Serum albumin hydrogels in broad pH and temperature ranges: characterization of their self-assembled structures and nanoscopic and macroscopic properties.

We report extended pH- and temperature-induced preparation procedures and explore the materials and molecular properties of different types of hydrogels made from human and bovine serum albumin, the major transport protein in the blood of mammals. We describe the diverse range of properties of these hydrogels at three levels: (1) their viscoelastic (macroscopic) behavior, (2) protein secondary structure changes during the gelation process (via ATR-FTIR spectroscopy), and (3) the hydrogel fatty acid (FA) binding capacity and derive from this the generalized tertiary structure through CW EPR spectroscopy. We describe the possibility of preparing hydrogels from serum albumin under mild conditions such as low temperatures (notably below albumin's denaturation temperature) and neutral pH value. As such, the proteins retain most of their native secondary structure. We find that all the combined data indicate a two-stage gelation process that is studied in detail. We summarize these findings and the explored dependences of the gels on pH, temperature, concentration, and incubation time by proposing phase diagrams for both HSA and BSA gel-states. As such, it has become possible to prepare gels that have the desired nanoscopic and macroscopic properties, which can, in future, be tested for, e.g., drug delivery applications.