PhD defence: The cooperative ionization transitions of hydrophobic polyelectrolytes

Sharp, cooperative, switch-like transitions are key in biological regulation. A textbook example is the sensitivity of the binding of oxygen onto the hemoglobin molecule found in red blood cells. It is advantageous for the efficiency of the capture, and subsequent release of oxygen to the body, for the hemoglobin molecule to prefer states where it is either fully unoccupied or fully occupied by oxygen. There are many other examples of the sensitivity of biological systems with respect to their surrounding conditions. It is generally assumed that only complex molecules such as proteins can display these types of cooperative transitions, which are often driven by an underlying conformational change in the protein’s structure.

Taking inspiration from the theoretical framework used to describe these cooperative transitions for biological systems, we posed the question: is it possible for simple synthetic systems to display this class of cooperative transitions? We used previous literature results on the interactions between a medium’s acidy (pH) and the behaviour of relatively simple polymeric acid or base molecules (hydrophobic polyelectrolytes) to show that this is, in fact, possible.

After detailing the conditions that lead to sharp, pH-induced transitions, for so-called hydrophobic polyelectrolytes, we synthesised our own polyelectrolytes and designed our own systems to probe and understand this behaviour further.  The findings related to the polyelectrolyte structures, and the nature of the systems they are dissolved in, are potentially relevant in drug and gene delivery applications and in the development of materials whose optical, electronic, or other properties can be switched by small concentration variations of specific molecules in the environment.