Counterion Diffusion Reveals Coil-to-Helix Transition in a Polyelectrolyte

Malgorzata Ciszkowska⁺ and Janet G. Osteryoung⁺⁺

⁺ Department of Chemistry, Brooklyn College, The City University of New York Brooklyn, New York 11210-2889
⁺⁺ Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695-8204.
 

Abstract

Diffusion of counterion is a useful and easily interpreted transport phenomenon, directly related to the interactions between polyelectrolytes and their counterions in solution.  The strength of electrostatic interactions between simple counterions and polyions is proportional to the charge density of polyelectrolyte, which in turn depends on the structure of polyion.  Thus the information on the transport properties of counterion in polyelectrolyte solution can result in the structural characterization of macroions.  In this work, steady-state voltammetry at microelectrodes is proposed as an accurate, sensitive and inexpensive method to study conformational transitions of ionic polymers, to investigate the interactions between probe ions and polyions during such transitions and to determine the structure of conformers in solutions.  The coil-to-helix transition of ionic polysaccharide kappa-carrageenan as a result of changes of the temperature is studied using thallium cation as a probe.  Structural characterization of conformers in solution is expressed by the distance between ionic groups, determined based on existing theories for counterion-polyelectrolyte interactions.  Experimental findings are compared with results from circular dichroism measurements, a well developed methodology for conformational transition studies.