Aggregation modes in sheets formed by protected beta-amino acids and beta-peptides

The crystal structures of four protected beta-amino acid residues, Boc-(S)-beta3-HAla-NHMe (1); Boc-(R)-beta3-HVal-NHMe (2); Boc-(S)-beta3-HPhe-NHMe (3); Boc-(S)-beta3-HPro-OH (6) and two beta-dipeptides, Boc-(R)-beta3-HVal-(R)-beta3-HVal-OMe (4); Boc-(R)-beta3-HVal-(S)-beta3-HVal-OMe (5) have been determined. Gauche conformations about the C(beta)-C(alpha) bonds (theta approximately +/-60 degrees) are observed for the beta3-HPhe residues in and all four beta3-HVal residues in the dipeptides and . Trans conformations (theta is approximately 180 degrees) are observed for beta3-HAla residues in both independent molecules in and for the beta3-HVal and beta3-HPro residues in and , respectively. In the cases of compounds , molecules associate in the crystals via intermolecular backbone hydrogen bonds leading to the formation of sheets. The polar strands formed by beta3-residues aggregate in both parallel (1,3,5) and antiparallel (2,4 fashion. Sheet formation accommodates both the trans and gauche conformations about the C(beta)-C(alpha) bonds.     

Self-assembling dipeptide-based nontoxic vesicles as carriers for drugs and other biologically important molecules

Self-assembling short peptides can offer an opportunity to make useful nano-/microstructures that find potential application in drug delivery. We report here the formation of multivesicular structures from self-assembling water-soluble synthetic amphiphilic dipeptides containing a glutamic acid residue at the C-terminus. These vesicular structures are stable over a wide range of pH (pH 2-12). However, they are sensitive towards calcium ions. This causes the rupturing of these vesicles. Interestingly, these vesicles can not only encapsulate an anticancer drug and a fluorescent dye, but also can release them in the presence of calcium ions. Moreover, these multivesicular structures have the potential to carry biologically important molecules like cyclic adenosine monophosphate (cAMP) within the cells keeping their biological functions intact. A MTT cell-survival assay suggests the almost nontoxic nature of these vesicles. Thus, these peptide vesicles can be used as biocompatible delivery vehicles for carrying drugs and other bioactive molecules.     

Kinetics and mechanism of O-O bond cleavage in the reaction of [RuIII(edta)(H2O)]- with hydroperoxides in aqueous solution

The reactions of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ((t)BuOOH) and potassium hydrogenpersulfate (KHSO(5)) were studied kinetically as a function of oxidant concentration and temperature (10-30 degrees C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOR)](2-) intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru(V)=O](-). Since [(edta)Ru(V)=O](-) was produced almost quantitatively in the reaction of 1 with the hydroperoxides (t)BuOOH and KHSO(5), the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] > [hydroperoxide] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS (*+). The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru(II)(edta)(H(2)O)](-) with H(2)O(2).     

A generalized supramolecular strategy for self-sorted assembly between donor and acceptor gelators

An effective supramolecular strategy for self-sorting between naphthalene-diimide (NDI) acceptor and dialkoxy-naphthalene (DAN) donor organogelators is reported. The concept is based on mismatch in the placement of the two amide functionalities in the donor and acceptor chromophores so that self-sorting ensured maximum effect of H-bonding.     

Cascade Energy Transfer and White-Light Emission in Chirality-Controlled Crystallization-Driven Two-Dimensional Co-assemblies from Donor and Acceptor Dye-Conjugated Polylactides

Achieving predictable and programmable two-dimensional (2D) structures with specific functions from exclusively organic soft materials remains a scientific challenge. This article unravels stereocomplex crystallization-driven self-assembly as a facile method for producing thermally robust discrete 2D-platelets of diamond shape from biodegradable semicrystalline polylactide (PLA) scaffolds. The method involves co-assembling two PLA stereoisomers, namely, PY-PDLA and NMI-PLLA , which form stereocomplex (SC)-crystals in isopropanol. By conjugating a well-known Förster resonance energy transfer (FRET) donor and acceptor dye, namely, pyrene ( PY ) and naphthalene monoimide ( NMI ), respectively, to the chain termini of these two interacting stereoisomers, a thermally robust FRET process can be stimulated from the 2D array of the co-assembled dyes on the thermally resilient SC-PLA crystal surfaces. Uniquely, by decorating the surface of the SC-PLA crystals with an externally immobilized guest dye, Rhodamine-B, similar diamond-shaped structures could be produced that exhibit pure white-light emission through a surface-induced two-step cascade energy transfer process. The FRET response in these systems displays remarkable dependence on the intrinsic crystalline packing, which could be modulated by the chirality of the co-assembling PLA chains. This is supported by comparing the properties of similar 2D platelets generated from two homochiral PLLAs ( PY-PLLA and NMI-PLLA ) labeled with the same FRET pair.     

Stimuli‐Responsive Self‐Assembly of a Naphthalene Diimide by Orthogonal Hydrogen Bonding and Its Coassembly with a Pyrene Derivative by a Pseudo‐Intramolecular Charge‐Transfer Interaction

A naphthalene diimide (NDI) building block containing hydrazide (H1) and hydroxy (H2) groups self‐assembled into a reverse‐vesicular structure in methylcyclohexane by orthogonal H‐bonding and π‐stacking. At an elevated temperature (LCST=43 °C), destruction of the assembled structure owing to selective dissociation of H2–H2 H bonding led to macroscopic precipitation. Further heating resulted in homogeneous redispersion of the sample at 70 °C (UCST) and the formation of a reverse‐micellar structure. In the presence of a pyridine (H3)‐functionalized pyrene (PY) donor, a supramolecular dyad (NDI–PY) was formed by H2–H3 H‐bonding. Slow transformation into an alternate NDI–PY stack occurred by a folding process due to the charge‐transfer interaction between NDI and PY. The mixed NDI–PY assembly exhibited a morphology transition from a reverse micelle (with a NDI–PY mixed‐stack core) below the LCST to another reverse micelle (with a NDI core) above the UCST via a “denatured” intermediate.     

Growth and Approximation of Generalized Bi-Axially Symmetric Potentials

The paper deals with growth estimates and approximation (not necessarily entire) of solutions of certain elliptic partial differential equations. These solutions are called generalized bi-axially symmetric potentials (GBASP's). To obtain more refined measure of growth, we have defined $q$-proximate order and obtained the characterization of generalized $q$-type and generalized lower $q$-type with respect to $q$-proximate order of a GBASP in terms of approximation errors and ratio of these errors in sup norm.     

Metal Bromide Controlled Interfacial Aromatization Reaction for Shape‐Selective Synthesis of Palladium Nanostructures with Efficient Catalytic Performances

Herein, the effect of diverse metal bromides for the shape evolution of palladium nanostructures (Pd NS) has been demonstrated. Aromaticity‐driven reduction of bromopalladate(II) is optimized to reproducibly obtain different Pd NS at the water/organic layer interface. In this soft interfacial strategy, a redox potential driven reaction has been performed, forming the thermodynamically more stable (>10⁴‐fold) PdBr₄^2? precursor from PdCl₄^2? by adding extra metal bromides. In the process, the reductant, Hantzsch dihydropyridine ester (DHPE), is aromatized. Interestingly, alkali metal bromides devoid of coordination propensity exclusively evolve Pd nanowires (Pd NWs), whereas in the case of transition metal bromides the metal ions engage the ‘N’ donor of DHPE at the interface, making the redox reaction sluggish. Hence, controlled Pd nanoparticles growth is observed, which evolves Pd broccolis (Pd NBRs) and Pd nanorods (Pd NRs) at the interface in the presence of NiBr₂ and CuBr₂, respectively, in the aqueous solution. Thus, the effect of diverse metal bromides in the reaction mixture for tailor‐made growth of the various Pd NS is reported. Among the as‐synthesized materials, the Pd NWs stand to be superior catalysts and their efficiency is almost 6 and 2.5 times higher than commercial 20 % Pd/C in the electrooxidation of ethanol and Cr^VI reduction reaction by formic acid, respectively.