Metallo-Supramolecular Cylinders Induce Intramolecular DNA Coiling
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porous, size-selective, zeolite particles to polymer matrices. However, the reliability of such membranes was compromised by poor polymer/zeolite adhesion and inadequate particle dispersion. In this research study, nonporous, fumed silica particles were added to disrupt the molecular bonding in a rigid glassy polymer. Fine silica particles subtly increased the size of the free-volume voids in the polymer matrix, as revealed by positron annihilation lifetime spectroscopy (PALS). Since it is through a network of these larger voids that the penetrants are assumed to diffuse, slightly increasing their size enhanced the membrane’s reverse-selectivity by further weakening the size-sieving character of the PMP and improved its permeability by enabling faster diffusion. Critical to this method, said the researchers, was the addition of particles comparable in size to the polymer molecule; adding an equivalent volume of particles larger than 50 nm in diameter did not improve permeability since it did not uniformly alter the polymer chain packing. Transmission electron microscopy imaging of the membranes revealed well-distributed, discrete silica particles and silica aggregates within the PMP matrix, consistent with the proposed mode of enhanced transport and selectivity by molecular-scale alteration of the polymer structure. JULIE NUCCI
Metallo-Supramolecular Cylinders Induce Intramolecular DNA Coiling Proteins regulate the structure and function of life encoded in DNA with their ability to recognize the genetic code in a sequence-selective manner and induce structural changes in DNA. The ability to turn processing of the genetic code on or off requires a variety of molecular tools. Advancing toward this goal, researchers from the University of Warwick in Coventry, England; Universitat de Barcelona, Spain; and the University of Bergen, Norway have employed synthetic supramolecular assemblies to selectively bind to the major groove of DNA, causing the DNA to coil significantly. (DNA’s surface is not a smooth cylinder but has two differently sized grooves.) Enantiomers are molecules that are nonsuperimposable mirror images of each other. In enantio-selective binding, one enantiomer binds but not the other. As reported in the April 16 issue of the Proceedings of the National Academy of Sciences, teams led by University of Warwick researchers A. Rodger and M.J. Hannon employed a variety of experimental techniques to explore the 492
enantio-selectivity of the binding of [Fe2 (C25H20N4)3]Cl4—a cylindrical tetra cationic dimetallo triple helicate—to DNA. Rodger and Hannon previously showed that a racemic mixture (i.e., contains equal amounts of both enantiomers) of [Fe2L3]Cl4 induced a dramatic intramolecular bending, but the resulting DNA coils contained only the M enantiomer. Small synthetic molecules designed to achieve DNA sequence selectivity typi-
cally bind to the minor groove of DNA. Recognition is generally limited to 2–3 base pairs due to size limitations. Sequence recognition within the major groove may prove more fr
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