Non-toxic and clean crosslinking system for protein materials: Effect of extenders on crosslinking performance

One of the biggest disadvantages of protein cross-linking method is the toxicity of cross-linking agents. In this paper, a clean and non-toxic cross-linking system for protein materials using citric acid (CA) as crosslinker and polyols as cross-linking extender was established. Incorporation of polyols could enhance cross-linking performance of CA by forming branched network among protein molecules. Effect of polyol structure on the cross-linking performance was studied systematically, and keratin fiber was used as an example of protein materials. Extenders containing both primary and secondary hydroxyl groups were more effective in enhancing cross-linking efficiency. Changes in the amounts of carboxyl group, amine group and sulfhydryl group of keratin fiber after cross-linking were measured via potentiometric and conductometric titrations to quantify the cross-linking efficiency. Cross-linking mechanism was also illustrated. With proper numbers of primary hydroxyl groups, secondary hydroxyl groups and backbone length, polyols could be efficient in enhancing performance of this cross-linking system. This novel clean cross-linking system could be used for not only protein engineering, but also other areas where utilization of cross-linking agents are contemplated.