Cooperative Ligand-Mediated Transitions in Simple Macromolecules.

In biology, ligand-mediated transitions (LMT), where the binding of a molecular ligand onto the binding site of a receptor molecule leads to a well-defined change in the conformation of the receptor, are often referred to as "the second secret of life." Sharp, cooperative transitions arise in many biological cases, while examples of synthetic cooperative systems are rare. This is because well-defined conformational states are hard to "program" into a molecular design. Here, we impose an external constraint in the form of two immiscible liquids that effectively define and limit the available conformational states of two different synthetic and relatively simple macromolecules. We show that the mechanism of the observed cooperative transitions with ligand concentration is the coupling of ligand binding and conformation, similar to more complex biological systems. The systems studied are (1) hydrophobic polyelectrolytes (HPE) which are (bio) polymers that consist of hydrophobic as well as ionizable (proton and hydroxyl ligand binding) functional groups. (2) Oligomeric metal chelators (OMC), which are oligomers composed of metal ion chelating repeating groups that are able to bind metal ions (considered as the "ligands"), resulting in gel-like networks of oligomers cross-linked by coordinated metal ions. We find that in HPE, interactions between ligands and individual macromolecules explain the observed cooperative transitions. For OMC, coordinated bonds significantly enhance the degree of cooperativity, compared to HPE.