j. Soc. Cosmet. Chem., 33, 385-406 (December 1982) The physical properties of alpha-keratin fibers PROFESSOR M. FEUGHELMAN, CSIRO, Division of Textile Physics, 338 Blaxland Road, Ryde, New South IVales 2112, Australia. INTRODUCTION The bulk of the material forming all alpha keratins such as mammalian hair, wools, horns, claws, nails, and quills is a biological polymer consisting of polypeptide chains. These chains, themselves the products of the condensation of amino acids, have the general formula O R• H ..... C--N--CH--C--N--CH ...... H O R• where R•, R2, are the side chains of the amino acids of which twenty different compositions exist in keratin, their proportion varying with the type of keratin. The term alpha refers to the distinct high angle X-ray diffraction pattern (the a-pattern) which differentiates these keratins from others such as feather keratin (1), The distinguishing feature of all keratins, when compared with other proteins, is the presence of a large proportion of the sulphur containing amino acid cystine. This amino acid with two amino and two carbonyl groups can form part of two adjacent polypeptide chains, creating a covalent crosslink via the disulphide group of the cystine residue. These disulphide linkages are associated with some 10% of the amino acid residues of the keratins and confer a high degree of the physical and chemical stability to the keratin fibers. As will be noted further in this review, these disulphide links play a basic role in the process of setting both in the hairdressing and wool textile industries. At the molecular level ce-keratin fibers can not only be considered to consist of networks of polypeptide chains crosslinked by the covalent disulphide bonds, but (as with all proteins) a large variety of hydrogen bonds as well as Van der Waals interactions exist both between and within the chains. Of special interest in the physical properties of ce-keratin fibers are the Coulombic interactions, also referred to as "salt links," which exist between charged basic side chains of lysine, arginine, and histidine and the acidic side-chains of glutamic and aspattic acids. These basic and acidic side chain groups represent about a quarter of the residues of the ce-keratin structure. Hydrophobic bonds, interactions between chains created by the presence of water, have also been detected for ce-keratin fibers immersed in water (2). 385

386 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In considering the physical properties of ce-keratin fibers it is necessary to recognize the fundamental role of water in the structure of the fiber. The properties of the fiber change markedly with the amount of water interacting with the structure of the fiber (3) to such a degree that the material may be considered as a keratin-water polymer. The penetration and the interaction of the water molecules depend not only on the existence of hydrophyllic sites within the keratin structure but also on the state of order within the different components of that structure. A high degree of order (crystallinity) as well as the presence of cross links may limit the ability of water to interact at what are chemically hydrophyllic sites. It is the lack of recognition for the role of order, on the sorption of water within the ce-keratin structure, that has led to some of the apparently anomalous conclusions, as to the placement of water based purely on the chemistry of the structure (4). In commercial application, human hair and to a lesser extent finger nails in the cosmetic industry, and wool fibers in the textile industry, are the most important of the ce-keratins. The prime physical properties in the application to these fibers are their mechanical properties. Regardless of the treatment that is applied to the fibers, in nearly all instances the result obtained and assessment made are directly related to the mechanical state of the fibers. For this reason the author has considered the mechanical properties of the ce-keratin fibers as the central physical properties of these fibers. The other properties such as X-ray diffraction, Infra-red absorption, electrical conduction, dielectric response, birefringence, and other measurements lead to our understanding of the relationship between the molecular and near molecular structure of ce-keratin fibers and their mechanical properties. It should be emphasised that the mechanical properties of a fiber are the summation of the properties of the whole fiber involving the molecular units both in the ordered (crystalline) regions of the fiber and the less ordered (amorphous) regions. Many of the physical properties dealt with in this review emphasize mainly one aspect of the fiber structure. High angle X-ray diffraction studies primarily lead to information about highly ordered regions of the fiber structure, whereas calorimetric measurements, being related to the freedom of movement of the molecular components within the fiber, respond more to the less ordered, more mobile regions. This review aims to broadly identify the molecular components and assemblies responsible for the variation of the physical properties of ce-keratin fibers with change of moisture content, temperature, and physical and chemical modification. The presence of swelling agents such as concentrated aqueous lithium bromide solutions and formic acid will be examined for their effect on the behavior of the fibers. Two broad structural features may be identified which control the physical behavior of any polymeric structure. The equilibrium structural organization of the fiber, which essentially relates to the equilibrium position of all the components of the fiber, is measured primarily by physical data which do not involve molecular movement. The structural dynamics of the system, that is the time dependent component of the physical behavior of the fiber, are measured by physical data which are sensitive to the freedom of movement of molecular groups within the fiber structure. Because molecular movement is mainly confined to the less ordered components of structure, and since the less ordered components are the main regions of absorption of plasticizers such as water and various alcohols, these plasticizers have a major control over the structural dynamics of the fiber. Figure 1 diagrammatically summarizes the relationship between the physical data obtained relevant to the equilibrium structural organization and structural dynamics of a fiber. Known structural modification by chemical and physical