Effect of Gold Nanoparticles and Coexisting Acetonitrile Solvent on the Structure of Bovine Serum Albumin

In the design of biocompatible protein-based drug delivery systems (DDS), the protein structural stability is important for its proper function. While organic solvents widely used during the preparation of protein-based DDS might induce structural changes in the proteins, the presence of nanoparticles (NPs) might additionally alter the protein structure. Although previous studies have reported various effects of NPs on protein structure in various environments, there is a lack of understanding of the effect of the coexisting organic solvent environment on designing a biocompatible DDS. In this study, we investigated the effect of 5 nm gold NPs (AuNPs) on albumin structural stability in the presence of organic solvent, such as acetonitrile (ACN). Bovine serum albumin (BSA) was chosen as an albumina transport protein model with high abundance in the blood, high stability, and possible applications in DDS. In addition, AuNPs were used in this study due to their possible applications in DDS, therapeutics, sensing, imaging, and biotechnology. ACN was chosen as an organic solvent with wide applications in DDS and the pharmaceutical industry, and its concentration was varied to investigate the effect of different solvent polarities. Circular dichroism spectroscopy showed that the presence of AuNPs caused partial unfolding of the BSA structure, with increased hydrodynamic diameter distribution in the presence of 0%-15% vol. ACN. While the most prominent effect was observed at 15% ACN, such results were associated with enhanced protein-ACN interactions in the presence of AuNPs, as revealed by MD simulations. Furthermore, while ACN molecules strongly interacted with the protein subdomains bound to AuNPs, water molecules had a higher propensity to interact with the subdomains located far from the NP. Overall, such distinct interactions of protein subdomains with water and coexisting organic solvents may enhance the protein's structural changes and unfolding in the presence of NPs, leading to a better design of drug loading into proteins.