Amyloidogenic model peptides as catalysts for stereoselective aldol reactions

In this work, the self-assembly in an aqueous medium of two compounds derived from amyloid-like structures, [RF]4 and P[RF]4 (where: R = arginine; F = phenylalanine; P = proline), was studied by using spectroscopy, microscopy, and scattering techniques. The peptides presented a similar critical aggregation concentration (cac) considering the same medium conditions (∼0.35 mmol L₁ at 25°C). The stability of the systems was analyzed by zeta potential measurements, which showed that under acidic and neutral conditions the colloidal particles remained stable, and for basic solutions, strongly interacting colloids and supramolecular aggregates in solution were observed. CD data confirmed the changes in the conformation of the secondary structures for both compounds by changing the pH of the solution. By SAXS, the curves were fitted using a fractal structure factor, and we observed that the size of the monomers that compose the fractal aggregates of [RF]4 is twice as large as that of P[RF]4. These compounds showed potential as catalysts in aldol reactions, using cyclohexanone and p-nitrobenzaldehyde. In this case, under optimized reaction conditions, 78% conversion (ee up to 85%) was observed in [RF]4 solutions, while P[RF]4 had only 51% conversion (ee up to 85%). Our results show, for the first time, a dependence of the morphology on the stereoselective aldol reactions, whereas the folding arrangement of the chiral assemblies in the polypeptide chains plays an essential role in the access of the reactants to the active surface sites. (author)