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.

Automated summary

The parameters for the formation of linear peptides and cyclic dimers (DKP) on different silica surfaces were studied, and the thermal treatments were found to control the formation. Characterization of the formed products was done using various spectroscopies and analysis techniques. The results showed that the presence of nearly-free silanols is crucial for the formation of linear peptides, while silanols involved in hydrogen bonding led to the cyclization of glycine to DKP. Liquid-phase deposition of glycine could also result in the formation of both DKP and linear polymers, depending on the loading and silica surface.