Tryptophan Gold Nanoparticle Interaction: A First-Principles Quantum Mechanical Study

The miniaturization of structures in the guise of nanoscale materials has created a so-called nanoplatform for applied biomedical research in developing novel and improved diagnostics and therapeutics. Nanoparticles offer large surfaces for reactions involving biomolecules, thus providing a promising pathway to combine biomolecular functionality with unique electronic and optical attributes of nanoparticles for creation of hybrid nanoscaffolds. Such scaffolds with specific features are of particular interest in recognition and modulation of biomacromolecules, protein interactions, protein nucleic acid interactions, enzyme activity, biomimetic reactions, etc. 3 Because of intriguing optoelectronic properties and biocompatibility, gold nanoparticles have widely been exploited for diagnostics and therapeutic applications. The long-term fascination for gold was substantially facilitated by the ease of their size-controlled synthesis and functionalization with multifunctional ligands such as amino acids, proteins, peptides, and DNA. It was previously established that serum proteins become associated with nanoparticles in biological medium and form protein ‘corona’, which defines the biological identity of the particles within the biological system thus making protein nanoparticle interaction of particular interest. The underlying fact is that the adsorption of proteins on nanoparticles can alter their three-dimensional structure, activity, and biological responses that can lead to unpredicted perturbations in the biosystem. Protein nanoparticle interactions and their effect on protein structure and activity are generally determined by various spectroscopic and thermodynamic methods. For example, intrinsic fluorescence quenching is a powerful spectroscopic tool for investigating protein structure and binding upon interaction with nanoparticles. The fluorescence of proteins is due to aromatic amino acid residues, among which tryptophan (Trp) has the stronger fluorescence and higher quantum yield attributed to the π π* transitions of the indole functional group. Trp is an essential amino acid of the human diet and is known to