Mechanistic Insights into the Silica-Mediated Synthesis of Glyceraldehyde from Glycolaldehyde and Hydroxymethylene

Minerals are crucial ingredients in prebiotic chemistry as they could have promoted the evolution of simple organic molecules toward proto-biomolecules that are on the route of the emergence of self-replicating information-rich macromolecules. In this respect, the formose reaction, involving the sequential autocatalytic condensation of formaldehyde, is the generally accepted pathway to sugar synthesis. Although obtained under controlled laboratory conditions, with enhanced sugar yields promoted by the presence of silicate in the reaction medium, it presents a number of limitations, and the underlying reaction mechanism remains an unsolved riddle. In this work, the focus is on the second step of the formose reaction, namely the synthesis of glyceraldehyde, which is accomplished by considering the reaction between glycolaldehyde and hydroxymethylene taking place on the edingtonite mineral. The reaction mechanism is explored by quantum chemical simulations performed at various degrees of sophistication to shed light on the thermochemical and kinetic feasibility of the reaction. The same pathway is also investigated in the gas-phase, in order to disentangle the role played by the zeolitic mineral. The obtained results show that the endothermic reaction between glycolaldehyde and hydroxymethylene yields glyceraldehyde by a submerged reaction path, both in the gas-phase and on the edingtonite surface. The mineral substrate provides further stabilization, by about 20 kcal mol-1, of all the species involved in the reaction pathway and acts as a scaffold favoring the interaction of the two reactants.