Potentiometric titration and conformation of synthetic and natural polyelectrolytes
From the analysis of the potentiometric titration data of polyelectrolytes, we can obtain the electrostatic free energy of the macromolecules. Therefore, potentiometric titration is a useful experimental procedure to study the conformation and conformational change of macromolecules. A few examples of the application of potentiometric titration to the studies on polypeptides, protein and DNA are discussed. When the potentiometric titration method is not applicable, the numerical solution of the Poisson—Boltzmann equation without use of the Debye—Hflckel approximation may be employed to estimate the electrostatic potential at the surface of macromolecules. To confirm the reliability in the values thus estimated without experiments and in the values of the electrostatic free energy calculated from the surface potential, the calculated potentiometric titration data are compared with experimental data of various polyelectrolytes and proteins. The relationship between potentiometric titration data and conformations of stereoregular polyelectrolytes is also discussed. INTRODUCTION A carboxylic acid in aqueous solution dissociates as —COOH—COO + H (1) Its thermodynamic dissociation constant, K, is expressed by K = (—COO)(H)/(—COOH) (2) where parentheses denote the activity of each species. Often, however, the conventional dissociation constant, K0, defined by K0 = [—COO ](H)/[—COOH] (2) is used instead of K since we do not know the activities (in parentheses) of —COO and —COOH but only their analytical concentrations (in brackets). Therefore, K0 is not always a true constant, while K must be. Equation 2' can be written pH = pK0 log [(1 — (3) 519