Mirror image nanostructures

Chiral molecules that self-assemble to form chiral supramolecular structures exhibit interesting structural features reminiscent of tertiary and quaternary structures of proteins and have applications in catalysis and nonlinear optics. Often, these structures are hierarchical, with their chiral structure difficult to interpret on the molecular scale. In this communication, we observe chiral assembling molecules that form well-defined helices with a pitch of 28 nm. We observe the behavior in both R- and S-enantiomers of the molecule, forming mirror image nanostructures. The molecular chirality is determined by the dimethyloctyl alkyl coil of the molecule and is located more than 4 nm from the hydrogen-bonding segment. The nanostructures observed are not hierarchical, which could be a result of the significant separation between the stereocenter and hydrogen-bonding dendron. The subtle structural modification at the periphery of the molecule biases the supramolecular assembly, which is driven primarily by strong hydrogen-bonding and pi-pi stacking interactions.     

Labeling of Polymer Nanostructures for Medical Imaging: Importance of crosslinking extent, spacer length, and charge density

Radiolabeling studies were employed to investigate the influence of structure on the efficiency of surface functionalization for poly(acrylic acid)-coated shell crosslinked nanoparticles (SCKs) with two types of amine-terminated DOTA chelators. An intricate interplay between the chemical and physical properties of both the DOTA derivative and the SCK nanostructures was revealed, demonstrating the importance of structural control.     

Pentavalent Organo-Vanadates as Transition State Analogues for Phosphoryl Transfer Reactions

Pentavalent organo-vanadates have been put forth as transition state analogues for a variety of phosphoryl transfer reactions. In particular, uridine 2',3'-cyclic vanadate (U>v) has been proposed to resemble the transition state during catalysis by ribonuclease A (RNase A). Here, this hypothesis is tested. Lys41 of RNase A is known to donate a hydrogen bond to a nonbridging phosphoryl oxygen in the transition state during catalysis. Site-directed mutagenesis and semisynthesis were used to create enzymes with natural and nonnatural amino acid residues at position 41. These variants differ by 10(5)-fold in their k(cat)/K(m) values for catalysis, but <40-fold in their K(i) values for inhibition of catalysis by U>v. Plots of logK(i) vs log(K(m)/k(cat)) for three distinct substrates [poly(cytidylic acid), uridine 3'-(p-nitrophenyl phosphate), and cytidine 2',3'-cyclic phosphate] have slopes that range from 0.25 and 0.36. These plots would have a slope of unity if U>v were a perfect transition state analogue. Values of K(i) for U>v correlate weakly with the equilibrium dissociation constant for the enzymic complexes with substrate or product, indicating that U>v bears some resemblance to the substrate and product as well as the transition state. Thus, U>v is a transition state analogue for RNase A, but only a marginal one. This finding indicates that a pentavalent organo-vanadate cannot necessarily be the basis for a rigorous analysis of the transition state for a phosphoryl transfer reaction.     

A templating approach for monodisperse self-assembled organic nanostructures

The precise structural control is known for self-assembly into closed spherical structures (e.g., micelles), but similar control of open structures is much more challenging. Inspired by natural tobacco mosaic virus, we present the use of a rigid-rod template to control the size of a one-dimensional self-assembly. We believe that this strategy is novel for organic self-assembly and should provide a general approach to controlling size and dimension.     

Synthesis, self-assembly, and characterization of supramolecular polymers from electroactive dendron rodcoil molecules

We report here the synthesis and self-assembly of a series of three molecules with dendron rodcoil architecture that contain conjugated segments of oligo(thiophene), oligo(phenylene-vinylene), and oligo(phenylene). Despite their structural differences, all three molecules yield similar self-assembled structures. Electron and atomic force microscopy reveals the self-assembly of the molecules into high aspect ratio ribbon-like nanostructures which at low concentrations induce gelation in nonpolar solvent. Self-assembly results in a blue-shifted absorption spectrum and a red-shifted, quenched fluorescence spectrum, indicating aggregation of the conjugated segments within the ribbon-like structures. The assembly of these molecules into one-dimensional nanostructures is a route to pi-pi stacked supramolecular polymers for organic electronic functions. In the oligo(thiophene) derivative, self-assembly leads to a 3 orders of magnitude increase in the conductivity of iodine-doped films due to self-assembly. We also found that electric field alignment of these supramolecular assemblies can be used to create arrays of self-assembled nanowires on a device substrate.