Interactions of aqueous amino acids and proteins with the (110) surface of ZnS in molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to analyze how amino acids and proteins interact with the ZnS (110) surface in water, revealing specific binding behaviors, the importance of cysteine, and differences from ZnO surfaces.

Contribution

It provides the first detailed molecular-level characterization of amino acid and protein interactions with ZnS surfaces, including the role of cysteine and water structuring.

Findings

Five amino acids do not bind to ZnS surface.

Binding energies for amino acids are up to 4.3 kJ/mol, with cysteine capable of covalent binding.

ZnS is more hydrophobic than ZnO, with distinct water layering and dynamics.

Abstract

The growing usage of nanoparticles of zinc sulfide as quantum dots and biosensors calls for a theoretical assessment of interactions of ZnS with biomolecules. We employ the molecular-dynamics-based umbrella sampling method to determine potentials of mean force for 20 single amino acids near the ZnS (110) surface in aqueous solutions. We find that five amino acids do not bind at all and the binding energy of the remaining amino acids does not exceed 4.3 kJ/mol. Such energies are comparable to those found for ZnO (and to hydrogen bonds in proteins) but the nature of the specificity is different. Cysteine can bind with ZnS in a covalent way, e.g. by forming the disulfide bond with S in the solid. If this effect is included within a model incorporating the Morse potential, then the potential well becomes much deeper - the binding energy is close to 98 kJ/mol. We then consider tryptophan cage, a protein of 20 residues, and characterize its events of adsorption to ZnS. We demonstrate the relevance of interactions between the amino acids in the selection of optimal adsorbed conformations and recognize the key role of cysteine in generation of lasting adsorption. We show that ZnS is more hydrophobic than ZnO and that the density profile of water is quite different than that forming near ZnO - it has only a minor articulation into layers. Furthermore, the first layer of water is disordered and mobile.