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 undertaken to determine the relationship between the quantities of hydroclaloric 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 carefully the part taken by these groups, a titration curve was obtained 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 suggested 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 similar to that obtained with other insoluble proteins, namely to increase 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 concentration of hydrogen ions and that combination with hydroxyl ions is affected by the concentration of cations.

Photochemical reactions in silk

“…It is seen that the stability of the silk to light increases WIth increase in pH of the solutions used for treatment of the silk. This is to be expected, since as shown by the titration curve of silk [6], the binding of cations also increases with increasing pH. In addition to pH, the chemical nature of the cationic substance may also influence the rate of the photochemical decomposition, but sufficient data are not available to enlarge upon this point.…”

“…• 'rbe cboice of tbis pH was based on tbe titration studies of GJeysteen snd Harris [6] in which it was sbown that removal of cations from silk becomes appreciable below about pH 3. and is not caused by the presence of free acid in the exposed samples. It was found that none of the silk samples kept in the dark exhibited any deterioration, showing that the deterioration of the irradiated silk ti:…”

“…Addition of enough dilute sodium hydroxide to raise the pH of the salt solution to 9.8 produced a significant increase in the stability of both, although an equivalent amount of sodium hydroxide alone had no measurable effect on the silk. The reason for this is found in the titration studies of silk [6], in which it was shown thu.t, at a given pH, silk combines with considerably more base in the presence of salts than in their absence.…”

“…The stabilities wer e lowered to about that of the original degummed silk, but they were still higher than those of the electro dialyzed or acid-treated samples. The pH of the water used for washing the samples was about 6.5, and accordingly, the silk: would have still retained appreciable amounts of cations [6], which would account fo r these differences. It may be that Bonicatti's [4] failUTe to observe the stabilizing effect of cations resulted from his use of degummed silk which might have contained appreciable amounts of cation.…”

“…The efficacy of the basic compounds in inhibiting decomposition is of considerable interest, especially since it has recently been found by Gleysteen and Harris [6] that most of the base taken up by silk is bound by the hydroxyl groups of the amino acid tyrosine. It is now shown that the photochemical behavior of silk is associated with the behavior of the tyrosine and that alteration of the tyrosine hydroxyl groups by reaction with various reagents is responsible £01' at least part of the stabilizing effects noted in the past.…”