Probing the inner local density of complex macromolecules by pyrene excimer formation

Abstract

The direct relationship existing between the average rate constant 〈k〉 for pyrene excimer formation and the local concentration [Py]_loc of ground-state pyrenyl labels covalently attached to a macromolecule was established for 55 pyrene-labeled macromolecules (PyLM). These PyLM belonged to three different families of macromolecules with the first representing short monodisperse linear chains end-labeled with pyrene (polystyrene, poly(ethylene oxide), and poly(N-isopropyl acrylamide)), the second representing long polydisperse linear chains randomly labeled with pyrene (poly(methyl acrylate), poly(methyl methacrylate), polystyrene, poly(butyl methacrylate), poly(methoxyethyl methacrylate), and poly(N-isopropyl acrylamide)), and the third being comprised of two series of pyrene end-labeled low generation dendrimers with a bis(hydroxymethyl)propionic acid or a polyamidoamine backbone. The assumption, that the polymeric segments probed by an excited pyrenyl label covalently attached to one of these macromolecules obeyed Gaussian statistics, enabled the calculation of their square root average squared end-to-end distance (L_Py), which was applied to calculate [Py]_loc. The log–log plots of 〈k〉 as a function of [Py]_loc yielded straight lines with a slope of unity for all families of macromolecules studied in four different organic solvents demonstrating the validity and generality of the 〈k〉-vs.-[Py]_loc relationship. Since an experimentalist knows how the the pyrenyl labels are covalently attached onto a macromolecule, [Py]_loc offers a means to probe the local density of a macromolecule, which can be employed to characterize its conformation in solution. Consequently, the 〈k〉-vs.-[Py]_loc relationship provides a novel experimental means to probe the conformation of macromolecules which should establish pyrene excimer formation as an appealing method for conformational studies of macromolecules in solution, which should nicely complement scattering techniques.