Physicochemical properties of pectic acid. I. Thermodynamic evidence of a pH‐induced conformational transition in aqueous solution

Abstract

SynopsisOn the basis of measurements of enthalpy of dissociation and of dilution, an intramolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution. Additional evidence is given by potentiometic, viscometric, and chiroptical results. The transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a AH value of about 500 cal/equiv and a AS value of 1.6 cal/equiv K in water at 25OC. The addition of salts increases the stability of the rigid conformation without changing the general features of the phenomenon. Dilatometric measurements suggest that the transition is accompanied by practically no change in the overall solvation of the polymer chain.

Effects of Polyelectrolyte Chain Stiffness, Charge Mobility, and Charge Sequences on Binding to Proteins and Micelles

“…However, if (pH -pK a ) g 2, we can assume complete ionization, i.e., no charge mobility. This is nearly the case for pectin, for which literature values of pK a vary from 4.0 to 4.4 in pure water, and from 3.5 to 4.0 at I ) 0.05 M. For pectin, pH c values ranged from 5.9 to 4.4, such that the condition (pH -pK a ) g 2 is satisfied at I e 0.1 M. 42 Conflicting values are found for the pK a of HA: Cleland 43 reported 2.8 < pK a < 3.2, depending on I and R; a more recent report gives pK a , in I ) 0.05 M, as 3.5 ( 0.1 independent of R, 44 but these values are also low enough to assume full ionization for HA over most of the pH c range. Therefore, the condition (pH -pK a ) g 2 was satisfied at I e 0.15 M. On the other hand, the larger values of pK a for PAA result in R at pH c ranging from 0.75 to 0.50 as I increases from 0.01 to 0.6 M. Despite this complication of determining the degree of charge for PAA at different condi- tions, we focus in this section on PAA because resolving effects of charge from chain stiffness for HA and pectin is even more problematic.…”

Polygalacturonase-Inhibiting Protein Interacts with Pectin through a Binding Site Formed by Four Clustered Residues of Arginine and Lysine

“…The lack of interaction at neutral pH cannot be ascribed to different protonation of positive residues (Arg and Lys) of PGIP or carboxylic groups of PGA. It is possible instead that a conformational transition of the PGA structure occurs and increases the pH, creating a charge distribution not suitable for PGIP binding (Cesaro et al, 1982). Indeed, we have observed by circular dichroism analysis that PGA at pH 7.0 in the presence of 100 mM calcium is subjected to a conformational change with respect to the form present at pH 5.0 (data not shown).…”

Potentiometric Titrations of Maleic Acid Copolymers in Dilute Aqueous Solution: Experimental Results and Theoretical Interpretation

“…[21] The ordered conformation was indicated as a rather rigid one, also on the basis of an accompanying relative increase of viscosity. The recent calculations of Benegas et al, [22] based upon the analysis of the pH titration of PA, confirmed the existence of such a transition, providing a relative increase of the stiffness parameter k from 0.1 (disordered) to 0.3 kcal Á mol À1 Á Å À2 for the helical conformation.…”