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Potentiometric titration of poly‐ S ‐carboxyethyl‐ L ‐cysteine
Author(s) -
Maeda Hiroshi,
Ikeda Shoichi
Publication year - 1971
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360101213
Subject(s) - chemistry , titration curve , potentiometric titration , titration , ionic strength , aqueous solution , polymer , random coil , gran plot , ionic bonding , analytical chemistry (journal) , ion , crystallography , chromatography , organic chemistry , circular dichroism
Potentiometric titration curves have been determined for aqueous solutions of poly‐ S ‐carboxyethyl‐ L ‐cysteine, which is subject to the β‐coil transition by a change in pH. Reversibility and time dependence of the titration curves are examined by different methods in order to establish the conditions for obtaining equilibrium curves. The β‐coil transition is manifest, at some region on the equilibrium titration curve, if pH – log (α/1 – α) is plotted against α. Assuming a value, 4.00, for pK int , the free‐energy change for the β‐coil transition of uncharged polymer has been evaluated from the extrapolation of the observed titration curves and is found to depend on the ionic strength and polymer concentration. The Henderson‐Hasselbach plot of the titration curve yields clearer distinction between the β‐form and random coil, and it permits estimation of the content of β‐form at, a given pH. Comparison of the conformational titration curve with the circular dichroic measurements leads to a value of −10,000° for [θ] 223 for the pure β‐structure. Precipitation which occurs at low degrees of ionization and, especially, at high ionic strength does not reveal any discontinuous change of the titration curve, which suggests that, the degree of ionization of the precipitated β‐form is not very different from that in solution.