Polycationic (mixed) core-shell dendrimers for binding and delivery of inorganic/organic substrates.

The convergent synthesis of a series of polycationic aryl ether dendrimers has been accomplished by a convenient procedure involving quantitative quaternarization of aryl(poly)amine core molecules. The series has been expanded to the preparation of the first polycationic, mixed core-shell dendrimer. All these dendrimers consist of an apolar core with a peripheral ionic layer which is surrounded by a less polar layer of dendritic wedges. These cationic, macromolecular species have been investigated for their ability to form assemblies with (anionic) guest molecules. The results obtained from UV/Vis and NMR spectroscopies, and MALDI-TOF-MS demonstrate that all the cationic sites throughout the dendrimer core are involved in ion pair formation with anionic guests giving predefined guest/host ratios up to 24. The large NMR spectroscopic shifts of resonances correlated with the groupings located in the core of the dendrimers, together with the relaxation time data indicate that the anionic guests are associated with the cationic core of these dendrimers. The X-ray molecular structure of the octacationic, tetra-arylsilane model derivative [Si(C6H3[CH2NMe3](2)-3,5)4]8+. 8I- shows that the iodide counterions are primarily located near the polycationic sphere. The new polycationic dendrimers have been investigated for their catalytic phase-transfer behavior and substrate delivery over a nanofiltration membrane.

Hinge-helix formation and DNA bending in various lac repressor-operator complexes.

The hinge-region of the lac repressor plays an important role in the models for induction and DNA looping in the lac operon. When lac repressor is bound to a tight-binding symmetric operator, this region forms an alpha-helix that induces bending of the operator. The presence of the hinge-helices is questioned by previous data that suggest that the repressor does not bend the wild-type operator. We show that in the wild-type complex the hinge-helices are formed and the DNA is bent, similar to the symmetric complex. Furthermore, our data show differences in the binding of the DNA binding domains to the half-sites of the wild-type operator and reveal the role of the central base-pair of the wild-type operator in the repressor-operator interaction. The differences in binding to the operator half-sites are incorporated into a model that explains the relative affinities of the repressor for various lac operator sequences that contain left and right half-sites with different spacer lengths.