Combination of wool protein with acid and base: The effect of temperature on the titration curve

Abstract

Equilibrium measurements are reported of the amounts of hydrochloric acid and potassium hydroxide bound by wool, as a function of pH and salt concentration at 0°, 25°, and 50° C. In order to eliminate from the measurements of base combination effects due to decomposition, a study was also m ade of the effect of temperature on the rate of decomposition of the disulfide bonds in wool and on the rate of solution of the fiber in alkaline solutions. The data support the assumption made in accounting for previously reported titration measurements at 0° C, that the carboxyl and amino groups of wool in the uncombined state are completely ionized. Thus changes in the pH coordinates of the titration curves brought about by changes in temperature are small in the pH range in which acid is combined, which indicates that combination with acid is equivalent to backtitration of the carboxyl groups, but are large in the pH range in which base is combined, which indicates that combination with base is equivalent to backtitration of amino groups.The heats of dissociation calculated for the two kinds of groups, approxim at ely 2,500 and 14,000 calories, respectively, are in good agreement with values for these groups in comparable compounds and in soluble proteins. The value obtained in the acid range also agrees with the results of calorimetric measurements on the combination of acid with wool.It is shown that approximately equal parts of the total heat changes in the acid range are associated with the dissociation of hydrogen ions and chloride ions from the fiber. An appreciable part of the total heat effect may be 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 provide evidence for or against the existence of "salt linkages" in wool.

Calculation of protein-anion affinity constants from acid titration data

“…+ an a~+ K'{; (5) This equation differs from eq 3 only by the presence of a correction term which affects appreciably only the upper part of the curve. This low-affinity form (eq 3), rather than the high-affinity form, is required because when salt is present the amounts of acid bound should be almost wholly given by the sum of the terms [WHAj and [WH+j.…”

“…The curves in this figure represent the theoretical relations which are obtained when values of KA', shown in figure 1 for a number of acids selected to cover a wide range of affinity, and the value of Kg' at 0°0 previously established [3,5] are inserted in eq 4. The logarithmic ordinate has been chosen because it yields approximation to a linear relation to pH [3], and furnishes a more critical visual criterion of fit than a linear ordinate allows.…”

“…The formulation of an equation in terms of a single dissociation constant disregards that influence. However, it was pointed out previously [2,3,5] that the oversimplified equations could be made to represent the entire course of the individual titration curves very closely if the concentration terms were permitted to enter into the equations as square roots. The constants KH ', KA, etc., must then be changed for numerical consistency in the computation to the corresponding square roots (KH')l\ (KA')~' etc., although the actual average values of the dissociation constants may be still given by the first powers [5/1].…”

Combination of wool protein with acids in mixtures, and its relation to the acid dyeing of wool

“…, " : " , The solutions were kept at 50 0 ±0.1° 0 for from 3 to ·5 days, periods which have been shown in earlier work [5,7] to be sufficient, for a close approach to equilibrium. T~e following measnr:ements were ~hen made on aliquots OT each; (a) pH' at roonl temperature; (b).…”

Combination of silk fibroin with acid and with base

“…The alkaline branches of the curves are less dissimilar and, in this case, at least, a part of the difference may be attributed to a t('mperature effect. It has been shown that temperature appreciably influences the binding of base by wool but has very little effect on the binding of acid [11].…”