The role of cysteine residues in the. protein binding kinetics and stability. on gold nanoparticles (AuNP) was studied using AuNP: localized surface plasmon resonance (LSPR): in combination with an organothiol (OT) displacement Method. GB3, the third IgG-binding domain of protein G, was used to model protein-AuNP adsorption. While wild.. type GB3 (GB3(0)) contains no cysteine residues, bioengineered GB3 variants containing one (GB3,) and two (GB3(2)) cysteine residues Were also tested: The cysteine content has no significant effect on GB3 binding kinetics with AuNPs, and most protein adsorption occurs within the first few seconds upon protein/AuNP mixing. However, the stability of GB3 on the AuNP surface against OT displacement depends strongly on the cysteine content and the age of the AuNP/GB3 mixture. The GB30 covered AuNPs can be completely destabilized and aggregated by OTs, regardless of the age of the GB3(0)/AuNP mixtures. Long-time incubation of GB3(1) or GB32 with AuNPs can stabilize AuNPs against the OT adsorption inducted aggregation. This study indicates that multiple forces involved in the GB3/AtiNP interaction, and covalent binding between cysteine and AuNP is essential for a stable protein/AuNP complex.