Probing Intramacromolecular Forces by Pyrene Excimer Fluorescence

INTRODUCTION Intramacromolecular forces (IMFs) of a macromolecule control its conformation and deformability under stress. For instance, shear will induce the elongation of a macromolecule only if it is subject to a stress that overcomes its IMFs. Since changes in the conformation of a macromolecule are usually associated with changes in the viscoelastic properties of its solution, the characterization of the IMFs has been the object of intense scientific scrutiny. Numerous procedures have been implemented to probe IMFs but they all involve tethering a nanometer-scale macromolecule of interest to two macroscopic surfaces which can be micron-sized latex or silica particles or 1 cm×1 cm mica plates, prying the two surfaces apart, and monitoring the reaction force exerted by the macromolecule onto the two surfaces. In these experiments, great care must be applied to ensure that the molecular linker tethering the macromolecule to the macroscopic surfaces does not affect the behavior of the macromolecule as well as prevents interactions between the macromolecule and the surfaces. Also the macroscopic surfaces are expected to have no effect on the IMFs of the macromolecule being investigated. By contrast, the present study investigates how excimer formation between the pyrenelabeled ends of a series of poly(ethylene oxide)s (Py2-PEO) can yield quantitative information on the IMFs experienced by these macromolecules. Instead of being tethered to macroscopic surfaces, the pyrenyl end groups are encapsulated inside nanometer-size sodium dodecyl sulfate (SDS) micelles whose electrostatic repulsion can be investigated by monitoring pyrene excimer formation. These experiments provide a first example where the IMFs of macromolecules freely floating in solution are being probed quantitatively without being tethered to a macroscopic surface.