Combination of Wool Protein with Acid and Base: The Effect of Tem perature on the Titration Curve

Abstract

A description has previously been given of the relation bctwcen the amounts of hydrochloric acid and of potassium Hydroxide bound by wool and the concentration of these substances in solution. These measurements were made at 0° C in order to avoid com plications due to decomposition. However, when wool is exposed to acid or base in the course of carbonizing, acid-dyeing, milling, and scouring much more elevated temperatures prevail. It is therefore desirable to determine the temperature dependence of the relations previously described. It is also desirable to compare the relative magnitudes of the effect of temperature on the amounts of acid and on the amounts of base combined, in order to establish the correctness of assumptions as to the mechanism of combination previously made in explaining the results obtained at 0' C. With the purpose of providing this information, measurements of the amounts of hydrochloric acid and potassium hydroxide bound by wool as a function of acidity and salt concentration, at 0°, 25° and 50° C, are now reported. In order to eliminate effects due to decomposition of the wool in alkaline solutions, a study was also made of the effect of temperature on the rate of decomposition of the wool. The results obtained support the assumption previously made that the carboxyl groups and amino groups of wool in the uncombined state are completely ionized. This conclusion follows from the observation that changes in the amounts of acid bound brought about by changes in temperature are small, which indi cates that combination with acid is equivalent to back-titration of the carboxyl groups, but changes in the amounts of base bound at at given hydrogen ion concentration are large, which indicates that combination with base is equivalent to back-titration of amino groups. Similar results obtained on analogous compounds, render it probable that the direction of the temperature effect on the curve of acid combination reverses at a temperature near 40° C. Thus curves at temperatures used in dyeing are probably not far different from the curves at 0° C, which have been studied in most detail. The heats of dissociation calculated from the magnitudes of the changes with temperature in the curves of combination for the two kinds of groups, are in good agreement with values for these groups in comparable compounds, and in soluble proteins. The value obtained in the acid range is also in good agreement with the results of calorimetric measurements on the combination of acid by wool. It is shown that approximately equal parts of the total heat changes in the acid range are associated with the dissociation of hy drogen ions and of chloride ions from the fibre. An appreciable part of the total heat effect is ascribed to a heat of transfer of the ions between the two phases of the heterogeneous titration system. Neither titrimetric nor calorimetric estimations of the heats of dissociation provides evidence for or against the existence of salt linkages in wool.

Combination of wool protein with weak acids

“…Details of the purification of the wool, of the determination of the acid combined, and of the pH have been described elsewhere [3,4]. Except where otherwise noted, all the reagents used were of reagent grade, and were used without further purification, except for the removal of traces of strong acids present as impurities.…”

“…Numerous studies have been made of the combination of wool with acids and bases [1,2,3,4,5,6]. 2 With the notable exception of some measurements by Speakman and Stott [1], practically all of these studies have been made with strong acids and bases which are totally dissociated into their constituent ions.…”

“…Estimates are available of the h eats of combination of a number of strong acids with wool [4,6]. Values between 2,000 and 12,000 calories have been obtained, the higher values being characteristic of the acids of high affinity.…”

Nature of the reaction of wool with alkali

“…As pointed out earlier [2], there existed the possibility that recombination between the aldehydic and sulfhydryl groups may have occurred under the conditions of the hydrolysis. However, it was subsequently shown [6] that few if any free sulfhydryl groups are present in the alkali-treated wool even prior to hydrolysis. In addition, this hypothesis, when applied to wool, would result in the rupture of the disulfide cross-links, a process which would be expected to yield a material of very low strength and extremely high alkalisolubility [7].…”

“…Since practically all of the sulfur in undegraded wool can be accounted for as cystine sulfur, it seems certain that this new amino acid is a product of the reaction of the cystine in wool with alkali. More recently Nicolet and Shinn [9] have offered an explanation for the formation of lanthionine, postulating as a primary reaction a split between carbon and sulfur as follows: (6) I I Cysteine Dehydroalanine…”