Proteins are essential biological catalysts for all extant life. It has been discussed that primordial proteins spontaneously formed on the prebiotic Earth, promoting chemical evolution to the origin of life through interactions with RNA (Orgel, 1986; Cech, 2009; Tagami et al., 2017). The presence of amino acids on the early Earth has been suggested through both laboratory simulation experiments and possible extraterrestrial delivery (Glavin et al., 2010; Furukawa et al., 2009, 2015; Elsila et al., 2016; Takeuchi et al., 2020; Zang et al., 2022). Glycine (Gly) and alanine (Ala) were typically two of the most common amino acids in meteorites and in the experimental products simulating early Earth, with glycine being the most common (Glavin et al., 2010, 2020; Furukawa et al., 2009, 2015; Martins et al., 2013; Takeuchi et al., 2020).
It is not clear how amino acids polymerize to form primordial proteins on the prebiotic Hadean Earth. Many geological settings, including tidal flats, deep-sea hydrothermal vents, sea-floor sediments, comet impacts, and geothermal areas on land, have been proposed (Lahav et al., 1978; Ferris et al., 1996; Huber and Wächtershäuser, 1998; Imai et al., 1999; Kawamura et al., 2005; Ohara et al., 2007; Otake et al., 2011; Sugahara and Mimura, 2015; Kitadai et al., 2017). Among the different settings previously investigated, evaporative environments are rather advantageous in amino acid oligomerization, forming longer peptides in simulating experiments (Rodriguez-Garcia et al., 2015; Sumie et al., 2023). The longest Gly peptides detected in previous prebiotic experiments are 39-mer, resulting from the effects of boric acid under evaporative conditions (Sumie et al., 2023). Evaporative environments could also promote reactions that form components of RNA, such as ribonucleotides and ribose 5-phosphate (Lohrmann and Orgel, 1968; Burcar et al., 2016; Hirakawa et al., 2022; Takabayashi et al., 2023). Previous studies on peptide synthesis in evaporative environments mostly focused on the oligomerization of Gly, and the effects of potentially available compounds in the reaction, such as sodium hydroxide, copper, clay minerals, metal oxides, and salts (Lahav et al., 1978; Schwendinger and Rode, 1989; Saetia et al., 1993; Bujdák and Rode, 2002; Leman et al., 2004; Kitadai et al., 2011; Rodriguez-Garcia et al., 2015; Campbell et al., 2019).
Proteins contain twenty kinds of different amino acids. The sequences of the different side chain functional groups in proteins provide essential characteristics in structures and as biological catalysts, including enzymatic properties. Thus, the formation of abiotic peptides composed of multiple amino acids is substantially important. Fewer research studies have investigated hetero-oligopeptide synthesis that contains multiple amino acid species compared to homo-oligopeptide synthesis (e.g., Parker et al., 2014; Rodriguez-Garcia et al., 2015; Greenwald et al., 2016). Previous wet-dry cycle experiments under alkaline conditions identified up to 20-mer glycine homo-oligopeptides with 4-mer of hetero-oligopeptides containing Gly and Ala (Rodriguez-Garcia et al., 2015). Gly would be a more reactive amino acid for oligomerization than Ala, valine, and methionine (Otake et al., 2011; Furukawa et al., 2012; Huang et al., 2017; Rodriguez-Garcia et al., 2015). Considering the various functions that different proteinic amino acid side chains play, several amino acids likely polymerized to form early peptide sequences as precursors to create some of the initial proteins on the prebiotic Earth. However, how multiple amino acids were incorporated into peptide sequences remains unclear. We designed an experiment that investigated the formation of peptides composed of two amino acids, which have different reactivities in peptide synthesis, under drying conditions at 90°C and 130°C to understand how the difference of time, temperature, and the molar ratio of starting amino acid monomers affects the compositions of product peptides.