Studies on silk proteins I. The properties and constitution of fibroin. The conversion of fibroin into a water-soluble form and its bearing on the phenomenon of denaturation

Abstract

Some of the constituent amino-acids of fibroin (degummed silk) are determined. Special attention is directed to histidine, owing to its use in the calculation of the molecular weight of fibroin. A value of 0⋅45% has been found by methods in which the histidine is isolated as nitranilate or di-(3:4-dichlorobenzenesulphonate). Other values obtained are serine 12⋅6%, threonine 1⋅5%, tyrosine 10⋅6%, and proline 1⋅5%. Hydroxyproline appears to be absent, but the presence of small amounts of some hydroxyamino-acid other than serine and threonine is indicated. The mean residue weight of fibroin is determined by three methods, one of which is a new method based on analysis of the complex formed between fibroin and cupri-ethylenediamine. This method gives a Cu:fibroin-N ratio of 1:1⋅92 and, if allowance is made for co-ordination with the tyrosine hydroxy1 group, an equivalence of 1⋅964 atoms of peptide-nitrogen to 1 atom of copper is obtained. The three methods give an average value of 78⋅0 for the mean residue weight of fibroin. This value, together with the most acceptable data for amino-acid constituents, indicate that the unidentified anhydro-residues (about 20%) have a mean residue weight of about 107. Evidence is presented that fibroin contains no amide-nitrogen. Methods for the determination of amide-nitrogen at present in use, which indicate a content of 1 to 2%, are considered to be unreliable. Fibroin dissolved in cupri-ethylenediamine gives, on neutralization and dialysis of the resulting solution, a water-soluble protein. The production of this water-soluble protein is attended by little or no degradation of the original fibroin as shown by determinations of fluidity, amino-nitrogen, and acid- and alkali-combining power. The water-soluble protein is precipitated by the normal protein-precipitating reagents, but in every instance examined the precipitated material exhibits an insolubility comparable with that of the original fibroin. Factors responsible for the solubilization process are investigated and data for molecular weight, titration values, ultra-violet absorption spectra, reducing activity, optical rotation, tryptic hydrolysis, and viscosity for both soluble and dispersed fibroin are given. Soluble fibroin has [α]_D¹⁵ — 53⋅1° and dispersed fibroin [α]_D¹⁵ — 58⋅9°, both in aqueous media. The preparation and properties of films and filaments of fibroin are described. Films of fibroin can be prepared that are water-soluble. On stretching, these films show strain-birefringence, acquire considerable tensile strength, and become insoluble in water, but X-ray examination gives the β-keratin pattern for both the stretched and unstretched films. Reasons are advanced for considering the water-soluble form of fibroin to be the native or renatured protein and the original protein to be the denatured form. The denaturation of fibroin is discussed on the basis that denaturation is essentially an unfolding of a coiled long-chain molecule. The subsequent aggregation of the uncoiled molecules to give an insoluble product is considered to be a secondary process. Some aspects of protein and polypeptide chains as macro-molecules are also discussed.