We demonstrate the effect of concentration of extracted keratin on the rebuilding of disulfur bond during dialysis against deionized water: cross-linked networks can be formed in the aggregate when the concentration is high, whereas at low concentration, intramolecular disulfur bond is preferred. This preliminary study illustrates, for the first time, the potential of controlling the cross-linking between keratin chains by facile means without addition of other chemicals, which is helpful to construct the biomacromolecular self-assemblies.
keratin, disulfur bond, rebuildingWool fiber is an excellent natural example of macromolecular self-assembly, where micro-scale keratin chains are assembled into macro-scale fibers through multiple hierarchies [1 -3] . It is expected that the researches of wool keratin will cast new light on the understanding of biomacromolecular self-assembly. There are several interactions stabilizing the structures in a wool fibril, including disulfur bond, hydrogen bond, salt bond and van der Waals interactions. Like other crosslinked materials, the disulfur bond-crosslinked wool fibril is not soluble in common solvents [4,5] . Several methods have been reported to describe the extraction of keratin from wool fiber. However, only the method that utilizes reductant to cleave the disulfur bond can preserve the original protein primary structure [6] . The reducing method is based on the chemistry of thiol-disulfide exchange [7,8] . Other methods will degrade the proteins severely that result in short irregular peptides.Usually, high concentration of denaturant (such as urea and guanidine) is needed when a reducing method is exploited to extract keratin from wool fiber [6] . Removal of reductant and denaturant from the keratin solution by dialysis against distilled water results in white aggregation of the keratin chains [4] .To increase the stability of the extracted keratin against cross-linking via disulfur bond in the absence of reductant and denaturant, many different approaches have been followed, most of which involve chemical modification of the cysteine residues of the keratin [9][10][11] . However, the chemical modification of keratin will eliminate the potential of being a building-block in the self-assemblies. To maintain the potential, treatment of keratin by chemical modification should be exclusive. The controlling of cross-linking between keratin chains is expected to be an important step in understanding how keratin assemblies into wool fibers. Here in this letter, we find that at low concentration, reduced keratin chains do not cross-link into networks via intermolecular disulfur bonds. Instead, they prefer to rebuild intramolecular disulfur bonds. We extracted clean white wool by reducing method with 0.2 mol/L 2-mercaptoethanol and 6 mol/L guanidine chlorate (Gua) in the protection of nitrogen atmosphere at 50℃ for 15 h. Gua was used here, since the hydrolysis of urea would generate ammonia, which resulted in degradation of keratin chains by an increased
