[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: We investigated the mechanical properties of single chains of dendronized polystyrenes with dendrons of generation 1 to 4 spin coated on the basal plane of graphite. A scanning force microscope was used both to image the chain relaxation upon annealing and to manipulate single chains directly. The two approaches show that conformations for higher dendron generations chains are inherently trapped at room temperature. This behavior is attributed to a glassy state of single dendronized polymer chains. Upon annealing, the chains relax and orient quite perfectly along the axes of the substrate.
[show abstract][hide abstract] ABSTRACT: Positively charged dendronized polymers with protonated amine groups at the periphery and different dendron generations are cylindrically shaped nanoobjects whose radii and linear charge densities can be varied systematically. These polyelectrolytes have been complexed with DNA and subsequently adsorbed on precoated mica substrates. The analysis of scanning force microscopy data indicates that DNA wraps around the dendronized polymers. The calculated pitch is 2.30 +/- 0.27 and 2.16 +/- 0.27 nm for DNA wrapped around dendronized polymers of generation two and four, respectively. The complex with the second generation has been shown to be negatively charged, which is consistent with the theory of spontaneous overcharging of macro-ion complexes, when the electrostatic contribution to the free energy dominates over the elastic energy. The complexes may be of interest for the development of nonviral gene delivery systems.
Journal of the American Chemical Society 07/2002; 124(24):6860-6865. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: In cells and viruses as well as non-viral gene delivery systems, DNA is complexed with different molecules to form highly condensed structures. A wide range of conditions that cause DNA to collapse into compact structures has been discovered . However, in most of these cases an exact description of these structures cannot be given. Since the complex stability is largely due to electrostatic forces, it can be modulated by varying the salt concentration. Apart from the biological aspects the study of the molecular structure of polyelectrolyte complexes may be used to improve our general understanding of polyelectrolyte interactions. Theoretical models reveal the structure of complexes formed between a stiff charged cylinder and an oppositely charged flexible or semiflexible polymer , . Here we determine the influence of salt on the structure of linearized pUC19 plasmid DNA and positively charged dendronized polymers of generation two (PG2).
[show abstract][hide abstract] ABSTRACT: Dendronization with the first generation (G1) building block 10b of the polystyrenes 13c and 14c carrying first (G1) and second generation (G2), amino-terminated appendant dendrons is reported. Comparison of the 1H and 13C NMR spectra of the resulting G2 and G3 polystyrenes (15, 17a) with those recorded of the same polymers independently prepared from the corresponding G2 and G3 macromonomers 16 and 12, respectively, proved the degree of coverage to be virtually 100%. This is the first example of a convergent synthesis of a G3 polymer, which will considerably widen the synthetic access into this class of macromolecules.