Use of potentiometric titration for determination of α‐ and ω‐peptide bonds in poly(aspartic acid) and poly(glutamic acid)

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Abstract
Polypeptides of dicarboxylic amino acids having the monomer units linked in α‐ and ω‐peptide bonds contain two kinds of carboxyls of different acidity. How well potentiometric titration can distinguish these two carboxyls and so characterize the nature of the peptide bonds is evaluated critically. An analysis of the equation describing the dependence of pH on the degree of neutralization based on neglecting the polymer effect and a discussion of the dissociation behavior of polyanions show that the method of evaluating experimental data found in the literature is incorrect. Nevertheless, if a conformational transition does not interfere, some useful and reliable information may be gained by this method; namely, an indication of the presence of two different peptide bonds, their mole ratio, and an approximate pK value for the carboxyl of the amino acid linked in the ω‐peptide bond. The presence of two types of carboxyls complicates the evaluation of the titration curves in the conformation studies.