Structural instability of human serum albumin during microparticles synthesis

Human serum albumin, the most abundant plasma protein, plays vital roles in maintaining oncotic pressure, modulates drugs pharmacokinetics and pharmacodynamics, and features antioxidant and enzyme-like properties. This protein also exhibits high-affinity binding to a wide range of endogenous and exogenous molecules. Combined with its excellent solubility, biocompatibility, biodegradability, low toxicity, and nonimmunogenicity, these properties make human serum albumin an attractive platform for biomedical applications, particularly in drug delivery. This study reports a detailed physicochemical characterization of human serum albumin microparticles obtained via the coprecipitation-cross-linking-dissolution method. The resulting submicron particles are peanut-shaped, exhibit uniform morphology, and show robust mechanical properties, including high stiffness and colloidal stability. However, we observed that the microparticle agglomeration process induces significant structural changes in the protein. Notably, Raman and FTIR spectroscopies highlighted a partial switch from α-helices to β-sheets secondary structure, which leads to diminished HSA binding capability as here demonstrated in the case of hemin. This trade-off between mechanical integrity and biological activity poses challenges for applications requiring native protein interactions, such as the HSA-dependent efficient drug binding and controlled release. Conversely, the reduced binding capability of HSA molecules localized on the surface of MPs implies that these HSA-MPs cannot bind to plasma ligands (e.g., drugs, heme, bacteria toxins). This prevents unwanted ligands from being transported to cellular targets, ensuring that only those located within the HSA-MP are transported. Our findings open new avenues for engineering microsystems that could couple the structural resilience of these microparticles with restored functional surfaces.