Structure, Activity, and Dynamics of Human Serum Albumin in a Crowded Pluronic F127 Hydrogel

Structure, activity, and dynamics of a plasma protein, human serum albumin (HSA), inside a crowded environment of F127 gel are studied by circular dichroism (CD), fluorescence correlation spectroscopy (FCS), and picosecond time-resolved fluorescence spectroscopy. For this purpose, the protein is covalently labeled by a maleimide dye, 7-(diethylamino)-3-(4-maleimidylphenyl)-4-methyl-coumarin (CPM). The circular dichroism (CD) spectra suggest that the protein is more structured in the gel reflecting about the biological activities of the protein. FCS results demonstrate that compared to that in bulk water (buffer solution), translational diffusion is about 59 times slower inside the F127 gel. This indicates higher translational friction (viscosity) sensed by the probe (CPM). On the contrary, rotational relaxation (and hence, rotational friction) is more or less similar in F127 gel and in bulk water. FCS results further indicate that the time scales of conformational relaxation of the protein are substantially slow inside the crowded environment of F127 gel. The fast component of conformational relaxation is retarded by ∼55 times, and the slow component by ∼20 times. Fluorescence maximum of CPM bound to HSA show a ∼5 nm red shift, implying that the microenvironment of the probe, CPM, is more polar inside the gel. Solvation dynamics of CPM-labeled HSA inside the gel (⟨τ_s⟩ ∼ 300 ps) is faster compared to that for the protein in bulk water (⟨τ_s⟩ ∼ 600 ps).