Spin-label electron paramagnetic resonance investigations of PAMAM dendrimer end-group structure and dynamics

Abstract

PAMAM dendrimers are nanoparticles containing a series of branching units emanating from an ethylene diamine initiator core. Control of the number of branching units during synthesis results in monodisperse macromolecules with a specified, but variable, number of terminal branches, to which various functionalities can be attached. The ability to attach large numbers of functional groups, in controlled ratios, to the dendrimer end-groups makes dendrimers attractive templates for a variety of applications. For example, partially glycosylated dendrimers are being explored as multivalent ligands for inhibitory and targeting purposes. In such applications the spatial distribution of functional groups on the dendrimers must be understood. Analytical studies aimed at elucidating the structure and dynamics of dendrimers have, to date, been very limited. In this dissertation, a spin-label electron paramagnetic resonance (EPR) approach is developed and applied to solve this problem.

The functionalization of dendrimers with TEMPO spin-labels in varying degrees of loading is described. Trends in the dipolar line-broadening of the EPR spectra in frozen solutions and spin exchange controlled lineshapes in fluid solutions are compared to structural models. New computational techniques are developed for interpreting spin-spin broadening in these complex molecular systems containing many spin-labels. The distribution of functional groups was found to be random for all cases tested, and the dimensionality of the space occupied by spin-labels was dependent on dendrimer size. Analysis of spin-spin broadening in the fluid solution for samples with less than full spin-label loading required the inherent heterogeneity of the spin environments to be explicitly taken into account.