The study of gold nanoclusters (AuNCs) has seen much interest in recent history due to their unique fluorescent properties and environmentally friendly synthesis method, using proteins as a growth scaffold. The differences in the physicochemical properties of lysozyme encapsulated AuNCs in comparison to natural lysozyme are characterised in order to determine the effects AuNCs has on natural protein behaviour. The molecules hydrodynamic radius (Dynamic Light Scattering), light absorbance (UV-Vis), electrophoretic mobility, relative density, dynamic viscosity, absorption (Quartz Crystal Microbalance) and circular dichroism (CD) characteristics were studied. It was found that lysozyme forms small dimer/trimer aggregates upon the synthesis of AuNCs within the protein. The diameter of Ly-AuNCs was found to be 8.0 nm across a pH range of 2-11 indicating dimer formation, but larger aggregates with diameters >20nm formed between pH 3-6. The formation of larger aggregates limits Ly-AuNCs use as a fluorescent probe in this pH range. A large shift in the protein’s isoelectric point was also observed, shifting from 11.0 to 4.0 upon AuNC synthesis. This resulted in major changes to the adsorption characteristics of lysozyme, observed using QCM. A monolayer of 8 nm was seen for Ly-AuNCs at pH 4, offering further evidence the proteins form small aggregates, unlike the natural monomer form of lysozyme. The adsorption of Ly-AuNCs was seen to decrease as pH was increased; this is in major contrast with lysozyme absorption behaviour. A decrease in α-helix content was observed from 25 % in natural lysozyme to 1 % in Ly-AuNCs. This coincided with an increase in β-sheet content after AuNCs synthesis indicating that the natural structure of lysozyme was lost. The formation of protein dimers, the change in protein surface charge from positive to negative, and secondary structure alteration caused by the AuNC synthesis must be considered before attempting to utilise Ly-AuNCs as in vivo probes.
- gold nanoclusters