Glycine Polymerization on Oxide Minerals

Abstract

It has long been suggested that mineral surfaces played an important role in peptide bond formation on the primitive Earth. However, it remains unclear which mineral species was key to the prebiotic processes. This is because great discrepancies exist among the reported catalytic efficiencies of minerals for amino acid polymerizations, owing to mutually different experimental conditions. This study examined polymerization of glycine (Gly) on nine oxide minerals (amorphous silica, quartz, α-alumina and γ-alumina, anatase, rutile, hematite, magnetite, and forsterite) using identical preparation, heating, and analytical procedures. Results showed that a rutile surface is the most effective site for Gly polymerization in terms of both amounts and lengths of Gly polymers synthesized. The catalytic efficiency decreased as rutile > anatase > γ-alumina > forsterite > α- alumina > magnetite > hematite > quartz > amorphous silica. Based on reported molecular-level information for adsorption of Gly on these minerals, polymerization activation was inferred to have arisen from deprotonation of the NH group of adsorbed Gly to the nucleophilic NH group, and from withdrawal of electron density from the carboxyl carbon to the surface metal ions. The orientation of adsorbed Gly on minerals is also a factor influencing the Gly reactivity. The examination of Gly-mineral interactions under identical experimental conditions has enabled the direct comparison of various minerals' catalytic efficiencies and has made discussion of polymerization mechanisms and their relative influences possible Further systematic investigations using the approach reported herein (which are expected to be fruitful) combined with future microscopic surface analyses will elucidate the role of minerals in the process of abiotic peptide bond formation.

A Possible Path to Prebiotic Peptides Involving Silica and Hydroxy Acid‐Mediated Amide Bond Formation

“…The interaction between silica and molecular components of peptide‐ and depsipeptide‐forming heterogeneous systems have been documented . Reactive hydroxy groups and Lewis acid and base terminations are expected to occur on other metal oxide surfaces and interact with HAs .…”

“…Earlier work has also suggested that organic acids esterified with surface hydroxyl groups are potentially important for mineral‐catalyzed amide bond formation under prebiotic conditions . Although silicates were present on early earth, and SiO 2 is a primary component of basalt and primary magmatic rock, silica has been thought to be of minor importance in the prebiotic formation of oligopeptides due to limited reactivity of the surface upon exposure to liquid‐phase solutions containing AAs . However, the interaction with coadsorbates or coreactants, such as esterified surface hydroxy groups, was not considered.…”

Catalysis in the Primordial World

“…A systematic study of the polymerization of glycine on mineral surfaces showed the order of catalytic efficiency: rutile > anatase > γ-alumina > forsterite > α-alumina > magnetite > hematite > quartz > amorphous silica. 98 Martra and coworkers prepared poly-Gly up to 16 units long by condensation of the amino acid vapour (130 °C) on TiO 2 (anatase) and amorphous SiO 2 surfaces. 99 That line of research 100 opens a number of possibilities for prebiotic synthesis, but it has to be taken into consideration that there is a quite significant difference in polymerization efficiency for different amino acids on different minerals, e.g.…”

Ligand Sequential Replacement on Chromium(III)–Aqua Complexes by l-Alanine and Other Biological Amino Acids: A Kinetic Perspective

“…16 Given that the amino acid molecules exhibit two functional groups with nitrogen and oxygen atoms capable of acting as electron-pair donors, they can be considered as suitable ligands for vacant-orbital transition metal ions. In particular, the reactions of complexation of Cr(III) by amino acids might be related to the problem of the origin of the first peptides on prebiotic Earth, 17 since transition metal ions have been shown to catalyze the formation of peptide bonds between the amino acid monomers acting as ligands. 18 The substitution reactions on aqua, [19][20][21][22][23][24][25][26] hydroxo, 27 and ammonia 28 complexes of Cr(III) by amino acids have been the subject of several kinetic studies.…”