Shear-driven solidification of dilute colloidal suspensions

We show that shear-induced solidification of dilute charge-stabilized colloids is due to the interplay between shear-induced formation and breakage of large non-Brownian clusters. While their size is limited by breakage, their number density increases with shearing time. Upon flow cessation, the dense packing of clusters interconnects into a rigid state by means of grainy bonds, each involving a large number of primary colloidal bonds. The emerging picture of shear-driven solidification in dilute colloidal suspensions combines the gelation of Brownian systems with the jamming of athermal systems.     

Crystallization mechanism of hard sphere glasses

In supercooled liquids, vitrification generally suppresses crystallization. Yet some glasses can still crystallize despite the arrest of diffusive motion. This ill-understood process may limit the stability of glasses, but its microscopic mechanism is not yet known. Here we present extensive computer simulations addressing the crystallization of monodisperse hard-sphere glasses at constant volume (as in a colloid experiment). Multiple crystalline patches appear without particles having to diffuse more than one diameter. As these patches grow, the mobility in neighboring areas is enhanced, creating dynamic heterogeneity with positive feedback. The future crystallization pattern cannot be predicted from the coordinates alone: Crystallization proceeds by a sequence of stochastic micronucleation events, correlated in space by emergent dynamic heterogeneity.     

Gas-phase reactions of the bare Th2+ and U2+ ions with small alkanes, CH4, C2H6, and C3H8: experimental and theoretical study of elementary organoactinide chemistry

The gas-phase reactions of two dipositive actinide ions, Th(2+) and U(2+), with CH(4), C(2)H(6), and C(3)H(8) were studied by both experiment and theory. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the bimolecular ion-molecule reactions; the potential energy profiles (PEPs) for the reactions, both observed and nonobserved, were computed by density functional theory (DFT). The experiments revealed that Th(2+) reacts with all three alkanes, including CH(4) to produce ThCH(2)(2+), whereas U(2+) reacts with C(2)H(6) and C(3)H(8), with different product distributions than for Th(2+). The comparative reactivities of Th(2+) and U(2+) toward CH(4) are well explained by the computed PEPs. The PEPs for the reactions with C(2)H(6) effectively rationalize the observed reaction products, ThC(2)H(2)(2+) and UC(2)H(4)(2+). For C(3)H(8) several reaction products were experimentally observed; these and additional potential reaction pathways were computed. The DFT results for the reactions with C(3)H(8) are consistent with the observed reactions and the different products observed for Th(2+) and U(2+); however, several exothermic products which emerge from energetically favorable PEPs were not experimentally observed. The comparison between experiment and theory reveals that DFT can effectively exclude unfavorable reaction pathways, due to energetic barriers and/or endothermic products, and can predict energetic differences in similar reaction pathways for different ions. However, and not surprisingly, a simple evaluation of the PEP features is insufficient to reliably exclude energetically favorable pathways. The computed PEPs, which all proceed by insertion, were used to evaluate the relationship between the energetics of the bare Th(2+) and U(2+) ions and the energies for C-H and C-C activation. It was found that the computed energetics for insertion are entirely consistent with the empirical model which relates insertion efficiency to the energy needed to promote the An(2+) ion from its ground state to a prepared divalent state with two non-5f valence electrons (6d(2)) suitable for bond formation in C-An(2+)-H and C-An(2+)-C activated intermediates.     

Phosphathiahelicenes: Synthesis and Uses in Enantioselective Gold Catalysis

Enantiomerically pure thiahelicenes displaying a terminal phosphole unit and a stereogenic phosphorus center have been prepared by oxidative photocyclization of a diaryl‐olefin precursor. Starting from one of these phosphathiahelicene oxides, the corresponding trivalent phosphine–Au^I complex is obtained with complete diastereoselectivity. It affords a new, excellent precatalyst for the enantioselective cycloisomerization of N‐tethered enynes (up to 96 % ee).     

Petrorhagiosides A – D,New γ‐Pyrone Derivatives from Petrorhagia saxifraga Link

Four novel γ‐pyrone (=4H‐pyran‐4‐one) metabolites, petrorhagiosides A–D, along with four known analogs, have been isolated from the MeOH extract of Petrorhagia saxifraga, a perennial herbaceous plant typical of Mediterranean vegetation. The structures of the new compounds were established on the basis of extensive spectroscopic analyses including 1D‐ an 2D‐NMR (¹H,¹H‐DQ‐COSY, TOCSY, HSQC, CIGAR‐HMBC, and HSQC‐TOCSY) experiments.     

Synthesis,characterization, and nucleotidic chain cleavage ability of uncharged water soluble poly(ethylene glycol)‐fullerene derivatives with an amphiphilic character

Water‐soluble fullerenes containing two poly(ethylene glycol) branches [Full‐(PEG)₂] were prepared starting from commercial poly(ethylene glycol)‐monomethyl ethers and C₆₀ [Full‐(PEG)₂]s chemical characterization was made by FT‐IR, NMR, and MALDI‐TOF mass spectrometric analyses. Their thermal stability was determined by TGA experiments. The capability of C₆₀‐derivatives to induce oligonucleotide cleavage under visible light irradiation was also ascertained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2154–2153, 2008     

Acid promoted cinnamyl ion mobility within peptide derived macrocycles

We are developing methods that restrict the conformational mobility of peptides and related heteropolymers while simultaneously altering their properties. Our experiments occur as processes wherein a conserved, lipophilic reagent is activated in stages to form composite products with unprotected polyamides in parallel. For each starting oligomer, the goal is to create not one, but rather a collection of products. The intent is for those materials to retain molecular recognition elements of the biopolymer, yet display that functionality as part of stable, cyclic structures having defined shapes and enhanced membrane solubility/permeability. Here we describe reagent 2 and its two-step integration into peptides to afford macrocyclic ethers (e.g., 4 when starting with W-W-Y). When those materials are treated with protic acid in anhydrous solvent, the cinnamyl unit migrates from the oxygen of tyrosine to distribute throughout the structure, forming new products via carbon/carbon bonding. These changes occur concomitantly with acid-promoted rearrangements/cyclizations of the dienyne appendage to generate mixtures containing unique macrocycles such as 15. Similar amalgamations of 2 with more diverse peptides is a means to begin accessing complex peptidomimetics systematically. From a library of screening fractions generated in this way, we have identified a small molecule that selectively promotes hippocampal neurogenesis in the adult mouse brain.     

A concise entry into nonsymmetrical alkyl polyamines

The synthesis of nonsymmetrical polyamines (PAs) has, up to now, been problematic due to lengthy synthetic procedures, lack of regioselectivity, and very poor atom economy. An innovative synthetic protocol for nonsymmetrical PAs using a modified Ugi reaction ( N-split Ugi) which simplifies the synthesis of these tricky compounds is described. We believe that this new synthesis may open the door for the generation of new and pharmacologically active PAs.     

Expanding the registry of aromatic amide foldamers: folding, photochemistry and assembly using diaza-anthracene units

The synthesis of various 1,8-diaza-4,5-dialkoxy-2,7-anthracene dicarboxylic acid derivatives and their incorporation into cyclic and helically folded aromatic oligoamides are reported. The ability of the diaza-anthracene monomers to undergo photoaddition or head-to-tail photodimerization was investigated in the solid state and in solution. Quantitative conversion of a monomer diester to the corresponding head-to-tail photodimer could be achieved in the solid state without protection from oxygen. The formation of an emissive excimer between two diaza-anthracene units appended at the end of a helically folded oligomer was demonstrated. Intramolecular photodimerization was not observed in this compound, possibly due to the low thermal stability of the head-to-head photoadduct. A cyclic oligoamide composed of two diaza-anthracene and two pyridine units was shown to adopt a flat conformation and to form columnar stacks in the solid state. Longer, noncyclic oligoamides composed of one or two diaza-anthracene units were shown to adopt helical conformations that exist preferentially as double helical dimers.