Combination of Silk Fibroin with Acid and with Base

Abstract

Silk contains a number of acidic and basic groups. The state of combination and the behavior of these groups are important in influencing the tensile and elastic properties of the fibroin and the adsorption of moisture and dyes.

As part of a broad research program of the Textile Foundation Research Associateship at the National Bureau of Standards on the acidic and basic properties of fibers, this investigation was under taken to determine the relationship between the quantities of hydro claloric acid and potassium hydroxide taken up by fibroin from solutions and the concentrations of those solutions. The effect of added potassium chloride was also studied. In general, it has been possible to correlate the several parts of the titration curves of proteins with their amino acid contents, but our knowledge of the rôle of the phenolic hydroxyl groups of tyrosine has always been uncertain. Silk fibroin, however, contains such a large proportion of tyrosine as to assure practically an indication of the rôle of the phenolic hydroxyl groups of tyrosine simply by the magnitude of the base-binding capacity. In order to investigate more care fully the part taken by these groups, a titration curve was ob tained for methylated fibroin in which most of the hydroxyl groups had been converted into nonreactive methoxyl groups.

The component parts of the titration curve of silk fibroin have been interpreted in terms of the composition of this protein. Thus the hydroxyl groups of tyrosine appear to bind base as shown by the fact that the base-combining capacity is much greater than can be accounted for by the other groups which are able to bind base.

The presence of free carboxyl groups in silk fibroin is sug gested by the shape of the titration curve. From the maximum amount of acid bound an estimate of the total content of free amino groups may be obtained. The effect of added salt is simi lar to that obtained with other insoluble proteins, namely to in crease the quantity of acid or base bound at a given pH, in a manner which suggests that the combination of hydrogen ions is affected by the concentration of anions as well as by the concen tration of hydrogen ions and that combination with hydroxyl ions is affected by the concentration of cations.