Adsorption of Zwitterionic and Capped Amino Acids to Graphene: A Molecular Dynamics Study

Pyrogenic carbon (PyC) materials are commonly used to immobilize environmental contaminants and could also bind amino acids or proteins (including proteinaceous contaminants), reducing their availability to plants and other organisms. This study determined the adsorption free energies of 20 amino acids to graphene (model PyC surface) and explored the effects of ionic strength on adsorption and the adsorption mechanisms, using umbrella sampling molecular dynamics. We simulated the interactions of amino acids with graphene in their zwitterionic and capped forms at ionic strength of 0 and 0.5 M (NaCl), and further simulated the interactions of tryptophan, tyrosine, arginine, and lysine at ionic strength of 0.1, 0.2, and 0.3 M (NaCl). The aromatic amino acids and arginine adsorbed to graphene most strongly with adsorption free energies in the range of -20 to -40 kJ/mol. The adsorption free energies of the capped amino acids were on average 12.6 ± 2.8 kJ/mol more negative than those of the zwitterionic amino acids. Ionic strength had significant effects on the adsorption of capped aspartic acid, arginine, and tryptophan, despite generally insignificant effects when averaged across all amino acids. This study provided insights into the fate and transport of amino acids and proteins in the environment.