We demonstrated that it is possible to identify the protein-nanoparticle interaction site at amino acid scale in solution. Using NMR, chemical shift perturbation analysis, and dynamic light scattering we have identified a specific domain of human ubiquitin that interacts with gold nanoparticles. This method allows a detailed structural analysis of proteins absorbed onto surfaces of nanoparticles in physiological conditions and it will provide much needed experimental data for better modelling and prediction of protein-nanopartide interactions. ; JRC.DG.I.5-Nanobiosciences