Chandrabhas Narayana

Allosteric Transition Induced by Mg2+ Ion in a Transactivator Monitored by SERS

2014-01-01T00:00:00Z

Allosteric Transition Induced by Mg2+ Ion in a Transactivator Monitored by SERS Kundu, Partha P.; Bhowmick, Tuhin; Swapna, Ganduri; Kumar, G. V. Pavan; Nagaraja, V.; Narayana, Chandrabhas We demonstrate the utility of the surface-enhanced Raman spectroscopy (SERS) to monitor conformational transitions in protein upon ligand binding. The changes in protein's secondary and tertiary structures were monitored using amide and aliphatic/aromatic side chain vibrations. Changes in these bands are suggestive of the stabilization of the secondary and tertiary structure of transcription activator protein C in the presence of Mg2+ ion, whereas the spectral fingerprint remained unaltered in the case of a mutant protein, defective in Mg2+ binding. The importance of the acidic residues in Mg2+ binding, which triggers an overall allosteric transition in the protein, is visualized in the molecular model. The present study thus opens up avenues toward the application of SERS as a potential tool for gaining structural insights into the changes occurring during conformational transitions in proteins. Restricted Access

Conformational change in a urea catalyst induced by sodium cation and its effect on enantioselectivity of a Friedel-Crafts reaction

2014-01-01T00:00:00Z

Conformational change in a urea catalyst induced by sodium cation and its effect on enantioselectivity of a Friedel-Crafts reaction Chittoory, Arjun K.; Kumari, Gayatri; Mohapatra, Sudip; Kundu, Partha P.; Maji, Tapas K.; Narayana, Chandrabhas; Rajaram, Sridhar While developing bis-camphorsulfonyl urea as a hydrogen-bonding catalysts, we discovered that the native conformation of the catalyst is unsuitable for inducing enantioselectivity. By complexing the catalyst with weakly Lewis acidic sodium cations, we were able to change the conformation of the catalyst and attain a significant improvement in the selectivity. We provide structural information from X-ray crystallography to show that the uncomplexed catalyst is indeed in an unfavorable conformation. Infrared and Raman spectroscopic studies show that sodium binds the catalyst through the carbonyl and sulfonyl groups. Simulated IR and Raman spectra match well with the experimentally recorded spectra, thereby corroborating the proposed conformational change. This result shows that weak Lewis acids can be used to tune the conformation of hydrogen-bonding catalysts and enhance the selectivity of reaction catalyzed by these systems. (C) 2014 Elsevier Ltd. All rights reserved. Restricted Access

Low cost, rapid synthesis of graphene on Ni: An efficient barrier for corrosion and thermal oxidation

2014-01-01T00:00:00Z

Low cost, rapid synthesis of graphene on Ni: An efficient barrier for corrosion and thermal oxidation Mogera, Umesha; Kurra, Narendra; Radhakrishnan, Dhanya; Narayana, Chandrabhas; Kulkarni, G. U. Graphene because of its inert and impermeable nature holds a great promise as a protective coating against corrosion and oxidation. It is envisioned that graphene coated metals will be increasingly used in metal and electronic industries. To date, mainly expensive chemical vapor deposition methods are being employed for its synthesis over large area involving hydrogen at high reaction temperatures. Here we report, a simple and rapid method of Joule heating a Ni foil coated with naphthalene in rotary vacuum to produce graphene without hydrogen gas. The graphene thus grown protects the Ni surface against corrosion and oxidation even at elevated temperatures. This synthetic approach has a great potential for widespread use as it is low cost and adaptable to metal surfaces of complex curvatures. (C) 2014 Elsevier Ltd. All rights reserved. Restricted Access

Nanocrystalline Ag microflowers as a versatile SERS platform

2014-01-01T00:00:00Z

Nanocrystalline Ag microflowers as a versatile SERS platform Mettela, Gangaiah; Siddhanta, Soumik; Narayana, Chandrabhas; Kulkarni, G. U. In this paper, the synthesis of Ag microflowers for use as manipulable and reusable substrates in surface enhanced Raman spectroscopy (SERS) is demonstrated, working with ultra-low volumes of the analyte. Flower-like AgBr crystallites with a growth direction of < 110 > were first obtained by thermolysing a complex obtained by the stabilization of (AgCl2)(-) anions with tetraoctylammonium bromide. NaBH4 reduction leads to the formation of porous Ag microflowers (50-100 mu m) with interconnected nanoparticles. The coupling of the nanoparticles in the microflower results in broadband extinction from visible to IR wavelengths, facilitating SERS using both red and green wavelengths. Using thiophenol as test analyte, uniform SERS enhancement factors in the range of 10(6)-10(8) have been achieved from different parts of the microflower. The microflowers have been used for labeled and non-labeled detection of both single- and double-stranded DNA and using simple manipulation techniques, SERS data have been collected from ultra-low volumes of the analyte solution (similar to 0.34 nL). The reusability of the substrate for SERS over multiple cycles involving a rapid and efficient wet chemical cleaning procedure is also demonstrated. Finally, by placing the microflower in a microfluidic device, chemical reactions have been examined in situ. Restricted Access