Sorption of Water Vapor by Keratin

The word “keratin” is usually applied to that group of fibrous proteins characterized by the presence of the amino acid cystine which enables the formation of disulfide bridges between neighboring polypeptide chains. In nature, the hardened form of keratin plays a protective role and appears in many forms of animal outer covering, such as wool, hair, nails, horn, feather, and quill, where a high degree of physical strength and chemical resistance are desirable. This variety provides a wide selection of geometrical forms and subtle differences in composition between keratins, which can be employed in the study of sorption mechanisms for this material. All forms of keratin contain hydrophilic amino acid residues and peptide groups which ensure significant adsorption of water vapor. Keratin differs from the soluble proteins because the presence of cross-links renders it insoluble in water; it is only soluble in reagents capable of rupturing the disulfide bridges. However, in common with other biopolymers and synthetic polymers having an affinity for water, keratin swells on adsorption over the entire humidity range.