Crosslinking structure of keratin. II. Intermolecular and intramolecular crosslinks in potassium‐cyanide‐treated wool fibers

Abstract

SynopsisThe crosslinking structure of keratin fibers has been investigated. The reaction of wool with aqueous KCN was studied by means of chemical and physical methods. Quantitative conversion of disulfide (SS) groups of the cystine (Cys) residues into stable thioether (S) linkages was confirmed. In terms of mechanically effective and noneffective crosslinkages, the amounts of intermolecular and intramolecular crosslink in the KCN-treated wools were determined from the analysis of the corresponding amino acids and mechanical experiment in which the shear modulus of swollen, rubberlike samples is determined from the relation between equilibrium stress and strain for simple extension of the fiber in a diluent. A modified elastic equation of state was used for the calculation of the number of intermolecular crosslink. The front factor in the equation used was determined by combining the results obtained from purely chemical kinetics and the values of shear modulus of the swollen wools. The ratio of the number of intermolecular and intramolecular SS crosslinkages in the Lincoln wool was found to be 64 : 36. The reactivity of the former is much higher than that of the latter, and these two types of SS crosslink form substantially the same type of S crosslink. The fraction of the Cys residues accessible to cyanide ions depends mainly on the reaction temperature. INTRODUCTIONThe reaction of keratins with aqueous potassium cyanide has been extensively studied.'-4 It has been postulated that the reaction proceeds via substitution mechanism of the type of reaction S, 2, in which cystine residues convert to lanthionine residues crosslinked by thioether linkage, and this causes negligible production of lysinoalanine resid~es.'-~.~ I t was decided, therefore, to investigate the crosslinking structure of keratins with particular emphasis on the use of potassium cyanide which reacts selectively with disulfide bonds in keratins and forms thioether crosslinkages.I t has been found that the rubbery conditions can be successfully achieved for swollen wools in a diluent system composed of equal volumes of 8M lithium bromide and diethylene glycol mono-n-butyl ether.6-8 From extension modulus measured in the diluent, crosslink density could be determined by using rubber elasticity theory for swollen network. Kajiyama et al.4 investigated crosslinking structure of wools treated with potassium cyanide, and the number of crosslinks was evaluated by measuring the degree of swelling in formic acid according to the method of Caldwell and Milligan.g They reported that a part of intermolecular disulfide crosslinks converts to intramolecular monosulfide crosslinks.Owing to the .complexity of keratin structure and the nonunifonnity throughout fiber structure, it is difficult to define the terms "intermolecular" and "intramolecular" crosslinkages in ~0 0 1 .~We can now differentiate only between " mechanically effective" and " noneffective" crosslinkages.This article deals with the application of the elastic equation of state derived from rubber...

Solubilization of keratins and functional properties of their isolates and hydrolysates

“…Reduction and oxidation of disulfide bonds belong to the common methods for keratin isolation. Reduction of keratin involves use of 2‐mercaptoethanol (Balaji et al, ; Fujii & Li, ; Kakkar, Madhan, & Shanmugam, ; Reichl, ; Schrooyen, Dijkstra, Oberthür, Bantjes, & Feijen, ; Tanabe, Okitsu, & Yamauchi, ; Yamauchi, Yamauchi, Kusunoki, Kohda, & Konishi, ), dithiothreitol (DTT), dithioerythritol (Vasconcelos et al, ; Yang, Zhang, Yuan, & Cui, ), thioglycolic acid (Hill et al, ; Zabashta, Kasprova, Senchurov, & Grabovskii, ), glutathione (Schrooyen, Dijkstra, Oberthür, Bantjes, & Feijen, ), salts of hydrocyanic acid (Arai, Sakamoto, Naito, & Takahashi, ), bisulfites (Tonin et al, ), and m‐ bisulphites (Aluigi et al, ; Vasconcelos et al, ) to solubilize the protein. Many keratins can remain trapped within the protective structure, and usually a hydrogen‐bond breaking agent, such as urea, thiourea, transition metal hydroxides, surfactants, and combinations thereof, are included in the extractant to unfold or denature the protein (Torchinsky, ).…”

Histochemical observation of the cell membrane complex of hair.

“…In addition, they found that there were at least two types disulfide groups in keratin fibers by the studies of thermoelastic examination of keratin fibers: one group being readily reduced by thioglycolic acid in hydrophilic environment and the other being readily reduced by tri-n-butylphosphine in the regions of hydrophobic environment (5)(6)(7)(8). The aim of this study is to clarify the fine structure of hair fiber components, especially CMC of cuticle, by dimensional changes resulting from reduction and grafting at ultrastructual level .…”