The Detection of Aromatic Amino Acids in CR Chondrites Suggests They are Prebiotically Plausible

Introduction: Amino acids (AAs) are the building blocks of proteins and are essential for life on Earth. Sources of AAs on early Earth range from exogenous sources such as delivery from extraterrestrial materials (i.e., meteorites, interplanetary dust particles, comets, etc.) to endogenous sources including chemical reactions in hydrothermal vents or atmospheric mixtures [1]. Analyses of AAs of extraterrestrial origin from meteorites and laboratory studies of simulated prebiotic environments revealed that a diverse set of abiotically produced AAs may have been present on early Earth. However only a fraction of these AAs is found within the proteins of living organisms. Of the 22 genetically encoded (proteinogenic) AAs, 20 occur in the standard genetic code (SGC) and are used by nearly all living organisms to build proteins. The universality of this AA alphabet suggests the presence and incorporation of these specific AAs occurred early in the evolution of life. Combined observational and experimental data on AA abundances in meteorites and laboratory studies suggest that only a subset of modern proteinogenic AAs are synthesized in measurable quantities by theorized prebiotic pathways [2]. Based on these results, many researchers have postulated that protein synthesis occurred in two phases. During the first phase, protein synthesis occurred with a limited AA alphabet, consisting of those AAs that are easily synthesized by prebiotic reactions or those that are abundant in meteorites. During the second phase, new AA biosynthetic pathways evolved thus expanding the AA alphabet to its modern form [3,4]. Statistical studies of AA frequency in a variety of diverse taxa bolster this claim. Several authors have found that “early” AAs decrease in frequency while “late” AAs increase, indicating that late proteinogenic AAs may not have been present in prebiotic environments during early protein synthesis [5-7]. While the chronological order of AA appearance can vary slightly based on the criteria used (i.e., thermodynamic properties versus trends in the genetic code), all methods predict the late arrival of aromatic AAs (Figure 1, Table 1). Examples of abiotic syntheses of phenylalanine, tyrosine, and tryptophan are rare in modern terrestrial environments and have not been easily recreated in laboratory settings [8,9]. Furthermore, aromatic AAs in meteorites have rarely been reported and, where reported, their abundances are significantly lower than abundances of ‘early’ proteinogenic AAs, such as glycine or alanine [10]. If prebiotic chemical reactions cannot produce aromatic AAs in meaningful quantities, an alternate explanation for their incorporation into the SCG is that they are biotic inventions.