Abstract
Titanium dioxide (TiO2) is probably one of the most widely used nanomaterials, and its extensive exposure may result in potentially adverse biological effects. Yet, the underlying mechanisms of interaction involving TiO2 NPs and macromolecules, e.g., proteins, are still not well understood. Here, we perform all-atom molecular dynamics simulations to investigate the interactions between TiO2 NPs and the twenty standard amino acids in aqueous solution exploiting a newly developed TiO2 force field. We found that charged amino acids play a dominant role during the process of binding to the TiO2 surface, with both basic and acidic residues overwhelmingly preferred over the non-charged counterparts. By calculating the Potential Mean Force, we showed that Arg is prone to direct binding onto the NP surface, while Lys needs to overcome a ~2 kT free energy barrier. On the other hand, acidic residues tend to form “water bridges” between their sidechains and TiO2 surface, thus displaying an indirect binding. Moreover, the overall preferred positions and configurations of different residues are highly dependent on properties of the first and second solvation water. These molecular insights learned from this work might help with a better understanding of the interactions between biomolecules and nanomaterials.
Highlights
Titanium dioxide (TiO2) is probably one of the most widely used nanomaterials, and its extensive exposure may result in potentially adverse biological effects
By turning on the electrostatic interactions, three distinctive water layers with different thickness were identified at the NP interface: a first layer (FL) that peaks at 1.85 nm with a thickness of 0.30 nm, a second layer (SL) that peaks at 2.13 nm with a thickness of 0.23 nm, and a third layer (TL) that peaks at 2.36 nm with a thickness of 0.22 nm
The strong partial electrostatic interactions of the titanium atoms impel the adsorption of water molecules onto the NP surface by establishing interactions with the oxygen atoms from water and its forming pattern is displayed in inset c in Fig. 2
Summary
The amorphous spherical TiO2 NP model with a radius of 17 Å was constructed. The Lennard-Jones (LJ) parameters for the TiO2 were taken from our recent work[23]. Umbrella sampling, which has been extensively used and is considered the gold standard free energy calculation method, was selected to estimate the free energy landscape associated with moving the different alpha-amino acids toward the NP This technique was considered adequate for our purposes since there are no ‘slow structural responses’ along the defined pathway. The PMF was calculated along a reaction coordinate defined by pulling the capped amino acids toward the NP surface with a harmonic force of 2000 kJ mol−1nm−2 applied on the heavy atoms of the side chain to bringing it from a distance of 2.6 nm to 1.8 nm (the radius of the NP was 1.7 nm); in some cases a larger harmonic force of 5000 kJ mol−1nm−2. The Weighted Histogram Analysis Method (WHAM)[40,41] is applied to calculate the free energy; the statistical uncertainty of the PMF was estimated by bootstrapping analysis[42] with an equilibration phase of 1 ns in length
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