Enhanced functionalization of Mn2O3@SiO2 core-shell nanostructures

Core-shell nanostructures of Mn2O3@SiO2, Mn2O3@amino-functionalized silica, Mn2O3@vinyl-functionalized silica, and Mn2O3@allyl-functionalized silica were synthesized using the hydrolysis of the respective organosilane precursor over Mn2O3 nanoparticles dispersed using colloidal solutions of Tergitol and cyclohexane. The synthetic methodology used is an improvement over the commonly used post-grafting or co-condensation method as it ensures a high density of functional groups over the core-shell nanostructures. The high density of functional groups can be useful in immobilization of biomolecules and drugs and thus can be used in targeted drug delivery. The high density of functional groups can be used for extraction of elements present in trace amounts. These functionalized core-shell nanostructures were characterized using TEM, IR, and zeta potential studies. The zeta potential study shows that the hydrolysis of organosilane to form the shell results in more number of functional groups on it as compared to the shell formed using post-grafting method. The amino-functionalized core-shell nanostructures were used for the immobilization of glucose and L-methionine and were characterized by zeta potential studies

Uptake of hydrophilic toxins in hollow silica shells obtained from core-shell nanostructures

Hollow shells of silica were obtained by the dissolution of the oxide core of Co3,O4@SiO2 core-shell nanostructures. These hollow shells were characterized using TEM, EDX and IR. TEM studies show that the size and shape of the hollow shells depend on the size and shape of the core-shell nanostructure from which they were obtained. These hollow shells have the potential to function as nanovessels which can entrap hydrophilic moieties (e.g. methyl orange) present in trace amount in the aqueous environment. From the spectrophotometric studies it was observed that up to 18% of the hydrophilic dye can be entrapped using these hollow shells

Quantum mechanical/molecular mechanical free energy simulations of the self-cleavage reaction in the hepatitis delta virus ribozyme

The hepatitis delta virus (HDV) ribozyme catalyzes a self-cleavage reaction using a combination of nucleobase and metal ion catalysis. Both divalent and monovalent ions can catalyze this reaction, although the rate is slower with monovalent ions alone. Herein, we use quantum mechanical/molecular mechanical (QM/MM) free energy simulations to investigate the mechanism of this ribozyme and to elucidate the roles of the catalytic metal ion. With Mg²⁺ at the catalytic site, the self-cleavage mechanism is observed to be concerted with a phosphorane-like transition state and a free energy barrier of ∼13 kcal/mol, consistent with free energy barrier values extrapolated from experimental studies. With Na⁺ at the catalytic site, the mechanism is observed to be sequential, passing through a phosphorane intermediate, with free energy barriers of 2–4 kcal/mol for both steps; moreover, proton transfer from the exocyclic amine of protonated C75 to the nonbridging oxygen of the scissile phosphate occurs to stabilize the phosphorane intermediate in the sequential mechanism. To explain the slower rate observed experimentally with monovalent ions, we hypothesize that the activation of the O2′ nucleophile by deprotonation and orientation is less favorable with Na⁺ ions than with Mg²⁺ ions. To explore this hypothesis, we experimentally measure the p<i>K</i>_a of O2′ by kinetic and NMR methods and find it to be lower in the presence of divalent ions rather than only monovalent ions. The combined theoretical and experimental results indicate that the catalytic Mg²⁺ ion may play three key roles: assisting in the activation of the O2′ nucleophile, acidifying the general acid C75, and stabilizing the nonbridging oxygen to prevent proton transfer to it

Controlling the size, morphology, and aspect ratio of nanostructures using reverse micelles: a case study of copper oxalate monohydrate

This study focuses on understanding the growth and control of nanostructures using reverse micelles. It has been earlier realized that parameters like surfactant, cosurfactant, and aqueous content influence the size and shape of the nanostructures obtained using reverse micelles. However, a concerted effort to understand the role of these factors on the growth of a specific nanomaterial is missing. In this study we have focused on one nanomaterial (copper oxalate monohydrate) and determined how the above-mentioned factors control the size, shape, aspect ratio, and growth of these nanostructures. Our results show that cationic surfactants (CTAB, TTAB, and CPB) favor the formation of nanorods of copper oxalate. The aspect ratio of these rods could be controlled to obtain nanocubes (~80-100 nm) and nanoparticles (~8-10 nm) in the CTAB system using longer chain cosurfactants like 1-octanol and 1-decanol, respectively. Nanocubes of ~50-60 and ~60-80 nm were obtained using nonionic surfactants Triton X-100 and Tergitol, respectively. The size of the nanostructures could also be controlled by varying the molar ratio of water to surfactant (W<SUB>0</SUB>) by using a nonionic (Triton X-100)-based reverse micellar system. The study espouses the versatility of the microemulsion method to realize a variety of nanostructures of copper oxalate monohydrate. Our results will be of use in extending these ideas to other nanomaterials

Synthesis of core-shell nanostructures of Co<SUB>3</SUB>O<SUB>4</SUB>@SiO<SUB>2</SUB> with controlled shell thickness (5-20 nm) and hollow shells of silica

Synthesis of uniform silica shell over Co3O4 nanoparticles was carried out using the colloidal solutions of Tergitol and cyclohexane. The shell could be controlled to a thickness of up to 20 nm by varying different parameters such as the amount of tetraethylorthosilicate, concentration of Co3O4 nanoparticles, reaction time and the presence of water and 1-octanol. Control of the amount of water (required for hydrolysis) appears to be the key factor for controlling the shell thickness. The methodology used is suitable to form shell over nanoparticles (present in powder form; synthesized at high temperature) which have high degree of agglomeration. Hollow shells of silica were obtained by the dissolution of the oxide core of Co3O4@SiO2 core-shell nanostructures. The composition of these core-shell nanostructures was confirmed by high-resolution transmission electron microscopy and elemental mapping by energy dispersive X-ray analysis. The hollow shells were characterized by using TEM, EDX and IR. Electron paramagnetic resonance studies of the core-shell nanostructures indicate the presence of free radicals on silica shell due to the presence of dangling bonds in the silica. Increase in the magnetic susceptibility was observed for these core-shell nanostructures

Inverse Thio Effects in the Hepatitis Delta Virus Ribozyme Reveal that the Reaction Pathway Is Controlled by Metal Ion Charge Density

The hepatitis delta virus (HDV) ribozyme self-cleaves in the presence of a wide range of monovalent and divalent ions. Prior theoretical studies provided evidence that self-cleavage proceeds via a concerted or stepwise pathway, with the outcome dictated by the valency of the metal ion. In the present study, we measure stereospecific thio effects at the nonbridging oxygens of the scissile phosphate under a wide range of experimental conditions, including varying concentrations of diverse monovalent and divalent ions, and combine these with quantum mechanical/molecular mechanical (QM/MM) free energy simulations on the stereospecific thio substrates. The <i>R</i>_P substrate gives large normal thio effects in the presence of all monovalent ions. The <i>S</i>_P substrate also gives normal or no thio effects, but only for smaller monovalent and divalent cations, such as Li⁺, Mg²⁺, Ca²⁺, and Sr²⁺; in contrast, sizable inverse thio effects are found for larger monovalent and divalent cations, including Na⁺, K⁺, NH₄⁺, and Ba²⁺. Proton inventories are found to be unity in the presence of the larger monovalent and divalent ions, but two in the presence of Mg²⁺. Additionally, rate–pH profiles are inverted for the low charge density ions, and only imidazole plus ammonium ions rescue an inactive C75Δ variant in the absence of Mg²⁺. Results from the thio effect experiments, rate–pH profiles, proton inventories, and ammonium/imidazole rescue experiments, combined with QM/MM free energy simulations, support a change in the mechanism of HDV ribozyme self-cleavage from concerted and metal ion-stabilized to stepwise and proton transfer-stabilized as the charge density of the metal ion decreases

Thio Effects and an Unconventional Metal Ion Rescue in the Genomic Hepatitis Delta Virus Ribozyme

Metal ion and nucleobase catalysis are important for ribozyme mechanism, but the extent to which they cooperate is unclear. A crystal structure of the hepatitis delta virus (HDV) ribozyme suggested that the <i>pro-R</i>_P oxygen at the scissile phosphate directly coordinates a catalytic Mg²⁺ ion and is within hydrogen bonding distance of the amine of the general acid C75. Prior studies of the genomic HDV ribozyme, however, showed neither a thio effect nor metal ion rescue using Mn²⁺. Here, we combine experiment and theory to explore phosphorothioate substitutions at the scissile phosphate. We report significant thio effects at the scissile phosphate and metal ion rescue with Cd²⁺. Reaction profiles with an <i>S</i>_P-phosphorothioate substitution are indistinguishable from those of the unmodified substrate in the presence of Mg²⁺ or Cd²⁺, supporting the idea that the <i>pro-S</i>_P oxygen does not coordinate metal ions. The <i>R</i>_P-phosphorothioate substitution, however, exhibits biphasic kinetics, with the fast-reacting phase displaying a thio effect of up to 5-fold and the slow-reacting phase displaying a thio effect of ∼1000-fold. Moreover, the fast- and slow-reacting phases give metal ion rescues in Cd²⁺ of up to 10- and 330-fold, respectively. The metal ion rescues are unconventional in that they arise from Cd²⁺ inhibiting the oxo substrate but not the <i>R</i>_P substrate. This metal ion rescue suggests a direct interaction of the catalytic metal ion with the <i>pro-R</i>_P oxygen, in line with experiments with the antigenomic HDV ribozyme. Experiments without divalent ions, with a double mutant that interferes with Mg²⁺ binding, or with C75 deleted suggest that the <i>pro-R</i>_P oxygen plays at most a redundant role in positioning C75. Quantum mechanical/molecular mechanical (QM/MM) studies indicate that the metal ion contributes to catalysis by interacting with both the <i>pro-R</i>_P oxygen and the nucleophilic 2′-hydroxyl, supporting the experimental findings