Human hair keratin and its interaction with metal ions

• Main Author: Low, Choon Teck
• Other Authors: Ng Kee Woei
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/142466
• Institution: Nanyang Technological University

Keratinous fibers from various sources had been used for the adsorption studies of metal ions such as Cu, Zn, Pb, Cr, Hg, Ag, and others. These fibers were found to chelate metal ions by introducing chemical treatment to the hair fibers which allows better penetration of the metal ions into the hair fiber, or by disrupting the disulfide bonds in hair which produces free thiol groups. However, there were no quantitative data to explain the improved binding of metal ions to keratinous fibers after chemical treatment. Furthermore, there were no articles that gave a direct correlation between the thiols in keratin to the binding of metal ions. Although it is well known that thiols in cysteine bind well to metal ions, cysteine residues in proteins might have a different characteristic under different conditions. This could be due to the folding of proteins or interference from neighboring groups affecting the reactivity of thiols in cysteine. Therefore, the hypothesis of this project is that thiols in keratin are the main functional group responsible for the binding of metal ions to human hair keratin. The objectives of this project are to find out the functionality of thiol in extracted human hair keratin for the binding of metal ions. Keratin were extracted from human hair fibers using reduction method through thiol disulfide exchange with another thiol compound. Quantification of protein and thiol concentration were done by Bradford and Ellman’s assays, respectively. The assays were selected as they are less prone to interference and requires relatively short incubation time below 20 minutes. The extracted keratins were subjected to different pH treatments and it was found that an acidic pH gave the best physical behavior and highest amounts of functional thiol groups, at about 1 mmol/g. Subsequently, keratin in acidic pH were used for the interaction studies with copper (II) ions to find out the relationship between copper and thiol concentrations. Results showed that the molar ratio between copper and thiol concentration has to be below 0.5 to prevent copper from oxidizing free thiol groups into disulfide which makes it unreactive. To further support the results of copper thiol binding in keratin, free thiol groups were capped with NEM. The capped keratin was subsequently subjected to different concentration of copper and it resulted in no interaction between copper and KIF. This result further validated the hypothesis that thiol is the main functional group in keratin involved in metal binding. Following the mechanisms of copper thiol binding in keratin, metal ions commonly found in electronic wastes such as nickel and aluminum were mixed with copper and interacted with keratin. Scarce metal such as silver were also interacted with keratin. Results from the experiment showed that both nickel and aluminum did not affect the binding of copper to keratin. Thus the binding of keratin appears to be highly selective towards copper in the mixed metal ions solution. A possible reason might be Hard Soft Acid Base (HSAB) whereby soft acid would prefer to bind to soft ligand and vice versa for hard acid base. Another possible explanation could be due to the intrinsic properties of the metal ions such as the stability of its oxidation states in aqueous solution. Lastly, keratin-associated proteins (KAPs) which have about 3 times more thiols than keratin were subjected to copper ions interaction. As expected, since KAPs have higher amounts of thiol, lower protein concentrations were required to achieve the same experimental outcomes of keratin. In conclusion, In conclusion, thiol groups in keratin were functional and responsible for the binding of keratin to metal ions. Between monovalent, divalent and trivalent metal ions, monovalent silver ions had the highest affinity towards thiol whereas trivalent aluminum had the poorest affinity. Lastly, keratin associated protein (KAP) displayed better efficacy in binding copper ions as compared to keratin due to its higher cysteine content. However, more focus was placed on keratin due to the presence of cell adhesion motif, LDV (Leucine-Aspartic acid-Valine) and the ability to form fibrous structure, for downstream biomedical application.