystems |date=January 1987 |volume=20 |issue=1 |pages=37–48 |doi=10.1016/0303-2647(87)90018-9 |pmid=3580532 |bibcode=1987BiSys..20...37T}} but it is unknown if this effect is large enough to affect the functionality of mirror biomolecules or explain homochirality in nature.{{cite journal |last1=Quack |first1=Martin |last2=Seyfang |first2=Georg |last3=Wichmann |first3=Gunther |title=Perspectives on parity violation in chiral molecules: theory, spectroscopic experiment and biomolecular homochirality |journal=Chemical Science |date=2022 |volume=13 |issue=36 |pages=10598–10643 |doi=10.1039/d2sc01323a |pmid=36320700 |pmc=9491092 |hdl=20.500.11850/569820 |hdl-access=free}}

Mirror animals would need to feed on reflected food, produced by reflected plants. Mirror viruses would not be able to attack natural cells, just as natural viruses would not be able to attack mirror cells.

Mirror life presents potential dangers. For example, a chiral-mirror version of cyanobacteria, which only needs achiral nutrients and light for photosynthesis, could take over Earth's ecosystem due to lack of natural enemies, disturbing the bottom of the food chain by producing mirror versions of the required sugars. Some bacteria can digest L-Glucose; exceptions like this would give some rare lifeforms an unanticipated advantage.

Direct application of mirror-chiral organisms can be mass production of enantiomers (mirror-image) of molecules produced by normal life.

• Enantiopure drugs - some pharmaceuticals have shown different activity depending on enantiomeric form,
• Aptamers (L-ribonucleic acid aptamers): "That makes mirror-image biochemistry a potentially lucrative business. One company that hopes so is Noxxon Pharma in Berlin. It uses laborious chemical synthesis to make mirror-image forms of short strands of DNA or RNA called aptamers, which bind to therapeutic targets such as proteins in the body to block their activity. The firm has several mirror-aptamer candidates in human trials for diseases including cancer; the idea is that their efficacy might be improved because they aren't degraded by the body's enzymes. A process to replicate mirror-image DNA could offer a much easier route to making the aptamers, says Sven Klussmann, Noxxon Pharma's chief scientific officer."{{cite journal |last1= Peplow |first1=Mark |title=Mirror-image enzyme copies looking-glass DNA |journal=Nature |date=16 May 2016 |volume=533 |issue=7603 |pages=303–304 |doi= 10.1038/nature.2016.19918 |pmid=27193657 |bibcode= 2016Natur.533..303P |doi-access=free}}
• L-Glucose, enantiomer of standard glucose, Tests showed that it tastes likes standard sugar, but is not metabolized the same way. However, it was never marketed due to excessive manufacturing costs.{{cite web |url=https://spinoff.nasa.gov/Spinoff2004/ch_4.html |title=A natural way to stay sweet |work=NASA |date=2004 |access-date=17 December 2024 }} More recent research allows cheap production with high yields; however the authors state that it is not usable as a sweetener due to laxative effects.{{cite journal |last1=Martinez |first1=RF |title=Short and sweet: (D)-glucose to (L)-glucose and (L)-glucuronic acid |journal=Angewandte Chemie International Edition |date=5 December 2013 |volume=53 |issue=4 |pages=1160–2 |doi=10.1002/anie.201309073 |pmid=24310928 |id=Epub 2013 Dec 5}}

The creation of a mirror human is the basis of the 1950 short story "Technical Error" by Arthur C. Clarke.{{Cite web |title=Technical Error |url=https://www.goodreads.com/book/show/18488976-technical-error |access-date=2023-10-27 |website=Goodreads |language=en}} In this story, a physical accident transforms a person into his mirror image, speculatively explained by travel through a fourth physical dimension. H. G. Wells' The Plattner Story (1896) is based on a similar idea.

In the 1970 Star Trek novel Spock Must Die! by James Blish, the science officer of the USS Enterprise is replicated in mirror form by a transporter mishap. He locks himself in the sick bay where he is able to synthesize mirror forms of basic nutrients needed for his survival.{{Cite web |title=Editions of Spock Must Die! by James Blish |url=https://www.goodreads.com/work/editions/2893169-spock-must-die |access-date=2023-10-27 |website=Goodreads }}

An alien machine that reverses chirality, and a blood-symbiont that functions properly only when in one chirality, were central to Roger Zelazny's 1976 novel Doorways in the Sand.{{Cite web |title=Doorways in the Sand |url=https://www.goodreads.com/book/show/61998.Doorways_in_the_Sand |access-date=2023-10-27 |website=Goodreads |language=en}}

On the titular planet of Sheri S. Tepper's 1989 novel Grass, some lifeforms have evolved to use the right-handed isomer of alanine.{{Cite web |title=Grass (Arbai, #1) |url=https://www.goodreads.com/book/show/104342.Grass |access-date=2023-10-27 |website=Goodreads |language=en}}

In the Mass Effect series, chirality of amino acids in foodstuffs is discussed often in both dialogue and encyclopedia files.

In the 2014 science fiction novel Cibola Burn by James S. A. Corey, the planet Ilus has indigenous life with partially-mirrored chirality. This renders human colonists unable to digest native flora and fauna, and greatly complicates conventional farming. Consequently, the colonists have to rely upon hydroponic farming and food importation.{{Cite web |last=Noble |first=Barnes & |title=Cibola Burn (Expanse Series #4){{!}}Paperback |url=https://www.barnesandnoble.com/w/cibola-burn-james-sa-corey/1117054786 |access-date=2023-10-27 |website=Barnes & Noble |language=en}}

In the 2017 Daniel Suarez novel Change Agent, an antagonist, Otto, nicknamed the "Mirror Man", is revealed to be a genetically-engineered mirror human. Serving as an assassin due to his complete immunity to neurotoxins, which he coats himself with in the form of a cologne-like aerosol, he views other humans with disdain and causes them to feel an inexplicable repulsion by his very presence.{{Cite web |title=Change Agent |url=https://www.goodreads.com/book/show/31396262-change-agent |access-date=2023-10-27 |website=Goodreads |language=en}}

The concept is used during Ryan North's 2023 run on Fantastic Four as an existential threat towards the human population.{{Cite web |date=2023-07-11 |title=Fantastic Four by Ryan North Vol. 1: Whatever Happened to the Fantastic Four? |url=https://mitpressbookstore.mit.edu/book/9781302932633 |access-date=2023-10-27 |website=mitpressbookstore.mit.edu |language=en}}

• Xenobiology
• Mirror matter – A hypothetical form of matter that interacts only weakly with normal matter, which could form mirror planets, potentially inhabited by mirror matter life.
• {{Section link|Peptidomimetic#D-peptides}}

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Category:Chirality

Category:Synthetic biology

Category:Hypothetical life forms