Template-directed synthesis of kinetically and thermodynamically stable molecular necklace using ring closing metathesis

We report the template-directed synthesis of a well-defined, kinetically stable [5]molecular necklace with dialkylammonium ion (R(2)NH(2)(+)) as recognition site and DB24C8 as macrocycle. A thread containing four dialkylammonium ions with olefin at both ends was first synthesized and then subjected to threading with an excess amount of DB24C8 to form pseudo[5]rotaxane, which in situ undergoes ring closing metathesis at the termini with second generation Grubbs catalyst to yield the desired [5]molecular necklace. The successful synthesis of [5]molecular necklace is mainly attributed to the self-assembly and dynamic covalent chemistry which allows the formation of thermodynamically most stable product. The self-assembly of the DB24C8 ring onto the recognition site known as templating effect was driven by noncovalent stabilizing interactions like [N(+)-H···O], [C-H···O] hydrogen bonds as well as [π···π] interactions which is facilitated in non-polar solvents. The reversible nature of olefin metathesis reaction makes it suitable for dynamic covalent chemistry since proof-reading and error-checking operates until it generates thermodynamically the most stable interlocked molecule. Riding on the success of [5]molecular necklace, we went a step further and attempted to synthesize [7]molecular necklace using the same protocol. This led to the synthesis of another thread with olefin at both ends but having six dibenzylammonium ions along the thread. However, the extremely poor solubility of this thread containing six secondary ammonium ions limits the self-assembly process even after we replaced the typical PF(6)(-) counter anion with a more lipophilic BPh(4)(-) anion. Although the poor solubility of the thread remains the bottleneck for making higher order molecular necklaces yet this approach of "threading-followed-by-ring-closing-metathesis" for the first time produces kinetically and thermodynamically stable, well-defined, homogeneous molecular necklace which was well characterized by one-dimensional, two-dimensional, variable temperature proton NMR spectroscopy and ESI mass spectroscopy.     

Covalent immobilization of active lysozyme on Si/glass surface using alkoxy Fischer carbene complex on SAM

A cross-metathesis reaction between an alkene terminated self-assembled monolayer (SAM) on glass/Si wafer and an alkene tethered Fischer carbene complex yielded a functionalized surface. Rapid aminolysis of the Fischer carbene moieties permit efficient anchoring of amine containing molecules on such a surface. Attachment of 1-pyrenemethylamine was thus monitored by ATR-IR spectroscopy and fluorescence microscopy. Similarly, BSA and lysozyme were individually grafted to such Fischer carbene modified surfaces using their pendant lysine residues. It has been demonstrated that the anchored lysozyme retains its bactericidal property.     

Synthesis,characterization, and properties of semifluorinated organo‐soluble new aromatic polyamides

A series of organo‐soluble new polyamides were synthesized by the direct polycondensation of different semifluorinated aromatic diamines, namely 4,4‐bis[3'‐trifluoromethyl‐4'(4“‐amino benzoxy)benzyl]biphenyl; 4,4”‐bis(aminophenoxy)‐3'3“‐trifluoromethyl terphenyl; 1,3‐bis[3'‐trifluoromethyl‐4'(4”‐amino benzoxy)benzyl]benzene; 2,6‐bis(3'‐trifluoromethyl‐p‐aminobiphenyl ether)pyridine; and 2,5‐bis(3'‐trifluoromethyl‐p‐aminobiphenyl ether)thiophene with 5‐t‐butyl‐isophthalic acid. The polymers were fully characterized by elemental analysis and IR, NMR spectroscopies. The synthesized polyamides were soluble in several organic solvents such as 1‐methyl‐2‐pyrrolidone, N,N‐dimethylformamide, N,N‐dimethylacetamide, tetrahydrofuran, and dimethyl sulfoxide at room temperature. They showed inherent viscosities of 0.42–0.63 dl/g. The polyamides exhibited weight‐average molecular weights of up to 233,000, which depended on the exact repeating unit structure. The polyamides synthesized from 4,4‐bis[3'‐trifluoromethyl‐4'(4”‐amino benzoxy)benzyl]biphenyl and 5‐t‐butyl isophthalic acid exhibited highest glass‐transition temperatures 261°C (evaluated by differential scanning calorimetry) in nitrogen. These polyamides showed good thermal stability up to 475°C for a 10% weight loss in air. The polyamides films were clear and flexible in nature with tensile strengths of up to 88 MPa, modulus of elasticity of up to 1.81 GPa, and elongations at break of up to 25%, which depended on the exact repeating unit structure. X‐ray diffraction measurements indicated that these polyamides were amorphous in nature. Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanistic studies of the selective reduction of ruthenium(III) containing trinuclear oxo complexes by l-ascorbic acid in aqueous solution

The reduction of the Ru(III) oxo-centred trinuclear acetate cations, [Ru₃(??₃-O)(??₂-CH₃CO₂)₆(H₂O)₃]⁺ and [Ru₂Cr(??₃-O)(??₂-CH₃CO₂)₆(H₂O)₃]⁺, by the biological reductant l-ascorbic acid was studied spectrophotometrically under pseudo first-order conditions over the ranges 3.05????pH????4.83 (acetate buffer), 15?°C??????????30?°C and at I?=?0.5?mol?dm^?3 (NaClO₄). The first electron transfer in the redox process resulted in mixed-valence species [Ru₂M(??₃-O)(??₂-CH₃CO₂)₆(H₂O)₃]⁰, where M?=?Ru or Cr, followed by the slow consecutive reduction of other Ru(III) ions. The kinetics of the formation of the mixed-valence species was studied in detail, and a mechanism in support of these data is proposed. The intricate mechanistic details of the subsequent reactions are unclear as the spectral characteristics of the species involved could not be resolved from those of the first intermediate. The final products, however, were found to be Ru(II) (and Cr(III) for the mixed-metal species) in acetate buffer. The electron-transfer mechanism has been proposed to be inner-sphere, as deduced from Marcus cross-relationship. In an aqueous acetate buffer at I?=?1.0?mol?dm^?3 (NaClO₄), the cyclic voltammograms of the complexes were found to be quasi-reversible and pH dependent and have values of 0.18 and 0.19?V (relative to SHE) at pH?=?3.41 for the [Ru₃(??₃-O)(??₂-CH₃CO₂)₆(H₂O)₃]⁺ and [Ru₂Cr(??₃-O)(??₂-CH₃CO₂)₆(H₂O)₃]⁺ cations, respectively.     

Surface‐enhanced Raman measurements and DFT calculations for l‐tryptophan of varying pH in silver sol

The interaction of l ‐tryptophan (Trp) with silver colloids was investigated at between pH values of 6.11 and 10.6 of the sol. The measurements of surface‐enhanced Raman bands of Trp in the colloidal solution indicate the evolution of interaction between the metal particles and the molecules with increasing pH values for the sol. The experimental observations were explained using the estimated atomic charge distribution in the zwitterionic and anionic forms of the residue, obtained by density functional theory calculations. The variation in the ratio of the spectral intensities of the Fermi resonance bands with the pH reflects the effect of the colloidal environment on Trp. The results obtained can be used as a marker for describing the nature of the interaction of silver colloids with the specific terminus of the residue, at varying pH environments. Copyright © 2012 John Wiley & Sons, Ltd.     

An alternate mode of binding of the polyphenol quercetin with serum albumins when complexed with Cu(II)

Polyphenols find wide use as antioxidants, cancer chemopreventive agents and metal chelators. The latter activity has proved interesting in many aspects. We have probed the binding characteristics of the polyphenol quercetin–Cu(II) complex with human serum albumin (HSA) and bovine serum albumin (BSA). Fluorescence studies reveal that the quercetin–Cu(II) complex can quench the fluorescence of the serum albumins. The binding constant (K_b) values are of the order of 10⁵ M^?1 which increased with rise in temperature in case of HSA and BSA interacting with the quercetin–Cu(II) complex. Displacement studies reveal that both the ligands bind to site 1 (subdomain IIA) of the serum albumins. However, thermodynamic parameters calculated from temperature dependent studies indicated that the mode of interaction of the complexes with the proteins differs. Both ΔH° and ΔS° were positive for the interaction of the quercetin–Cu(II) complex with both proteins but the value of ΔH° was negative in case of the interaction of quercetin with the proteins. This implies that after chelation with metal ions, the polyphenol alters its mode of interaction which could have varying implications on its other physicochemical activities.     

Universality of the plastic instability in strained amorphous solids

By comparing the response to external strains in metallic glasses and in Lennard-Jones glasses we find a quantitative universality of the fundamental plastic instabilities in the athermal, quasistatic limit. Microscopically these two types of glasses are as different as one can imagine, the latter being determined by binary interactions, whereas the former is determined by multiple interactions due to the effect of the electron gas that cannot be disregarded. In spite of this enormous difference the plastic instability is the same saddle-node bifurcation. As a result, the statistics of stress and energy drops in the elastoplastic steady state are universal, sharing the same system-size exponents.     

Microfluidic sorting with blinking optical traps

It is shown that by appropriately choosing the periodicity of a blinking optical trap only larger sized colloidal spheres can be selectively trapped out of a mixed population. This happens because smaller sized, more agile, spheres escape out of the trap volume during the off period of the trap beam. Therefore, by scanning an array of blinking traps over a mixed sample, bigger spheres can be forced to move with the traps and eventually could be taken to the output side. Experimental demonstration of sorting between 1 μm and 2 μm diameter silica spheres is presented.     

Projectile Impact on a Thin. Flexible Structure: A Singular Dynamic Contact Phenomenon

ABSTRACT

In this paper we demonstrate an unexpected type of dynamic contact phenomenon which arises when a large object impacts a thin flexible structure. It is shown that the contact pressure distribution is singular at the edges of the contact region when the region is expanding with a speed which is higher than the wave speed in the structure. This type of phenomenon will occur in a number of structural collision problems in which one of the objects such as a cable or a thin plate is much more flexible than the other. Head injury problems represent a typical example

The solution for a very simple model of such a system, namely that of a rigid cylindrical impaclor striking an infinitely long string, is given in detail to illustrate the phenomenon.     

Quark dynamics, thermal QCD, and the gravity dual

We perform a holographic renormalization of the supergravity action and compute the stress tensor of the dual gauge theory incorporating the logarithmic running of the gauge coupling. From the stress tensor we obtain the shear viscosity and the entropy of the medium at temperature T, and investigate the ratio η/s.