Orig Life Evol Biosph DOI 10.1007/s11084-014-9359-4 PREBIOTIC CHEMISTRY
Peptide Formation Mechanism on Montmorillonite Under Thermal Conditions Shigeshi Fuchida & Harue Masuda & Keiji Shinoda
Received: 25 March 2014 / Accepted: 23 May 2014 # Springer Science+Business Media Dordrecht 2014
Abstract The oligomerization of amino acids is an essential process in the chemical evolution of proteins, which are precursors to life on Earth. Although some researchers have observed peptide formation on clay mineral surfaces, the mechanism of peptide bond formation on the clay mineral surface has not been clarified. In this study, the thermal behavior of glycine (Gly) adsorbed on montmorillonite was observed during heating experiments conducted at 150 °C for 336 h under dry, wet, and dry–wet conditions to clarify the mechanism. Approximately 13.9 % of the Gly monomers became peptides on montmorillonite under dry conditions, with diketopiperazine (cyclic dimer) being the main product. On the other hand, peptides were not synthesized in the absence of montmorillonite. Results of IR analysis showed that the Gly monomer was mainly adsorbed via hydrogen bonding between the positively charged amino groups and negatively charged surface sites (i.e., Lewis base sites) on the montmorillonite surface, indicating that the Lewis base site acts as a catalyst for peptide formation. In contrast, peptides were not detected on montmorillonite heated under wet conditions, since excess water shifted the equilibrium towards hydrolysis of the peptides. The presence of water is likely to control thermodynamic peptide production, and clay minerals, especially those with electrophilic defect sites, seem to act as a kinetic catalyst for the peptide formation reaction. Keywords Peptide formation . Glycine . Montmorillonite . Chemical evolution . Dehydration
Introduction The processes of emergence and evolution of life on the early Earth are still undefined. The formation of organic polymers is an essential process to produce the precursors of life that lead to the production of proteins, DNA and RNA. The oligomerization of amino acids, i.e., peptide formation (n amino acids→peptide+n−1 H2O), is an important reaction to produce primitive proteins (Miller and Bada 1988). Therefore, it is of interest to determine the physico-chemical conditions that promote the oligomerization reaction to better understand the conditions on primitive Earth when life was born. S. Fuchida (*) : H. Masuda : K. Shinoda Department of Geosciences, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan e-mail: [email protected]
S. Fuchida et al.
High temperature conditions are thermodynamically favorable for peptide formation because the reaction is endothermic (Shock 1992; Lemke et al. 2009). Protein-like polymers (proteinoids) are synthesized through melting of an anhydrous mixture containing acidic and basic amino acids, such as aspartic acid, glutamic acid and lysine, at high temperature (120– 200 °C) (Harada and Fox 1960; Fox and Nakashima 1967; Melius and Srisomsap 1997). However, the proteinoids only contain a small amount of peptide bonds, and they are mainly composed of ester-like bonds (Andini et al. 1975; Fitz et al. 2007). Short peptides have been synthesized by heating a solution containing highly concentrated amino acids in a flow reactor, which simulates the rapid heating and cooling conditions of hydrothermal vents (Imai et al. 1999; Cleaves et al. 2009). However, the formation rates were very low (
Peptide Formation Mechanism on Montmorillonite Under Thermal Conditions Shigeshi Fuchida & Harue Masuda & Keiji Shinoda
Received: 25 March 2014 / Accepted: 23 May 2014 # Springer Science+Business Media Dordrecht 2014
Abstract The oligomerization of amino acids is an essential process in the chemical evolution of proteins, which are precursors to life on Earth. Although some researchers have observed peptide formation on clay mineral surfaces, the mechanism of peptide bond formation on the clay mineral surface has not been clarified. In this study, the thermal behavior of glycine (Gly) adsorbed on montmorillonite was observed during heating experiments conducted at 150 °C for 336 h under dry, wet, and dry–wet conditions to clarify the mechanism. Approximately 13.9 % of the Gly monomers became peptides on montmorillonite under dry conditions, with diketopiperazine (cyclic dimer) being the main product. On the other hand, peptides were not synthesized in the absence of montmorillonite. Results of IR analysis showed that the Gly monomer was mainly adsorbed via hydrogen bonding between the positively charged amino groups and negatively charged surface sites (i.e., Lewis base sites) on the montmorillonite surface, indicating that the Lewis base site acts as a catalyst for peptide formation. In contrast, peptides were not detected on montmorillonite heated under wet conditions, since excess water shifted the equilibrium towards hydrolysis of the peptides. The presence of water is likely to control thermodynamic peptide production, and clay minerals, especially those with electrophilic defect sites, seem to act as a kinetic catalyst for the peptide formation reaction. Keywords Peptide formation . Glycine . Montmorillonite . Chemical evolution . Dehydration
Introduction The processes of emergence and evolution of life on the early Earth are still undefined. The formation of organic polymers is an essential process to produce the precursors of life that lead to the production of proteins, DNA and RNA. The oligomerization of amino acids, i.e., peptide formation (n amino acids→peptide+n−1 H2O), is an important reaction to produce primitive proteins (Miller and Bada 1988). Therefore, it is of interest to determine the physico-chemical conditions that promote the oligomerization reaction to better understand the conditions on primitive Earth when life was born. S. Fuchida (*) : H. Masuda : K. Shinoda Department of Geosciences, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan e-mail: [email protected]
S. Fuchida et al.
High temperature conditions are thermodynamically favorable for peptide formation because the reaction is endothermic (Shock 1992; Lemke et al. 2009). Protein-like polymers (proteinoids) are synthesized through melting of an anhydrous mixture containing acidic and basic amino acids, such as aspartic acid, glutamic acid and lysine, at high temperature (120– 200 °C) (Harada and Fox 1960; Fox and Nakashima 1967; Melius and Srisomsap 1997). However, the proteinoids only contain a small amount of peptide bonds, and they are mainly composed of ester-like bonds (Andini et al. 1975; Fitz et al. 2007). Short peptides have been synthesized by heating a solution containing highly concentrated amino acids in a flow reactor, which simulates the rapid heating and cooling conditions of hydrothermal vents (Imai et al. 1999; Cleaves et al. 2009). However, the formation rates were very low (
