Thermodynamic Analysis of the Conformational Transition in Aqueous Solutions of Isotactic and Atactic Poly(Methacrylic Acid) and the Hydrophobic Effect.

Ksenija Kogej

doi:10.3390/polym8050168

Abstract

The affinity of amphiphilic compounds for water is important in various processes, e.g., in conformational transitions of biopolymers, protein folding/unfolding, partitioning of drugs in the living systems, and many others. Herein, we study the conformational transition of two isomer forms of poly(methacrylic acid) (PMA), isotactic (iPMA) and atactic (aPMA), in water. These isomers are chemically equivalent and differ only in the arrangement of functional groups along the chain. A complete thermodynamic analysis of the transition of the PMA chains from the compact to the extended form (comprising the conformational transition) in water in the presence of three alkali chlorides is conducted by determining the free energy, enthalpy, and entropy changes of the process as a function of temperature, and therefrom also the heat capacity change. The heat capacity change of the transition is positive (+20 J/K mol) for aPMA and negative (−50 J/K mol) for iPMA. This result suggests a different affinity of PMA isomers for water. The conformational transition of iPMA is parallel to the transfer of polar solutes into water, whereas that of aPMA agrees with the transfer of nonpolar solutes into water.

Highlights

An important point in investigating the solution behavior of polyelectrolytes is the relation between structure and properties
Very appropriate samples for such studies are those with the same chemical structure but different spatial orientation of functional groups along the chain, such as the three stereoregular forms of poly(methacrylic acid), PMA [1]: isotactic, syndiotactic, and the usual heterotactic PMA: iPMA, sPMA, and aPMA, respectively
∆Htr and ∆Str increase with increasing temperature. This results in a positive (∆cp )tr value, which is characteristic for the transfer of nonpolar solutes into water or to protein unfolding. These features may be related to the presence of the methyl groups in the aPMA chain

Summary

Introduction

An important point in investigating the solution behavior of polyelectrolytes is the relation between structure and properties. The conformational transition entails a cooperative change of the shape of the polymer chain from a compact to a more open (extended) form. This feature has been and still is a focus of extensive investigations with both natural and synthetic polymers. The compact conformation is of a particular interest because the native conformations of biological macromolecules (e.g., globular proteins) are compact, i.e., tightly packed. Such compact coils or globules, as they are often called, offer sites for incorporation of smaller nonpolar (hydrophobic) solutes like drug molecules. When a change in conformation is induced by some environmental factor (most often pH or temperature) the polymer shape changes to a more open one and the nonpolar solute

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Discussion

Conclusion