Potentiometric titration of poly-S-carboxyethyl-L-cysteine

✍ Scribed by Hiroshi Maeda; Shoichi Ikeda

• Publisher: Wiley (John Wiley & Sons)
• Year: 1971
• Tongue: English
• Weight: 603 KB
• Volume: 10
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360101213

✦ Synopsis

Potentiometric titration curves have been determined for aqueous solutions of poly-S-carboxyethyl-L-cysteine, which is subject, to the p-coil transition by a change in pH. Iteversibility and time dependence of the titration curves are examined by different methods in order to establish the condilions for obtaining equilibrium curves. The 0-coil transition is manifest at some region on the equilibrium titration curve, if pHlog (01/1 -0 1 ) is plotted against 01. Assuming a value, 4.00, for pKi,t, the free-energy change for the p-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 p-form and random coil, and it permits estimation of the content of @-form a t a given pH. Comparison of the conformational titration curve with the circular dichroic measurements leads to a value of -10,000'

for [e]223 for the pure p-structure. Precipitation which occurs a t low degrees of ionization and, especially, a t high ionic strength does not reveal any discontinuous change of the titration curve, which suggests t.hat the degree of ionization of the precipitated 8-form is not very different, from that in solut,ion.