Molecular weight distribution and solution properties of silk fibroins with different dissolution conditions

Four regenerated silk fibroin (SF) samples were prepared under different dissolution conditions and their molecular weight (MW) distributions and solution properties in water and formic acid were examined. SFL, produced by dissolving in LiBr aqueous solution for 6h, showed the highest MW level. In the three SFC samples, produced by dissolving SF in CaCl₂/H₂O/EtOH solution for dissolution times ranging from 3 to 180min, the MW of the SFs decreased with increasing dissolution time and a new band appeared at low MW. Interestingly, SFL presented as a relatively transparent aqueous solution with 10-30nm particle size, whereas the three SFC samples exhibited a turbid solution with 100-300nm particle size. SF formic acid solutions showed a higher viscosity than SF aqueous solutions and exhibited almost Newtonian fluid behavior, whereas SF aqueous solutions displayed abrupt shear thinning in the low shear rate region (0.1-3s^-1).