Porphyrins with multiple crown ether voids: novel systems for cation complexation studies

Porphyrins appended with crown ether, benzo-15-crown-5, at the methine positions have been synthesized and characterized. The fully and partially substituted porphyrins and their metallo (Co, Cu, and Zn) derivatives describe one or more ether cavities in the periphery that are capable of recognizing spherical cations. The ability of these macrocycles to complex cations (Na+, K+, Mg2+, Ca2+, Ba2', and NH4+) is investigated by use of visible, 'H NMR, ESR, and emission spectral studies. The tetrasubstituted crown porphyrin (TCP) exhibits very high selectivity for K+. The cations (K', Ba2+, and NH4+) that require two crown ether cavities for complexation promote dimerization of the porphyrins. The ESR study of the cation-induced porphyrin dimers reveals axial symmetry with the porphyrin planes separated by -4.2 A. This distance increases from the fully substituted to partially substituted porphyrins. The cations (K', Ba2+, and NH4') quench efficiently the fluorescence of the free base porphyrins and their metallo derivatives. The quenching process is attributed to the steric geometry of the dimers.

Photochemical Reactions in Oriented Systems

Modification of chemical reactions through the use of constrained and/or organized media has attracted a great deal of attention recently. Results from our laboratory in this direction which include a study of photochemical reactions in solid state and in cyclodextrins are presented here. A study of solid state photochemical behavior of coumarins has provided information regarding subtler aspects of topochemical postulates of photodimerization. Results pertaining to geometrical criteria for photodimerization and "chloro" as a crystal engineering group are discussed. As a part of an attempt to correlate chemical reactivity with molecular packing in the solid state, photooxidation of diarylthioketones in the solid state has been investigated. The observed differences in the reactivity of these crystals are rationalized in terms of crystal packing. Though cyclodextrins have been extensively studied, very few photochemical reactions involving molecules complexed to cyclodextrins have been examined. In this connection, the utility of cyclodextrins in bringing about selectivity in photochemical reactions through the study of excited state behavior of olefins and aryl alkyl ketones has been demonstrated in our laboratory.

An alternative approach to selective sea water oxidation for hydrogen production

Sea water electrolysis is one of the promising ways to produce hydrogen since it is available in plentiful supply on the earth. However, in sea water electrolysis toxic chlorine evolution is the preferred reaction over oxygen evolution at the anode. In this work, research has been focused on the development of electrode materials with a high selectivity for oxygen evolution over chlorine evolution. Selective oxidation in sea water electrolysis has been demonstrated by using a cation-selective polymer. We have used a perm-selective membrane (Nafion®), which electrostatically repels chloride ions (Cl−) to the electrode surface and thereby enhances oxygen evolution at the anode. The efficiency and behaviour of the electrode have been characterized by means of anode current efficiency and polarization studies. The surface morphology of the electrode has been characterized by using a scanning electron microscope (SEM). The results suggest that nearly 100% oxygen evolution efficiency could be achieved when using an IrO2/Ti electrode surface-modified by a perm-selective polymer.

Photocatalytic activity of Ag-substituted and impregnated nano-TiO2

Ag-substituted (Ag sub) and Ag-impregnated (Ag imp), anatase phase nano-TiO2 have been synthesized by solution combustion technique and reduction technique, respectively. The catalysts were characterized extensively by powder XRD, TEM, XPS, FT-Raman, UV absorption, FT-IR, TGA, photoluminescence, BET surface area and isoelectric pH measurements. These catalysts were used for the photodegradation of dyes and for the selective photooxidation of cyclohexane to cyclohexanone. The photoactivities of the combustion-synthesized catalysts were compared with those of commercial Degussa P 25 (DP 25) TiO2, and Ag-impregnated DP 25 (Ag DP). For the photocatalytic degradation of dyes, unsubstituted combustion-synthesized TiO2 (CS TiO2) exhibited the highest activity, followed by 1% Ag imp and 1% Ag sub. For the photoconversion of cyclohexane, the total conversion of cyclohexane and the selectivity of cyclohexanone followed the order: 1% Ag sub > DP 25 > CS TiO2 > 1% Ag imp > 1% Ag DP. The kinetics of the photodegradation of dyes and of the photooxidation of cyclohexane were modeled using Langmuir–Hinshelwood rate equation and a free radical mechanism, respectively, and the rate coefficients were determined. The difference in activity values of the catalysts observed for these two reactions and the detailed characterization of these catalysts are described in this study.

Synthesis of Azo Coupled Crown Ethers Containing Transition Metal and other Chromophoric Environments and Their Ion Complexation Properties

The crown ethers, 2,3-benzo-1,4,7,10,13-pentaoxa-cyclopentadeca-2-ene and 2,3, ll,12-dibenzo-l,4,7,10,13,16-hexaoxscyclooctadeca-2,11-diene are incorporated into H,N'-ethylenebis(acetylacetoneimino) nickel(II) and copper(II), phenol, and β-naphthol by diazo coupling reactions. The selective nature of the coupling reaction has-been demonstrated by the isolation of both asymmetric mono- and symmetric bis(glyoxalarylcrownhydrazoneimino) metal(II) complexes. An interesting binuclear complex containing two intramolecularly rearranged (glyoxal-hydrazonearylimino) metal(II) groups joined by 18-crown-6 result8 when bis(arenediazonium)-18-crown-6 is coupled with the metal(I1) Schiff bases. The substituted ethers form cationic salts with NaClO4, KCNS, NH4CNS, 14g(CNS)2 and Ca(CNS)2. All the synthesised ethers exhibit ion selectivity sequence as K+ > Na+ and Ca2+ > Mg2+.

Role of the Central Metal Ion and Ligand Charge in the DNA Binding and Modification by Metallosalen Complexes

Several metal complexes of three different functionalized salen derivatives have been synthesized. The salens differ in terms of the electrostatic character and the location of the charges. The interactions of such complexes with DNA were first investigated in detail by UV−vis absorption titrimetry. It appears that the DNA binding by most of these compounds is primarily due to a combination of electrostatic and other modes of interactions. The melting temperatures of DNA in the presence of various metal complexes were higher than that of the pure DNA. The presence of additional charge on the central metal ion core in the complex, however, alters the nature of binding. Bis-cationic salen complexes containing central Ni(II) or Mn(III) were found to induce DNA strand scission, especially in the presence of co-oxidant as revealed by plasmid DNA cleavage assay and also on the basis of the autoradiogram obtained from their respective high-resolution sequencing gels. Modest base selectivity was observed in the DNA cleavage reactions. Comparisons of the linearized and supercoiled forms of DNA in the metal complex-mediated cleavage reactions reveal that the supercoiled forms are more susceptible to DNA scission. Under suitable conditions, the DNA cleavage reactions can be induced either by preformed metal complexes or by in situ complexation of the ligand in the presence of the appropriate metal ion. Also revealed was the fact that the analogous complexes containing Cu(II) or Cr(III) did not effect any DNA strand scission under comparable conditions. Salens with pendant negative charges on either side of the precursor salicylaldehyde or ethylenediamine fragments did not bind with DNA. Similarly, metallosalen complexes with net anionic character also failed to induce any DNA modification activities.

Microbially induced separation of arsenopyrite and bioremediation of arsenic

This paper discusses the role of the mineral-adapted acidiphilic microorganism. Acidithiobacillus ferrooxidans in the beneficiation of arsenopyrite-containing multisulfides (pyrite and chalcopyrite) and the bioremediation of the resulting arsenical waste water. It was found that adaptation to minerals alters the surface properties of the microorganism. Bacterial adaptation to arsenopyrite and controlled bacterial adhesion to mineral surfaces lead to selectivity in arsenopyrite separation. Bioremoval of arsenic ions (both arsenite and arsenate ions) by Acidithiobacillus ferrooxidans is also discussed.

Polypyrrole based amperometric glucose biosensors

Biosensors have gained immense acceptance in the field of medical diagnostics, besides environmental, food safety and biodefence applications due to its attributes of real-time and rapid response. This synergistic combination of biotechnology and microelectronics comprises a biological recognition element coupled with a compatible transducer device. Diabetes is a disease of major concern since the ratio of world population suffering from it is increasing at an alarming rate and therefore the need for development of accurate and stable glucose biosensors is evident. There are many commercial glucose biosensors available yet some limitations need attention. This review presents a detailed account of the polypyrrole based amperometric glucose biosensors. The polymer polypyrrole is used extensively as a matrix for immobilization of glucose oxidase enzyme owing to its favourable features such as stability under ambient conditions, conductivity that allows it to be used as an electron relay, ability to be polymerized under neutral and aqueous mild conditions, and more. The simple one-step electrodeposition on the electrode surface allows easy entrapment of the enzyme. The review is structured into three categories (a) the first-stage biosensors: which report the studies from the inception of use of polypyrrole in glucose biosensors during which time the role of the polymer and the use of mediators was established. This period saw extensive work by two separate groups of Schuhmann and Koopal who contributed a great deal in understanding the electron transfer pathways in polypyrrole based glucose biosensors, (b) the second-stage biosensors: which highlight the shift of polypyrrole from a conventional matrix to composite matrices with extensive use of mediators focused at improving the selectivity of response, and (c) third-stage biosensors: the remarkable properties of nanoparticles and carbon nanotubes and their outstanding ability to mediate electrontransfers have seen their indispensable use in conjugation with polypyrrole for development of glucose biosensors with improved sensitivity and stability characteristics which is accounted in the review, which thus traces the evolution of polypyrrole from a conventional matrix, to composites and thence to the form of nanotube arrays, with the objective of addressing the vital issue of diabetes management through the development of stable and reliable glucose biosensors.

Mycobacterial Stress Regulation: The Dps ``Twin Sister'' Defense Mechanism and Structure-Function Relationship

In this work, we have tried to emphasize the connection between mycobacterial growth and regulation of gene expression. Utilization of multiple carbon sources and diauxic growth helps bacteria to regulate gene expression at an optimum level so that the inhospitable conditions encountered during nutrient depletion can be circumvented. These aspects will be discussed with respect to mycobacterial growth in subsequent sections. Identification and characterization of genes induced under such conditions is helpful to understand the physiology of the bacterium. Although it is necessary to compare the total expression profile of proteins as they transit from vegetative growth to stationary phase, at times a lot of insights can be deciphered from the expression pattern of one or two proteins. We have compared the protein expression and sigma factor selectivity of two such proteins in M. smegmatis to understand the differential regulation of genes playing diverse function in the same species. Some newer insights on the structure and function of one of the Dps proteins are also explained.

Catalytic cyclopropanation of olefins using copper(I) diphosphinoamines

Catalytic cyclopropanation reactions of olefins with ethyl diazoacetate were carried out using copper(I) diphosphinoamine (PPh2)(2)N(R) (R = Pr-i, H, Ph and -CH2-C6H4-CH=CH2) complexes at 40 degrees C in chloroform. High yields of the cyclopropanes were obtained in all cases. The rate of the reaction was influenced by the nuclearity of the complex and the binding mode of the ligand which was either bridging or chelating. Comparison of isostructural complexes shows that the rate follows the order R = Pr-i > H > Ph, where R is the substituent on the N. However, cyclopropane formation versus dimerization of the carbene, and trans to cis ratios of cyclopropane was similar in all cases. The nearly identical selectivity for different products formed was indicative of a common catalytic intermediate. A labile "copper-olefin" complex which does not involve the phosphine or the counterion is the most likely candidate. The differences in the reaction rates for different complexes are attributed to differences in the concentration of the catalytically active species which are in equilibrium with the catalytically inactive copper-phosphinoamine complex. To test the hypothesis a diphosphinoamine polymer complexed to copper(I) was used as a heterogeneous catalyst. Leaching of copper(I) and deactivation of the catalyst confirmed the proposed mechanism. (C) 2008 Elsevier B. V. All rights reserved.

Detection of SFBC-OFDM signals in frequency- and time-selective MIMO channels

Use of space-frequency block coded (SFBC) OFDM signals is advantageous in high-mobility broadband wireless access, where the channel is highly time- as well as frequency-selective because of which the receiver experiences both inter-symbol interference (ISI) as well as inter-carrier interference (10). ISI occurs due to the violation of the 'quasi-static' fading assumption caused due to frequency- and/or time-selectivity of the channel. In addition, ICI occurs due to time-selectivity of the channel which results in loss of orthogonality among the subcarriers. In this paper, we are concerned with the detection of SFBC-OFDM signals on time- and frequency-selective MIMO channels. Specifically, we propose and evaluate the performance of an interference cancelling receiver for SFBC-OFDM which alleviates the effects of ISI and ICI in highly time- and frequency-selective channels.

ICI-ISI mitigation in cooperative SFBC-OFDM with carrier frequency offset

Synchronization issues pose a big challenge in cooperative communications. The benefits of cooperative diversity could be easily undone by improper synchronization. The problem arises because it would be difficult, from a complexity perspective, for multiple transmitting nodes to synchronize to a single receiver. For OFDM based systems, loss of performance due to imperfect carrier synchronization is severe, since it results in inter-carrier interference (ICI). The use of space-time/space-frequency codes from orthogonal designs are attractive for cooperative encoding. But orthogonal designs suffer from inter-symbol interference (ISI) due to the violation of quasi-static assumption, which can arise due to frequency- or time-selectivity of the channel. In this paper, we are concerned with combating the effects of i) ICI induced by carrier frequency offsets (CFO), and ii) ISI induced by frequency selectivity of the channel, in a cooperative communication scheme using space-frequency block coded (SFBC) OFDM. Specifically, we present an iterative interference cancellation (IC) algorithm to combat the ISI and ICI effects. The proposed algorithm could be applied to any orthogonal or quasi-orthogonal designs in cooperative SFBC OFDM schemes.

Self-acylation properties of type II fatty acid biosynthesis acyl carrier protein

Acyl carrier protein (ACIP) plays a central role in many metabolic processes inside the cell, and almost 4% of the total enzymes inside the cell require it as a cofactor. Here, we report self-acylation properties in ACPs from Plasmodium falciparum and Brassica napus that are essential components of type II fatty acid biosynthesis (FAS II), disproving the existing notion that this phenomenon is restricted only to ACPs involved in polyketide biosynthesis. We also provide strong evidence to suggest that catalytic self-acylation is intrinsic to the individual ACP. Mutational analysis of these ACPs revealed the key residue(s) involved in this phenomenon. We also demonstrate that these FAS 11 ACPs exhibit a high degree of selectivity for self-acylation employing only dicarboxylic acids as substrates. A plausible mechanism for the self-acylation reaction is also proposed.

A bile acid derived potassium ion sensor

A chenodeoxycholic acid based K+ ion sensor has been designed using a modular approach in which a fluorophore and a cation receptor are attached to the bile acid backbone. In the absence of K+ the fluorescence of the molecule is quenched because of through-space, photo-induced electron-transfer from the aza-crown unit. Fluorescence enhancement was observed upon titration with K+ (and other alkali metal ions too). In methanol, good selectivity towards the sensing of K+ has been observed.

Design of a compact wideband bandpass filter

This article presents the analysis and design of a compact multi-layer layer, high selectivity wideband bandpass filter using stub loaded and `U' shaped resonators over a slotted bottom ground plane. While the resonators with folded open circuit stub loadings create the desired bandpass characteristics. the IT shaped resonators reduce the size of filter. The slotted bottom ground plane is used to enhance the coupling to achieve the desired bandwidth. The proposed filter has been analyzed using circuit model, and the results were verified through full wave simulations and measurements. The fabricated filter is compact and measures a size of 18 mm x 25 mm x 1.6 MM. (C) 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1387-1389, 2010: Published online in Wiley InterScience (www.interscience.wiley.com).