Solvent controlled mechanistic dichotomy in a Au(III)-catalyzed, heterocyclization triggered, Nazarov reaction

Tandem Au(III)-catalyzed heterocyclization/Nazarov cyclizations leading to substituted carbocycle fused furans are described. An interesting dichotomy of reaction pathways as a function of solvent, confirmed by the isolation and trapping of reaction intermediates, provided a basis for computational studies that supported the experimental findings.     

Bulk synthesis of polymer-inorganic colloidal clusters

We describe a procedure to synthesize colloidal clusters with polyhedral morphologies in high yield (liter quantities at up to 70% purity) using a combination of emulsion polymerization and inorganic surface chemistry. We show that the synthesis initially used for silica-polystyrene hybrid clusters can be generalized to create clusters from other inorganic and polymer particles. We also show that high yields of particular morphologies can be obtained by precise control of the inorganic seed particle size, a finding that can be explained using a hard-sphere packing model. These clusters can be further chemically modified for a variety of applications. Introducing a cross-linker leads to colloidal clusters that can be index matched in an appropriate solvent, allowing them to be used for particle tracking or optical studies of colloidal self-assembly. Also, depositing a thin silica layer on these colloids allows the surface properties to be controlled using silane chemistry.     

Catalyst-Substrate Effects on Biocompatible SABRE Hyperpolarization

The hyperpolarization technique, Signal Amplification by Reversible Exchange (SABRE), has the potential to improve clinical diagnosis by making molecular magnetic resonance imaging in vivo a reality. Essential to this goal is the ability to produce a biocompatible bolus for administration. We seek here to determine how the identity of the catalyst and substrate affects the cytotoxicity by in vitro study, in addition to reporting how the use of biocompatible solvent mixtures influence the polarization transfer efficiency. By illustrating this across five catalysts and 8 substrates, we are able to identify routes to produce a bolus with minimal cytotoxic effects.     

A new paramagnetic intermediate formed during the reaction of nitrite with deoxyhemoglobin

The reduction of nitrite by deoxygenated hemoglobin chains has been implicated in red cell-induced vasodilation, although the mechanism for this process has not been established. We have previously demonstrated that the reaction of nitrite with deoxyhemoglobin produces a hybrid intermediate with properties of Hb(II)NO(+) and Hb(III)NO that builds up during the reaction retaining potential NO bioactivity. To explain the unexpected stability of this intermediate, which prevents NO release from the Hb(III)NO component, we had implicated the transfer of an electron from the β-93 thiol to NO(+) producing ·SHb(II)NO. To determine if this species is formed and to characterize its properties, we have investigated the electron paramagnetic resonance (EPR) changes taking place during the nitrite reaction. The EPR effects of blocking the thiol group with N-ethyl-maleimide and using carboxypeptidase-A to stabilize the R-quaternary conformation have demonstrated that ·SHb(II)NO is formed and that it has the EPR spectrum expected for NO bound to the heme in the β-chain plus that of a thiyl radical. This new NO-related paramagnetic species is in equilibrium with the hybrid intermediate "Hb(II)NO(+) ? Hb(III)NO", thereby further inhibiting the release of NO from Hb(III)NO. The formation of an NO-related paramagnetic species other than the tightly bound NO in Hb(II)NO was also confirmed by a decrease in the EPR signal by -20 °C incubation, which shifts the equilibrium back to the "Hb(II)NO(+) ? Hb(III)NO" intermediate. This previously unrecognized NO hemoglobin species explains the stability of the intermediates and the buildup of a pool of potentially bioactive NO during nitrite reduction. It also provides a pathway for the formation of β-93 cysteine S-nitrosylated hemoglobin [SNOHb:S-nitrosohemoglobin], which has been shown to induce vasodilation, by a rapid radical-radical reaction of any free NO with the thiyl radical of this new paramagnetic intermediate.     

Rules for anionic and radical ring closure of alkynes

This work reexamined the stereoelectronic basis for the "favored attack trajectories" regarding the nucleophilic and radical cyclizations of alkynes. In contrast to the original Baldwin rules, the acute attack angle of a nucleophile leading to the proposed endo-dig preference for the formation of small cycles is less favorable stereoelectronically than the alternative obtuse trajectory leading to the formation of exo-dig products. For smaller cycles, this intrinsic stereoelectronic preference can be masked by the greater thermodynamic stability of the less strained endo-products. Unbiased comparison of competing cyclization attacks has been accomplished via dissection of the activation barrier into the intrinsic barrier and thermodynamic component via Marcus theory. Intrinsic barriers of thermoneutral reactions strongly favor exo-dig closures, in full accord with the greater magnitude of two-electron bond forming interactions for the obtuse trajectory. This analysis agrees very well with experimental observations of efficient 3-exo-dig and 4-exo-dig cyclizations predicted to be unfavorable by the Baldwin rules and with the calculated 3-exo-/4-endo-, 4-exo-/5-endo-, and 5-exo-/6-endo-dig selectivities in the cyclizations of carbon-, nitrogen-, and oxygen-centered nucleophiles. The generality of these predictions is confirmed by analogous trends for the related radical cyclizations where the stereoelectronically favorable exo-closures are also preferred kinetically, with a few exceptions where a large difference in product stability skews the intrinsic stereoelectronic trends.     

Conformational changes monitored by fluorescence study on reconstituted hemoglobins

Intrinsic steady state fluorescence measurements were performed on a series of reconstituted metal ion and hybrid hemoglobins (Hbs). At 296 nm excitation, the spectrum exhibits a broad and asymmetric feature in the case of copper and nickel reconstituted hemoglobins. Deconvolution of the fluorescence bands clearly reveals the existence of two definite peaks. A similar trend was also observed for hybrid hemoglobins (CuNi, NiCu, CuFe-CO, and NiFe-CO). A guassian fit of the fluorescence bands in these proteins again yields two prominent peaks, which are assigned as due to two different tryptophan (Trp) environments. A relative ratio of the amplitudes of these peaks indicates the percentage of T-character in these molecules. This is in support to our previous findings by other spectroscopic studies on the same molecules. These studies therefore, suggest the presence of two different environments of a tryptophan thereby revealing structural heterogeneity among the subunits.     

Magnetic pair making and breaking effect of Ru in La0.7Sr0.3Mn0.9Ru0.1O3 and La0.5Sr0.5Co0.9Ru0.1O3

The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La_0.7Sr_0.3MnO₃ and La_0.5Sr_0.5CoO₃ has been investigated. It is found that substitution of 10% Ru at the Mn site of La_0.7Sr_0.3MnO₃ decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔT_c80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La_0.5Sr_0.5CoO₃. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru⁺⁴/Ru⁺⁵. The e_g and t_2g parentage of Ru⁺⁵ is similar to Mn⁺⁴ and therefore, Ru⁺⁵ ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co⁺³: t_2g⁵e_g¹), forcing a double exchange FM state to anti-FM state.     

Identification and Structure Elucidation of a Novel Antifungal Compound Produced by Pseudomonas aeruginosa PGPR2 Against Macrophomina phaseolina

Pseudomonas aeruginosa PGPR2 was found to protect mungbean plants from charcoal rot disease caused by Macrophomina phaseolina. Secondary metabolites from the culture supernatant of P. aeruginosa PGPR2 were extracted with ethyl acetate and the antifungal compound was purified by preparative HPLC using reverse phase chromatography. The purified compound showed antifungal activity against M. phaseolina and other phytopathogenic fungi (Fusarium sp., Rhizoctonia sp. Alternaria sp., and Aspergillus sp.). The structure of the purified compound was determined using ¹H, ¹³C, 2D NMR spectra and liquid chromatography-mass spectrometry (LC-MS). Spectral data suggest that the antifungal compound is 3,4-dihydroxy-N-methyl-4-(4-oxochroman-2-yl)butanamide, with the chemical formula C₁₄H₁₇NO₅ and a molecular mass of 279. Though chemically synthesized chromanone derivatives have been shown to have antifungal activity, we report for the first time, the microbial production of a chromanone derivative with antifungal activity. This ability of P. aeruginosa PGPR2 makes it a suitable strain for biocontrol.     

Assessment of Factors Influencing the Agrobacterium-mediated in planta Seed Transformation of Brinjal (Solanum melongena L.)

An efficient and reproducible in planta transformation method was developed for brinjal using seed as an explant. The brinjal seeds were infected with Agrobacterium tumefaciens EHA 105 harbouring pCAMBIA 1301-bar plasmid, and the transformants were selected against BASTA®. Several parameters influencing the in planta seed transformation such as pre-culture duration, acetosyringone concentration, surfactants, duration of sonication, vacuum pressure and vacuum duration have been evaluated. The putatively transformed (T ₀) brinjal plants were screened by GUS histochemical analysis. Among the different combinations and concentrations tested, when the 18-h pre-cultured brinjal seeds were sonicated for 20 min and vacuum infiltered for 3 min at 500 mm of Hg in Agrobacterium suspension containing 100 μM acetosyringone, 0.2 % Silwett L-77 favoured the Agrobacterium infection and showed maximum transformation efficiency. Among the five brinjal varieties evaluated, Arka Samhitha showed maximum transformation efficiency at 45.66 %. The transgene was successfully transmitted to progeny plants (T ₁) which was evidenced by GUS histochemical analysis, polymerase chain reaction and Southern hybridisation. The in planta protocol developed in the present study would be beneficial to transfer the economically and nutritionally important genes into different varieties of brinjal, and the transgenic brinjal plants can be produced in less time (approximately 27 days).