Insulin and Human Serum Albumin Interactions with Core–Shell Fe₃O₄@SiO₂ Nanoparticles Functionalized with Carboranes

In a biological medium, nanoparticles (NPs) can spontaneously interact with proteins, adsorb onto their surface, and cause conformational and orientation changes of the proteins. As a result, the protein function is influenced in a complex manner. Therefore, a detailed understanding of the nature and specificity of protein-nanoparticle interactions is crucial for the application of functional NPs in medicine. In the presented work, we studied the interactions of GMA-treated SiO₂ NPs with the Fe₃O₄ core and attached carborane compounds (Fe₃O₄/TEOS/TMSPM/GMA/Carborane), designed for boron neutron capture therapy, with human serum albumin (HSA) and insulin. We combined different techniques: spectrofluorometry, circular dichroism spectroscopy, and isothermal titration calorimetry to address this issue. The results show that the adsorption of protein onto the NP surface is enthalpy-entropy-driven, with ensuing structural changes of the protein. As for albumin, the percentage of the α-helix structure in the protein is significantly reduced from 87.59 (free protein) to 40.9% for an NP concentration of 1.8 mg/mL, while the content of the β-sheet and random coil increases from 0.48 to 8.78% and from 11.93 to 50.32%, respectively. The interaction between NPs and small protein-insulin is weaker than that for HSA, confirming less negative Δ<i>H</i> and a 15% decrease in the α-structure content for the highest concentration of NPs. For both proteins, the exposure on Fe₃O₄/TEOS/TMSPM/GMA/Carborane affects the polarity of the microenvironment around Trp, which is consequently exposed to a more hydrophobic environment. Calculated values of the radius of gyration and the minimum distance between the proteins and the NPs indicate a stronger interaction and closer binding proximity to the NPs, corroborating experimental observations of the higher binding affinity of HSA to NPs.