Probing the dynamics of domain III of human serum albumin entrapped in sol–gel derived silica using a Sudlow’s site II specific fluorescent ligand

Previous studies from our lab reported on the use of time-resolved fluorescence anisotropy (TRFA) to probe the dynamics of domains I and II within the model protein, human serum albumin (HSA), in solution and when entrapped into sol–gel derived silica. In order to further our understanding of the dynamics within this multi-domain protein, TRFA was used to measure the dynamics of domain III of the protein. For this purpose, the fluorescence ligand dansylsarcosine (DS), which has a 400-fold higher emission intensity in the bound state relative to the free state and an emission lifetime of >22 ns when bound to Sudlow’s site II (domain III) in HSA, was selected. This probe is able to accurately report on slow rotational motions (up to 300 ns correlation time) and the bound form of the probe can be selectively measured at 475 nm, ensuring that the dynamics reflect only the properly folded form of the protein. The mobility of HSA with bound dansylsarcosine (HSA–DS) was evaluated in solution and after entrapment in sol–gel derived silica prepared from sodium silicate under varying ionic strength and pH conditions. The results here show that (1) the 43 ns global rotational correlation time of HSA in buffered solution can be accurately measured via labeling with DS with no interference from faster local or segmental motions; (2) the global motion of HSA in silica is greatly hindered immediately after encapsulation, with no correlation time faster than 300 ns discernable, indicative of strong templating of the silica around domain III of the native protein; and (3) the addition of salt and variation of pH have essentially no effect on HSA mobility, ruling out electrostatics as the primary interaction restricting HSA motion. The results from this study are compared to past studies using intrinsic tryptophan fluorescence (domain II) or fluorescein-labeled HSA (domain I), and demonstrate that motion observed using such probes likely reflects differential mobility of the three domains, consistent with domain III of HSA adsorbing to or templating with silica upon entrapment while the other domains protrude into the pore. Restricted motion of domain III of HSA was also observed in silica materials derived from diglycerylsilane or tetraethylorthosilicate, showing that templating is not dependent on the silica precursor or processing conditions.