Double-Helical Ultrastructure of Polycationic Dendronized Polymers Determined by Single-Particle Cryo-TEM

The ultrastructure of cationic dendronized polymers (denpols) of third and fourth generations (PG3 and PG4) in water was determined by using single‐particle cryo‐transmission electron microscopy (cryo‐TEM). At concentrations in the region of 50 mg L−1, networks of double‐stranded fibers were revealed that exhibit well‐defined diameters of 5.9 nm±0.4 nm for PG3 and 7.4 nm±0.4 nm for PG4. The structure varies with progression along the fibers, and includes a double helix with a pitch of 7.0±0.4 nm for PG3 and 9.0±0.4 nm for PG4. The formation of the double strands is attributed to the hydrophobic effect and limited crowding in the dendron shell of the third and fourth generation denpols investigated. From solutions of lower concentrations (around 10 mg L−1), isolated molecular fibers were adsorbed onto high‐energy surfaces and examined by performing scanning force microscopy (SFM) on mica, and after staining, TEM on glow‐discharged carbon films. In both cases, characteristic undulations of single strands were observed, which are attributed largely to the adsorption process. Dendronized polymers dimerize to double helices: The simple charging of dendronized polymers is enough to force two polymer strands in buffered water into dimeric aggregates, parts of which assume a double‐helical structure of unprecedented dimensions (see figure), as revealed by image analysis and the three‐dimensional reconstruction of cryo‐TEM data. The double‐helix formation is not driven by hydrogen bonding or π–π stacking.