Chemistry and Biochemistry

Wally Paxton

Wally Paxton

Research:

My research involves i) designing and controlling the self-assembly of stimuli-responsive amphiphilic block copolymers. Such materials can be used to create assemblies that respond to environmental stimuli (for example, changes in temperature, pH, or oxidation/reduction reactions). We intend to use these materials as synthetic matrices and scaffolds for ii) controlling the chemical and electronic properties of interfaces and iii) incorporating functional biomolecules – enzymes or membrane proteins – into stimuli that change the properties of the polymer matrix.

 

Designing and Controlling the Self-Assembly of Stimuli-Responsive Polymers. Amphiphilic molecules, like surfactants and lipids, spontaneously organize into aggregated structures from micelles to cell-membranes. Polymeric amphiphiles offer a rich library of self-assembling molecules that may be finely tuned for specific applications. Incorporating switchable functional groups that are responsive to pH, temperature, or oxidizing/reducing conditions makes it possible to switch between different kinds of assemblies. Students interested in this project will develop skills designing, synthesizing, and interrogating stimuli-responsive molecules and the resulting structures, which will be useful for a range of applications from sensors to drug delivery.

 

Tuning the Chemical and Electronic Properties of Interfaces. Some polymer amphiphiles comprising micelles and vesicles readily adsorb and fuse onto hydrophilic surfaces, forming layered structures with tunable thicknesses on the nanometer scale, analogous to supported lipid bilayers. These adsorbed layers can be used as models of cell surfaces and also allow tuning the chemical properties of interfaces. Understanding how these adsorbed layers change the electronic properties of the interface can be especially important for developing new kinds of sensors. Students interested in this project will develop expertise in forming supported polymeric and hybrid (lipid/polymer) bilayers and measuring their electronic properties using electrochemical methods, including cyclic voltammetry and impedance spectroscopy.

 

Schematic representation of a cyborg material consisting of a localized biotic catalyst – an enzyme or a membrane transport protein (purple) – bound to a bilayer membrane prepared from abiotic stimuli-responsive polymers. The action of the catalyst produces localized changes (e.g. pH, heat, or electrochemical) that will alter the morphology (i.e. the shape parameter) of the individual polymers (left) and, by extension, the synthetic membranes they comprise (right).

 

Integrating Biological Function in Stimuli-Responsive Vesicles and Bilayers. A wide range of polymers exist that respond to changes in pH, temperature, or oxidizing/reducing conditions. It is now possible to create materials that change their equilibrium shape to favor the formation of micelles, bilayers, or other structures depending on their local environment. Stimuli-responsive polymers (e.g. containing pH-sensitive polyacrylic acids) coupled to heterogeneous catalysts, including enzymes (e.g. urease), make it possible to translate chemical signals that polymer amphiphiles don’t recognize (e.g. urea) into stimuli the polymer amphiphiles do recognize (e.g. increase in pH). Clever designs of such systems would enable the development of materials that respond via catalysis to stimuli that are (otherwise) unrelated to the normal stimuli these materials are sensitive to. Such materials would find tremendous value as self-actuating materials, biosensors, and drug delivery vehicles.