Cyclic or Linear? Parameters Determining the Outcome of Glycine Polymerization in Silica Surface Prebiotic Scenarios.

The parameters that determine the formation of linear peptides and cyclic dimers (diketopiperazine, DKP) on silica surfaces of different surface area, silanol and siloxane ring populations, controlled by thermal treatments, are investigated upon glycine deposition from gas and liquid phases. The formed products were characterized by infrared and Raman spectroscopies, X-ray diffraction and thermogravimetric analysis. The results reveal the importance of "nearly-free" silanols to form ester centers as primers for the formation of linear peptides over DKP, on surfaces with medium silanol density (1.4 to 2.7 nm-2 ). Quenched reactivity is seen on isolated silanols (density≤0.7 nm-2 ), while silanols involved in hydrogen bonding (density of 4.5 nm-2 ) weakly interact with Gly resulting in its cyclization to DKP. Deposition of glycine from liquid phase may also form both DKP and linear polymers, depending on its loading and silica surface. These conclusions demonstrate the complexity of glycine surface chemistry in the polymerization reaction and highlight the interest of a surface science approach to evaluate geochemical prebiotic scenarios.

Emergence of Order in Origin-of-Life Scenarios on Mineral Surfaces: Polyglycine Chains on Silica.

The polymerization of amino acids (AAs) to peptides on oxide surfaces has attracted interest owing to its high importance in biotechnology, prebiotic chemistry, and origin of life theories. However, its mechanism is still poorly understood. We tried to elucidate the reactivity of glycine (Gly) from the vapor phase on the surface of amorphous silica under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy reveals that Gly functionalizes the silica surface through the formation of ester species, which represent, together with the weakly interacting silanols, crucial elements for monomers activation and polymerization. Once activated, ß-turns start to form as initiators for the growth of long linear polypeptides (poly-Gly) chains, which elongate into ordered structures containing both ß-sheet and helical conformations. The work also points to the role of water vapor in the formation of further self-assembled ß-sheet structures that are highly resistant to hydrolysis.

Polypeptide Chain Growth Mechanisms and Secondary Structure Formation in Glycine Gas-Phase Deposition on Silica Surfaces.

Peptide formation by amino acids condensation represents a crucial reaction in the quest of the origins of life as well as in synthetic chemistry. However, it is still poorly understood in terms of efficiency and reaction mechanism. In the present work, peptide formation has been investigated through thermal condensation of gas-phase glycine in fluctuating silica environments as a model of prebiotic environments. In-situ IR spectroscopy measurements under a controlled atmosphere reveal that a humidity fluctuating system subjected to both temperature and water activity variations results in the formation of more abundant peptides compared to a dehydrated system subjected only to temperature fluctuations cycles. A model is proposed in which hydration steps result in the hydrolysis and redistribution of the oligomers formed during previous deposition in dry conditions. This results in the formation of self-assembled aggregates with well-defined secondary structures (especially ß-sheets). Upon further monomers feeding, structural elements are conserved in newly growing chains, with indications of templated polymerization. The structural dynamics of peptides were also evaluated. Rigid self-assembled structures with a high resistance to further wetting/drying cycles and inaccessibility to isotopic exchange were present in the humidity fluctuating system compared to more flexible structures in the dehydrated system. The resistance and growth of self-assembled structures were also investigated for an extended duration of Gly deposition using isotope labeling.