In Silico Prebiotic Chemistry: Aluminosilicate Surfaces As Promoters for the Peptide Bond Formation

The route for which basic molecular building blocks such as amino acids and nucleobases were joined in a proper and controlled way in order to make the first active biopolymers during primitive Earth is an intriguing question that nowadays still remains open in the area of the prebiotic chemistry. Indeed, even for the condensation of glycine (the simplest amino acid) the reaction occurring in highly diluted water solution is thermodynamically disfavoured. An early suggestion form Bernal in 1951 (Bernal, 1951) advocated the special role of mineral clays as promoters for the condensation of monomer building blocks since they provide adsorption sites that, on one hand, may immobilize, concentrate and protect amino acids and peptides from hydration and, on the other hand, may induce a lowering of the activation barrier because of the presence at the surface of catalytic active sites. Along this line, Orgel (Orgel, 1998) stated that successive cycles of condensation occurring on mineral surfaces causes elongation of the synthesized peptide which remains almost irreversibly adsorbed, so that its destructive hydrolysis, will become more and more improbable. In the present contribution, a detailed theoretical mechanistic study addressed to the peptide bond formation catalyzed by an aluminosilicates surface is presented. A large and realistic cluster model cut out from the sanidine feldspar surface rich in both Lewis and Brønsted sites has been adopted as a catalytic surface. The free energy profiles for the condensation of glycine molecules on the sanidine surface yielding glycylglycine (Figure 1) and glycylglycylglycine as reaction products have been simulated using the ONIOM2[B3LYP/6–31 + G(d,p):MNDO] level of theory. Results indicate that the catalytic interplay between Lewis and Brønsted sites is a key factor to favour the reactions (Rimola, et al. 2007). Additionally, theoretical results show that purely London forces between the biomolecules and the surface play a crucial role in the condensation processes because they greatly stabilize the peptide at the surface, as suggested by Orgel (Orgel, 1998). Finally, further discussion concerning the controversy between peptide polymerization vs peptide hydrolysis is also addressed by the explicit introduction of water molecules in the reaction process.