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...
Titration curves of wool with 18 strong acids at 0°, 25°, or 50° C have been added to the data for 19 others presented earlier. Several have been investigated at more than one t emperature. The reversibility of the equilibria m easured has been demonstrated quantitatively. N ew anion-wool dissociation constants, based on modifications of equations previously used to calculate anion-wool affinities, are tabulated for 33 anions, and h eats of dissociation of a few anionwool complexes are also given. The previously reported t endency of affinity to rise with molecular-weight is confirmed ; fairly consist ent relationships between the affinity and the molecular weights of strong organic acids appear.
The fastness of wool dyes to washing varies widely not only among different classes of dyes but even among members of a given class. These variations are manifestations of the differences in affinities of the dyestuffs for the fibers. A possible basis for understanding such differences in affinities was described in two earlier papers in which it was shown that wool immersed in hydro chloric acid solutions combines not only with hydrogen ions but with chloride ions as well. As a consequence of this, it appeared that the affinities for wool of the anions of different acids might vary considerably and that therefore the positions of the titration curves with respect to the pH axis might vary by correspondingly large amounts, according to the acid used. The determination of the titration curve of wool with a gvien acid could then be used to measure the affinity of the anion of the acid for the fiber. Measurements have therefore been made by Research Associates of the Textile Foundation at the National Bureau of Standards of the combination of wool with nineteen different acids, including, in addition to some of the common mineral acids, several of the simpler aromatic sulfonic, carboxylic, and phenolic acids and a soluble mono-azo acid dye. Wide differences were found between the positions of the resulting nineteen titration curves. A gen eralized form of the equations previously developed to account for the effect of the concentration of chloride ion on the amounts of hydrochloric acid combined was utilized to calculate the affinity for wool of each of seventeen anions involved. Predictions, based on the equations, as to the effects of variations of anion concentrations and of temperature were tested and confirmed. Measurements of the combination of a number of the same acids with a soluble protein, crystalline egg albumin, were also made. Since qualitatively similar differences in the corresponding titration curves were found with both proteins, it is concluded that the property of combining with anions is not restricted to insoluble proteins, such as wool, but is an inherent property of proteins in general. The affinity of the anions appears to increase with their dimensions, and is higher in aromatic than in aliphatic ions of the same size. These relationships are used as the basis of a consid eration of the nature of the forces involved in the binding by pro teins of anions, including the anions of acid dyes.
The rates of hydrolysis by dilute acids of both a dissolved protein (egg albumin) and an insoluble protein (wool) are shown to depend not only on the temperature and acidity but also on t he acid used. When hydrolyzed at 65° C by certain strong monobasic acids of high molecular weight, the amide and the peptide bonds are broken over 100 times as fast as when they are hydrolyzed with hydrochloric acid. Even among the common mineral acids, large differences appear. These differences in hydrolytic effectiveness parallel differences in the affinities of the anions of the acids for protein. A further reason for attributing t his effect to the anions is the attainment, with anions of high affinity, of a maximum rate of amide hydrolysis at r elatively low concentrations, stoichiometrically equivalent to the sum of the amino plus the amide groups. A similar limiting anion concentration or maximum rate of hydrolysis of t he much more numerous peptide groups is not observed. On the basis of details of t he dependence of the rate of hydrolysis on concentration of effective anions and hydrogen ions, a m echanism which involves combination of the groups hydrolyzed with hydrogen ions, is proposed.At low concentrations of effective anions, amide hydrolysis is catalyzed more strongly than peptide hydrolysis. By keeping the concentration slightly below stoichiometric equivalence to the sum of the amino plus amide groups the amide groups may be rapidly hydrolyzed without extensive hydrolysis of the peptide bonds in the protein. Practical applications are suggested.
Evidence presented in previous papers supported the view that wool immersed in solutions containing hydrochloric lWit! combines stoichiometrically not only with the hydrogen ions of the acid but with chloride ions as well. As a consequence it appeared that the specific affinities for wool of the anions of different acids might vary considerably, and that therefore the positions of the titration curves of this protein with respect to the pH axis might vary by correspondingly large amounts according to the acid used.The present paper describes measurements of the combination of wool with 19 different acids, ranging in complexity from some of the mineral acids most commonly used through the simpler aromatic sulfonic, carboxylic, and phenolic acids to a soluble monoazo acid dye. It is shown that wide differences exist between the positions with respect to the pH axis of the titration curves of wool obtained with different strong acids, and that these differences may be ascribed to wide variations in the anion dissociation constants characterizing the corresponding protein-anion combinations. Equations previously derived to account for effects caused by variations in chloride concentration have been generalized for use in calculating these dissociation constants. A scale of relative affinities of anions for wool, based on these constants, and applicable to acid dyes, is proposed. Predic· tions as to the effects of variations of anion concentration and of temperature, based on the same generalized equations, have been tested and confirmed.Measurements of the combination of a number of the same acids with a soluble protein, crystalline egg albumin, have also been made. Since qualitatively similar differences in the positions, with respect to the pH axis , of the titration curves obtained with different acids are found with both proteins, it is concluded that the property of combining with anions is not restricted to insoluble proteins. The affinity of anions for proteins of both classes appears to increase with the dimensions of the anion, and is higher in aromatic than in aliphatic ions of the same size. The bearing of these relationships on the well known specific effects of ions on proteins and on the nature of the forces involved in the binding of anions by proteins is considered.
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