There are twenty genetically encoded amino acids that create the thousands of different proteins that are known in all the different kingdoms of living organisms. There are many other amino acids naturally occuring on Earth, that do not become translationally incorporated into proteins, so called nonproteinogenic amino acids. Two primary theories as to why proteins all use the same twenty amino acids are that 1.) These twenty amino acids created the most fit organisms for our planetary conditions over millions of years of evolution, and 2.) These were the twenty amino acids that were plentifully available when proteinogenic life evolved. However, neither of these hypotheses provide valid reasoning supporting that these same twenty amino acids would be solely used if life were to have evolved extrateresstrially. The conservative hypothesis of the possiblilty of extraterrestrial life posits that liquid water is a fundamental requirement. Mars, Enceladus, and Europa are considered the most likely bodies in our Solar System to contain liquid water now, or in the past. Mars, Enceladus, and Europa all have quite different environments than Earth; most notably their thin atmosphere allows a great deal of UV and cosmic radiation to impact the surface. The composition of the possible subsurface oceans of Europa and Enceladus is also unknown, with high salinity, non-neutral pH, and high concentrations of antifreeze molecules being potential components.<br/><br/>In this work we analyzed the stability of proteinogenic amino acids versus the stability of nonproteinogenic amino acids in liquid water when exposed to a variety of such potential extraterrestrial conditions. Aqueous 100 μM amino acid solutions were subjected to a variety of irradiation and chemical exposures, and their degradation quantified using UPLC-MS. Halogenation of certain aromatic nonproteinogenic amino acids caused them to be significantly more stable to irradiation than their nonhalogenated counterparts. Additionally, other functional substitutions to tryptophan, phenylalanine, and tyrosine caused increased or decreased stability as compared to native Trp, Phe, and Tyr. The results indicate that certain nonproteinogenic amino acids are more stable under environmental conditions that may be encountered on Mars, Enceladus, or Europa, implying that nonproteinogenic amino acids may play a role in the potential evolution of extraterrestrial living organisms.

Stability of nonproteinogenic amino acids under extraterrestrial conditions

255th National Meeting (2018)

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