Regulation of the Enzymatic Activities of Lysozyme by the Surface Ligands of Ultrasmall Gold Nanoclusters: The Role of Hydrophobic Interactions

Nanomaterials for biological applications would inevitably encounter and interact with biomolecules, which have a profound impact on the properties, functions, and even fates of both nanomaterials and biomolecules. Among the biomolecules, lysozyme (Lys) is of great importance in defending the bacterial intruder and maintaining health. Here, the interactions between fluorescent gold nanoclusters (AuNCs) (∼2 nm) capped with different surface ligands and Lys were thoroughly investigated. Fluorescence spectroscopic studies showed that dihydrolipoic acid (DHLA)-capped and glutathione (GSH)-capped AuNCs both quenched the intrinsic fluorescence of Lys by different quenching mechanisms. Agarose gel electrophoresis and zeta-potential assays showed that statistically one DHLA-AuNC could bind one Lys, while one GSH-AuNC could bind 3–4 Lys, providing new examples for the concept of a "protein complex". Activity assays indicated that DHLA-AuNCs heavily inhibited the enzymatic activity of Lys, while GSH-AuNCs had little effect. By synchronous fluorescence and circular dichroism spectroscopic studies, it was deduced that both AuNCs would interact with Lys by electrostatic attractions due to the distinct surface charges, and then DHLA-AuNCs would further interact with Lys by hydrophobic interactions, probably due to the hydrophobic carbon chain of DHLA and the hydrophobic side chains of amino acid residues in Lys, which was proved by the significant secondary structure changes caused by DHLA-AuNCs. Meanwhile, conformational changes induced by GSH-AuNCs with zwitterionic ligands were neglectable. Therefore, this work provided a comprehensive study of the consequences and mechanisms of the interactions between Lys and AuNCs, which was essential for the design and better use of nanomaterials as biological agents.