Probing the Role of Small Molecules and Amino Acids in Protein Adsorption on Gold Nanoparticles: A Thermodynamic and Mechanistic Insight

A molecular-level understanding of amino acid and small-molecule adsorption onto gold nanosurfaces is essential for designing nanomaterials with tailored biointerfacial properties. Till now, the interactions of gold nanoparticles (GNPs) with complex biomolecules such as peptides, proteins, and nucleic acids have been extensively studied under varying physicochemical conditions (including particle size, surface charge, functionalization, pH, and ionic strength). However, the fundamental mechanistic pathways governing the adsorption of their molecular building blocks─amino acids─onto GNP surfaces remain inadequately understood. Herein, we investigate the thermodynamics and mechanistic aspects of small molecule and amino acid adsorption on citrate-capped gold nanoparticles (cit-GNPs, ∼15 nm) using a combination of Dynamic Light Scattering (DLS), Zeta-potential analysis, and UV-Vis spectroscopy. Amino acids representing four distinct chemical classes─positively charged, negatively charged, polar neutral, and sulfur-containing─were selected to dissect the contributions of Coulombic forces, van der Waals interactions, and hydrogen bonding. Adsorption isotherms fitted using a modified Langmuir model yielded equilibrium constant (<i>K</i>_eq) surface coverage and standard Gibbs free energy changes (Δ<i>G</i>°), which ranged from -37 to -50 kJ mol^-1, consistent with physisorption-dominated binding. The cationic amino acids exhibited the highest affinity toward negatively charged cit-GNPs, while sulfur-containing residues (e.g., cysteine and others) showed exceptional binding strength due to thiol-gold interactions and sulfur's polarizability. Further temperature-dependent adsorption studies revealed exothermic, enthalpy-driven behavior with a positive entropy change indicative of increased configurational freedom at the nano-bio interface. These findings offer fundamental insights into amino acid-nanoparticle interactions, with implications for the rational design of gold-based nanostructures in nanomedicine.