Utilização de métodos computacionais para descrever a interação da quitosana com contaminantes

Abstract

In this work, computational methods based on classical and quantum dynamics simulation were used to obtain the molecular properties of the three distinct aggregates, which are formed by the chitosan biopolymer with the organic molecules of the herbicides dicylofop, methyldicylofop, and glyphosate. Chitosan, is a biodegradable material, 100% biocompatible with humans, with pharmaceutical and technological use, with high chelating potential of herbicides, dyes and even heavy metals. It was chosen to remove these herbicides from aquatic environments through the simulation of the biosorption process. This study used classical and quantum molecular dynamics simulations for the characterization of the interactions involved in these adsorption processes. The systems modeled by classical dynamics with the biopolymer and the herbicides were solvated with water and a small excess (neutralizing) of sodium (Na+ ) and chloride (Cl- ) ions were added to achieve the ionic strength of aquatic environments. The negative charges of the specific groups of glyphosate (CO2 - and PO3H - ) and dicylofop (CO2 - ) influenced the biosorption of these herbicides and increased the degree of attractive interactions, due to more numerous and stronger electrostatic interactions with the amino-proton groups (NH3 + ) of chitosan compared to the methyldicylofop herbicide. The binding free energies of glyphosate [-29 (3) kJ/mol] and methyldiclofop [-18 (7) kJ/mol] with chitosan, obtained by LIE method in equilibrium classical molecular dynamics (DM), confirmed the spontaneity of both biosorptions, with glyphosate showing a higher degree of adsorption. The mean force potentials (PMF) calculated from the directional molecular dynamics (DMP) with the umbrella sampling methodology revealed that dicyclofop (-110 (2) kJ/mol) also showed a higher degree of biosorption than methyldicyclofop (-48 (4) kJ/mol). Furthermore, through the Coulomb and Lenard-Jones potentials it was possible to observe a greater contribution of electrostatic interactions than Van der Waals interactions for diclofop. In contrast, the Van der Waals interactions contributed more to the formation of the complex methyldicyclofop-biopolymer. Furthermore, these intermolecular interactions were confirmed by the latest semiempirical quantum dynamics method in implicit solvent with the GFN2-xTB force field. This quantum method also confirmed the stability of this complex, which exhibited the lowest RMSD value = 0.5 (1) nm during the simulation compared to the classical RMSD which was equal to 1.1 (1) nm. The number of hydrogen bonds (of 44 (4), and standard deviation of 9.46%) around the aggregate stabilized this complex influenced the RMSD value obtained in DM. Based on all this information, chitosan is presented here as an excellent bioadsorbent material for herbicides due to its high degree of chelation of these compounds, besides being a biologically compatible material for humans.