PROBING INDIVIDUAL NON-COVALENT INTERACTIONS IN SUPRAMOLECULAR POLYMERS BY AFM
Unlike in traditional polymers, in which monomers are covalently linked to form long chain macromolecules, “reversible or supramolecular polymers” rely on the temporary aggregation of bi-functional low molar mass molecules, via non-covalent interactions, e.g. hydrogen bonds. These materials behave like polymers, while, at the same time, certain typical polymeric properties, such as chain entanglements, are absent. The sharp dependence of the “strength” of reversible, non-covalent molecular interactions on temperature, choice of medium, and concentration opens entirely new ways of controlling material properties.
However, the understanding of many of the unusual properties of supramolecular polymers is still in its infancy, in particular on the single molecule level. Using advanced nanotechnological tools, such as atomic force microscopy (AFM) as ultra-sensitive force probe, the aggregation behavior of the building blocks of supramolecular polymers connected by hydrogen bonds can be interrogated at the molecular level, as shown by us for the first time.
Specifically, we measured the rupture force of one individual complex of two quadruply hydrogen-bonded partners in ureido-4[1H]-pyrimidinone (UPy) supramolecular polymer chains in various media and at various temperatures by AFM (Figure 1). Temperature-controlled single molecule force spectroscopy (SMFS) measurements by AFM allowed us to extend the experimentally accessible loading rates and hence to cross regimes from thermodynamic non-equilibrium to quasi-equilibrium states. Central to this new approach is the application of the time-temperature superposition principle to supramolecular bond rupture forces on the single molecule level. In addition, in the presence of building blocks that form reversible polymer chains, individual supramolecular polymer chains with up to 15 repeat units length switched in series have been stretched successfully (Figure 2). These SMFS experiments have opened a new pathway to elucidate materials properties of H-bonded supramolecular polymers from a true molecular perspective.
Figure 1. a) Schematic of the AFM force pulling experiment between poly(ethylene glycol)-UPy disulfide self-assembled both on Au-coated AFM tip and Au substrate. b) Master plot of crossover from loading rate independence to loading rate dependence of rupture forces of single (UPy)2 complexes in hexadecane at a reference temperature Tref = 301 K (the dashed line serves to guide the eye).
Figure 2. Force-extension curves measured between a) individual UPy units tethered with a PEG spacer to a gold surface and b) surface immobilized UPy units in the presence of a PEG-containing bi-functional bis-UPy derivative in hexadecane. The fits of the data to the modified freely jointed chain model, which provide evidence for the stretching of one individual PEG filament, are shown as solid lines. The increase in stretching length between a) (one PEG spacer) and b) (4 PEG spacers) shows that indeed single supramolecular polymers have been probed.
Publication S. Zou, H. Schönherr, G. J. Vancso: Angew. Chem. Int. Ed. 2005, 44, 956 and S. Zou, H. Schönherr, G. J. Vancso: J. Am. Chem. Soc. 2005, 127, 11230.