Imparting nanoparticle function with size-controlled amphiphilic polymers

Herein we report our method of water solubilization and subsequent functionalization of a variety of nanoparticle systems with amphiphilic polymers containing build-in "chemical handles". We have used these polymers, which have narrow polydispersity indices, to impart water solubility and chemical sensitivity toward targeted species (here: pH). These material systems have high chemical conjugation efficiencies in aqueous conditions which may be used to create a variety of chemical and biological multifunctional materials.     

Quasi-static response and texture evolution of α- and γ-RDX: a comparative study

In this paper, we undertake a comparative study of the stress–strain response and slip activity of α- and γ-polymorph of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) under pressure loading using a rate-dependent single-crystal plasticity model. Texture evolution studies are performed to further understand the effects of the dominant slip systems. The simulations indicate that the difference in elastic moduli and lattice parameters for α- and γ-RDX lead to different elastic–plastic constitutive response in the two polymorphs. γ-RDX exhibits more plastic slip compared to α-RDX for loading on (1 1 1) plane and the two polymorphs have different sets of dominant slip systems. We observe that the high-pressure slip system (0 0 1)[0 1 0] that is determined using molecular dynamics simulations is the most dominant slip system for this orientation. Whereas, for loading on (2 1 0) plane, α-RDX has marginally higher plastic slip than γ-RDX, though the same slip system is dominant for both the polymorphs. The texture evolution for loading on (1 1 1) and (2 1 0) planes follow the path towards the most dominant slip systems for both the polymorphs. We predict that the larger plastic slip in γ-RDX for loading on (1 1 1) plane might play an important role in understanding the reduced sensitivity for shock loading on (1 1 1) plane, when compared to (2 1 0) for which γ-RDX has lesser plastic slip, and (1 0 0) which is purely elastic.     

Two [Fe(IV)=O Trp*] intermediates in M. tuberculosis catalase-peroxidase discriminated by multifrequency (9-285 GHz) EPR spectroscopy: reactivity toward isoniazid

We have characterized the intermediates formed in the peroxidase cycle of the multifunctional heme-containing enzyme KatG of M. tuberculosis. Selected Trp variants from the heme proximal (W321F) and distal (W107F and W91F) sides were analyzed together with the wild-type enzyme with regard to the reaction with peroxyacetic acid and hydrogen peroxide (in the catalase-inactive W107F). The 9 GHz EPR spectrum of the enzyme upon reaction with peroxyacetic acid showed the contribution of three protein-based radical species, two Trp* and a Tyr*, which could be discerned using a combined approach of multifrequency Electron Paramagnetic Resonance (EPR) spectroscopy with selective deuterium labeling of tryptophan and tyrosine residues and site-directed mutagenesis. Trp321, a residue in H-bonding interactions with the iron through Asp381 and the heme axial ligand His270, was identified as one of the radical sites. The 9 GHz EPR signal of the Trp321 radical species was consistent with an exchange-coupled species similar to the oxoferryl-Trp radical intermediate in cytochrome c peroxidase. On the basis of the possibility of distinguishing among the different radical intermediates of the peroxidase cycle in M. tuberculosis KatG (MtKatG), we used EPR spectroscopy to monitor the reactivity of the enzyme and its W321F variant with isoniazid, the front-line drug used in the treatment of tuberculosis. The EPR experiments on the W321F variant preincubated with isoniazid allowed us to detect the short-lived [Fe(IV)=O Por*+] intermediate. Our results showed that neither the [Fe(IV)=O Por*+] nor the [Fe(IV)=O Trp321*+] intermediates were the reactive species with isoniazid. Accordingly, the subsequent intermediate (most probably the other Trp*) is proposed to be the oxidizing species. Our findings demonstrate that the protein-based radicals formed as alternative intermediates to the [Fe(IV)=O Por*+] can play the role of cofactors for substrate oxidation in the peroxidase cyle of KatGs.     

pH-driven conformational switch of "i-motif" DNA for the reversible assembly of gold nanoparticles

Herein we demonstrate that gold nanoparticles conjugated to "i-motif" DNA behave like a pH dependent switch that undergoes reversible aggregations which can be easily visualized by the naked eye.     

Synthesis of novel cone-configurated hexa-tert-butyl-trimethoxy-calix[6]arenes bearing tris(bipyridyl) pendants and their use in recognition and ionic speciation

A series of novel cone-configurated p-tert-butyl-trimethoxycalix[6]arenes bearing three 2,2′-bipyridyl units at their lower rim have been synthesized. The ¹H NMR and ¹³C NMR spectra of synthesized derivatives revealed that the ring inversion in calix[6]arene could be suppressed by the introduction of three 2,2′-bipyridyl moieties at the lower rim of calix[6]arene scaffold which fixes it into its cone configuration. The complexation ability of the synthesized receptors (5a–d) towards Fe(II) ion was investigated by UV-Visible titration to reveal that the synthesized receptors interact with Fe(II) in a 1:1 binding stoichiometry and respond to a specific oxidation state of the metal ion. The observations have significance for studies directed at the design of molecular receptors for ionic speciation through molecular recognition.     

Surface modification of silica nanoparticles to reduce aggregation and nonspecific binding

In this article, a systematic study of the design and development of surface-modification schemes for silica nanoparticles is presented. The nanoparticle surface design involves an optimum balance of the use of inert and active surface functional groups to achieve minimal nanoparticle aggregation and reduce nanoparticle nonspecific binding. Silica nanoparticles were prepared in a water-in-oil microemulsion and subsequently surface modified via cohydrolysis with tetraethyl orthosilicate (TEOS) and various organosilane reagents. Nanoparticles with different functional groups, including carboxylate, amine, amine/phosphonate, poly(ethylene glycol), octadecyl, and carboxylate/octadecyl groups, were produced. Aggregation studies using SEM, dynamic light scattering, and zeta potential analysis indicate that severe aggregation among amine-modified silica nanoparticles can be reduced by adding inert functional groups, such as methyl phosphonate, to the surface. To determine the effect of various surface-modification schemes on nanoparticle nonspecific binding, the interaction between functionalized silica nanoparticles and a DNA chip was also studied using confocal imaging/fluorescence microscopy. Dye-doped silica nanoparticles functionalized with octadecyl and carboxylate groups showed minimal nonspecific binding. Using these surface-modification schemes, fluorescent dye-doped silica nanoparticles can be more readily conjugated with biomolecules and used as highly fluorescent, sensitive, and reproducible labels in bioanalytical applications.     

Hydroxyapatite coating on stainless steel pre-coated with bovine serum albumin at ambient conditions

A biomimetic process for coating of nanosized hydroxyapatite on stainless steel, which capitalises the dual nature of the protein bovine serum albumin in both metal binding and a strong affinity for calcium ions, has been developed. The novelty of the process lies in pre-conditioning the metallic surface using the above protein prior to its mineralization with hydroxyapatite at ambient conditions. The microporous morphology of these coatings may provide favourable solubility and resorbability as desired by many orthopaedic and orthodontic applications.     

CFD–DEM study of effect of bed thickness for bubbling fluidized beds

The effect of bed thickness in rectangular fluidized beds is investigated through the CFD–DEM simulations of small-scale systems. Numerical results are compared for bubbling fluidized beds of various bed thicknesses with respect to particle packing, bed expansion, bubble behavior, solids velocities, and particle kinetic energy. Good two-dimensional (2D) flow behavior is observed in the bed having a thickness of up to 20 particle diameters. However, a strong three-dimensional (3D) flow behavior is observed in beds with a thickness of 40 particle diameters, indicating the transition from 2D flow to 3D flow within the range of 20–40 particle diameters. Comparison of velocity profiles near the walls and at the center of the bed shows significant impact of the front and back walls on the flow hydrodynamics of pseudo-2D fluidized beds. Hence, for quantitative comparison with experiments in pseudo-2D columns, the effect of walls has to be accounted for in numerical simulations.