The Role of Cyclic Diketopiperazine in the Formation of Polypeptides on Silica Surfaces

Diketopiperazine (DKP), a recurring product of silica-catalyzed amino acid polymerization, is a molecule of interest in the origin of life studies, as well in the industrial medicinal chemistry. Its role in the peptide bond formation reactions is controversial as it is considered sometimes as a stable, dead-end product for the oligomerization reaction. We tried to elucidate the parameters that govern the DKP opening on silica surfaces of low and high surface areas in the frame of glycine polymerization reaction, when DKP is adsorbed from gas and liquid phases, or subjected to temperature and humidity fluctuations cycles. The formed products were characterized by infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. The results reveal that DKP represents an efficient intermediate for the polymerization reaction leading to the formation of linear peptides on silica surfaces. Abundant oligomers with β-sheet secondary structures are formed depending on the DKP loading and the silica surface. The crucial role of the silica surface including the "nearly free" silanols in the adsorption, opening, and reaction of DKP to form linear chains was shown. These conclusions highlight the complexity of the DKP surface chemistry in the polymerization reaction and can improve the understanding when dealing with geochemical prebiotic scenarios.