Combination of wool protein with acid and base: hydrochloric acid and potassium hydroxide

Abstract

As an initial part of a general program for the study of the acidic and basic characteristics of textile fibers, a study has been made of the dependence on pH of the amounts of hydrochloric acid and of potassium hydroxide taken up by wool from aqueous solutions. The effect on this dependence of the maintenance of a constant ionic strength by additions of a neutral salt, potassium chloride, has also been determined. Most of the measurements were made at 0° C to minimize the effects of decomposition brought about by exposure to extreme concentrations of acid or base.The maximum acid-binding capacity, independent of ionic strength, is 0.82 millimole per gram; the maximum base-binding capacity is greater than 0.78 millimole. With salt absent, no appreciable binding of acid or base occurs in the pH interval, 5 to 10, but the amount bound increases very sharply as these limits are exceeded. When salt is present, the amount of acid or base bound changes with pH more gradually, and there is no wide region in which combination fails to occur; the point of zero combination is sharply defined and is near pH 6.4. The positions of the titration curves with respect to the pH axis are different at every ionic strength. The differences are larger than can be attributed to the effect of salts on the dissociation of acids; thus, in dilute solutions an n-fold change in the total concentration of chloride ions produces a change almost as great as would be produced by a similar n-fold change in the concentration of hydrogen ions. This approach to stoichiometric dependence of the acid bound on the concentration of anions as well as of hydrogen ions accounts for the greater steepness of the titration curve when the source of both ions is the acid alone.The dependence of acid bound on anion concentration or base bound on concentration of cations is explained by treating the electrostatic restrictions arising from the existence of two phases as a case of partial dissociation of protein salts. A possible alternative analysis by means of the Donnan equilibrium is also presented, and factors to be considered in making a final choice between the two treatments are described in detail. Either analysis predicts that the positions of the curves wi, th res~t to the pH axis should, at high salt concentrations, approach a limit which should correspond to the titration curve of the same protein in the dissolved state. This prediction is supported by the fact that the data for wool agree very closely at high salt concentrations with those for a similar but soluble protein, egg albumin.On the basis of this comparison, a detailed analysis is undertaken of the composition of the titration curve in terms of the constituent di-acidic and di-basic amino acids of wool. This analysis leads to the conclusion that the binding of acid and base by wool occurs at the free carboxyl, imidazole, amino, and guanidino groups, but that no combination of base with the tyrosine hydroxyl group takes place in the pH range of this investigation.I The state of comb...

Combination of silk fibroin with acid and with base

“…At the lower values of pH the curves tend to join each other, as indeed they must if the curve for constant ionic strength be extended to that point at which no salt is present. As in the case of wool [3], the central portion of both the acid and the alkaline regions of the curve for solutions of constant ionic strength is not as steep as the curve for solutions without salt, and a greater amount of acid or base is bound at a given pH. The effect of the presence of salt on the slope and /1 position of the titration curve is greater than could reasonably be accounted for by the influence of the salt on the dissociation constants of the groups which are titrated.…”

“…, Figures in brackets indicate the literature references at the end of this paper. 71 and, more recently, by Steinhardt and Harris [3]. The titration of silk fibroin is, in some respects, similar to the titration of wool, but differs in other respects because of decided differences in the number and kinds of constituent amino acid residues.…”

“…A few det erminations of the chloride titer of the acid solutions served to indicate that chloride ions are bound in quantities approximately equivalent to the amount of hydrogen ion bound. A detailed discussion of the effect of salt and a theoretical treatment of the titration data for another fibrous protein, wool, have been given elsewhere by Steinhardt and Harris [3], who showed that the quantity of acid bound depends upon the concentraaltU8tun]…”

“…The silk was degummed by treatment with a commercial enzyme preparation, Enzo. 3 The degumming bath contained 5 g of Enzo per liter and was maintained at a temperature of 65° C. The ratio of silk to solution was about 1:40. Each lot of silk was given three consecutive treatments of half an hour each with constant agitation; after each treatment the silk was quickly rinsed in boiling water and was then thoroughly washed in water at room temperature.…”

“…The pH values of the solutions were determined with a glass electrode following the method used by Steinhardt and Harris [3].…”