Estudo quântico dos fatores cinéticos e termodinâmicos no controle seletivo da Síntese Homoquiral da Alanina

Abstract

Biological homochirality arouses interest since its discovery more than a century ago, either for its singularity in theoretical field and for its technological applications. Aiming to contribute to a deeper comprehension of homochirality, this work investigated pathways to a potential homochiral synthesis of Alanine, the simplest chiral amino acid found in nature, as theoretical-computational model of study. Two main reactional structures were selected based in experimental and theoretical findings, and then were studied using stationary and dynamic quantum mechanics methods. In the first reactional structure, E-ethanimine e Z- ethanimine, reacted with carbon monoxide forming R and S stereocenters of an Alanine precursor. In the second one, the ethanimines reacted with carbon dioxide also being analyzed the chiral stereocenters formed. Each one of these structures generated four reactions in total, a combination of two possible imines with two possible stereocenters. Then were proceeded the calculations of molecular structures and energies of the stationary states (reactants, products, Van der Waals complexes and transition states) for all the reactions, as well the kinetic and thermodynamic parameters. The kinetic and thermodynamic parameters pointed that the reactions with Z-ethanimine were favored, as well the reactions involving CO. Although, there were not found any preferential channels for the formation of any of the chiral centers, R or S. Metadynamic simulations were carried out to evaluate the enantiomeric control of the reactions. In the statistical step of the essay, 80 simulations were proceeded, resulting that the proposed intermediates in the stationary study exist just for some steps, being then converted into other intermediates, also potential precursors of alanine, but achiral. After some more steps of simulation, these also degrade to simpler and less reactive substances, without retaining the chiral configuration in any case. Finally, this study describes possible reactional pathways of Alanine precursors synthesis, although could not obtain any evidence of favoring the formation of one of the Alanine stereoisomers.