Polymer association in poor solvents: from monomolecular micelles to clusters of chains and phase separation

Abstract

SUMMARY: We report some recent results obtained in our laboratory on the poor-solvent behavior of macromolecules. We first discuss the globular collapse of short chains that, unlike long ones, may form compact ordered states. We then address the collapse of random AB copolymers, which may provide significant clues to understanding biophysical issues such as the protein folding problem or the DNA arrangement in a living cell. Afterwards, we turn to the many-chain problem of homopolymer aggregation into polymolecular micelles or clusters of chains and eventually phase separation. The unifying feature of our approach consists in the self-consistent free-energy minimization with inclusion of intra-and inter-molecular interactions, whenever they are required, that enables us to describe the chain conformation in detail. General introductionThe collapse of a polymer chain to the globular state in poor solvents has received a wide interest both as a natural extension of the study of long-range a interactions beyond the good-solvent conditions 1,2) , and as a possible key to understand complex biophysical phenomena, in particular the native state of globular proteins or the conformation of DNA in a living cell 1) . Furthermore, it is a prerequisite for a description of coil aggregation leading to the formation of polymolecular micelles and eventually precipitation. While the general features of the collapse of long homopolymer chains (i. e., formed by identical repeat units) are relatively well understood 1, 3-7) , this is not so for short chains, in particular with reference to their local conformation. Most approaches are instead mainly focused on the overall features of the resulting globule. Therefore, in this Feature Article we discuss some recent theoretical results obtained in our laboratory on the poor-solvent behavior which stress the importance of the intramolecular degrees of freedom.Coil collapse is usually compared to the condensation of a vapor to the liquid phase. However, it was suggested 1, 3) that a slim chain may undergo other transitions to some "ordered" state, somehow akin to the formation of a crystal, or at least of a liquid crystal with nematic order. This indeed turns out to be the case for short chains 8) , as we shall report in the present paper. Also, the process of aggregation and precipitation confronts us with open issues, concerning for example the coil conformation near the critical point. Thus, we shall also address many-chain systems. Homopolymers are clearly a poor model for proteins, that may be regarded as copolymers with very specific monomer sequences (the aminoacid residues). Therefore, later efforts were directed to the study of appropriate copolymers [9][10][11][12][13][14][15][16] . For instance, simulation studies usually considered chains formed by unlike units, whereas other approaches took for simplicity AB copolymers consisting of hydrophobic and hydrophilic units only. Accordingly, we shall also report our recent work on the collapse of random copolymers formed by two di...

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