The Effect of Global Compaction on the Local Secondary Structure of Folded Dendrimers

The effect of nonlocal interactions on the local structural propensities of folded dendrimers was evaluated in this work by comparing, under identical conditions, the conformational properties of isomeric dendrimers differing in their global packing efficiency. Accordingly, a modular synthesis of two series of dendrimers up to the third generation was developed to provide efficient access to isomeric dendrimers displaying different levels of overall compaction. Dendrimer compaction levels were adjusted by connecting the folded dendrons to 1,3,5-benzenetricarbonyl chloride, as the central core, via either a 2- or a 4-aminobenzamide linkage to induce relatively "compacted" or "expanded" conformations, respectively. The hydrodynamic volumes of the dendrimers were measured by time-resolved fluorescence anisotropy (TRFA) measurements as a function of the dendrimer series, generation level, and solvent. Packing efficiencies (compaction levels) were estimated by the ratio (Vh/Vvw) of the experimental hydrodynamic volume (Vh) to the calculated van der Waals volume (Vvw). The extent and stability of local helical bias was measured using circular dichroism and correlated with the packing efficiency (Vh/Vvw). These studies suggested that compaction plays an extremely important role in determining the secondary structural preferences of the dendrimers; however, the nature of compaction was more important than the extent of compaction.