Calculation of protein-anion affinity constants from acid titration data

Abstract

It has been shown earlier that the titration curves of wool and other proteins obtained with different strong acids differ widely in position with respect to the pH coordinate. By assuming that these differences were due to combination of the protein with anions as well as with hydrogen ions, it was possible to calculate from the pH of the midpoint of each curve numerical values of tbe affinity of each anion for wool. In the present paper modifications of the equations for calculating anion affinity are described. It is shown that the new equations describe the titration curves as a whole instead of merely the positions of their midpoints. The new forms are also shown to describe the effects of the presence of salts on the titration curves at least as adequately as did the earlier ones. CONTENTS Page

The combination of fibrous proteins with acids

“…Titrations carried out at constant ionic strength by Steinhardt (1942) and by Steinhardt & Harris (1940) The influence of the intrinsic affinity of the anion of an acid on the position of the titration curve with respect to the pH axis, is shown clearly by equation (11). For example, if the chloride ion is replaced by an ion of greater affinity (less positive A fa) the whole curve is moved to regions of higher pH.…”

“…Recently, Steinhardt (1941Steinhardt ( ,1942 and Steinhardt & Harris (1940) have advanced very different ideas. It will be of help in presenting them if the physical meaning of the equations developed above is summarized.…”

“…p K u> is given the value 4*2, and in one of the latest papers (Steinhardt 1942) a value of 100 is adopted for the ratio (K^/K^ )*.…”

“…I t is then found th a t a plot such as th a t in figure 1 leads to a titration curve with a slope of about -2 instead of -1 for pure acid, and a slope of -1 instead of -for titra tion in the presence of a constant concentration of anions. To correct for this, Steinhardt (1942) proposes the introduction of all activities and constants as square roots, and suggests th a t this is necessary because the state of ionization of the fibre as a whole has not been taken into account.…”

Analysis of the selective combination of wool with acids in mixtures

“…When wool is allowed to attain equilibrium with a dilute solution of an acid, the amount combined (given as a fraction of the maximum of about 0.83 mmole/g) is described with good approximation [2J by the equation Amount combin ed B [2] . .…”