Multiple Phases of Gels and Biological Implications

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Abstract
Synthetic polymer gels are known to exist in two phases - swollen and collapsed [1,2]. Volume transition occurs between the phases either continuously or discontinuously. More than two phases are found in gels consisting of copolymers of randomly distributed positively and negatively charged groups. They are characterized by distinct degrees of swelling; the gel can take one of a set of swelling values, but none of the intermediate values. The number of phases depends on the composition of positively and negatively charged monomers: the number decreases from a maximum of seven at a particular composition to one at pure cationic or anionic gel compositions. In these gels, polymer segments interact with each other through attractive or repulsive electrostatic interactions and through hydrogen bonding. The combination of these forces appears to result in multiple phases. The multiple phases seem to have important biological implications. The finding demonstrates that a polymer system can take a stable state other than swollen and collapsed phases, which were known only to proteins. Studying them may lead us to the principle with which linear chains of amino acids form unique and stable structures—a question of protein folding—and how they create marvelous functions such as molecular recognition, catalysis, and motion. The main portion of this article was previously published in Nature [3]. This article is the same as the lecture note given at the 34th IUPAC International Symposium on Macromolecules held in Prague, 1992.