Stabilization of DOPA Zwitterions on Laser-Generated Gold Nanoparticles: ONIOM Computational Study of the Charge-Dependent Structural and Electronic Changes of DOPA Adsorbed on the Gold Nanosurface

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A stable colloidal solution of gold nanoparticle–3,4-dihydroxyphenylalanine (DOPA) zwitterion conjugates (Au NP–z-DOPA) was prepared using nanosecond-laser ablation of a gold target in an aqueous solution of zwitterionic DOPA (z-DOPA). Spectroscopic data revealed that Au NPs strongly interact with z-DOPA, which lead to a significant change in the electronic structure of z-DOPA. The adsorbed z-DOPA is highly stable against oxidation in aqueous solution, indicating a significant stabilizing effect of the Au NPs surface on this zwitterion. The electronic structures and geometries of z-DOPA and the other forms of DOPA (including uncharged, cationic, and anionic forms) adsorbed on the Au(111) nanosurface were determined by ONIOM calculations. The geometry and electronic structure of each DOPA form are significantly affected by the surface upon adsorption. The analysis of the frontier orbitals confirmed the significant stabilizing effect of the Au NPs on z-DOPA. The calculations are consistent with the variations observed in the recorded absorption spectra of z-DOPA due to its interaction with Au NPs. The Au nanosurface does not necessarily have a stabilizing effect on all DOPA forms. The cytotoxicities of z-DOPA and Au NP–z-DOPA against the Jurkat T-cells were evaluated.