Hydrophobic Interactions in Donor-Disulphide-Acceptor (DSSA) Probes Looking Beyond Fluorescence Resonance Energy Transfer Theory

Donor –linker –acceptor (DSSA) is a concept in fluorescence chemistry with acceptor being a fluorescent compound (FRET) or quencher. The DSSA probes used to measure thiol levels in vitro and in vivo. The reduction potential of these dyes are in the range of ?0.60 V, much lower than the best thiol reductant reported in literature, the DTT (?0.33 V). DSSA disulphide having an unusually low reduction potential compared to the typical thiol reductants is a puzzle. Secondly, DSSA probes have a cyclized rhodamine ring as acceptor which does not have any spectral overlap with fluorescein, but quenches its absorbance and fluorescence. To understand the structural features of DSSA probes, we have synthesized DSSANa and DSSAOr. The calculated reduction potential of these dyes suggest that DSSA probes have an alternate mechanism from the FRET based quenching, namely hydrophobic interaction or dye to dye quenching. The standard reduction potential change with increasing complexity and steric hindrance of the molecule is small, suggesting that ultra- low Eo’ has no contribution from the disulphide linker and is based on structural interactions between fluorescein and cyclized rhodamine. Our results help to understand the DSSA probe quenching mechanism and provide ways to design fluorescent probes.     

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T₁) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T₁ state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.     

A quantum-classical approach to the photoabsorption spectrum of pyrazine

We have used the time-dependent discrete variable representation (TDDVR) method to simulate the photoabsorption spectrum of pyrazine. The time-dependent molecular dynamics of pyrazine after excitation to the S2 electronic state is considered as a benchmark to investigate the S2 absorption spectrum. We have carried out the dynamics on a basic four-mode model of pyrazine with the inclusion of five major modes as well as the rest of the vibrational modes as bath modes. Investigations reveal the effect of bath modes such as energy and population transfer from the subsystem to the bath. Calculated results demonstrate excellent agreement with traditional quantum-mechanical findings during the entire propagation and converge to the exact quantum results when enough gridpoints are used. It appears that TDDVR, as a numerical quantum dynamics methodology, is a good compromise between accuracy and speed.     

Induced Smectic-G Phase Through Intermolecular Hydrogen Bonding,Part XIII: Impact of a Nematogen on Phase Behaviour of Hydrogen-Bonded Liquid Crystals

A series of hydrogen-bonded mesogens, p-( p′-methoxybenzylidene)-cyanoaniline:p-n-alkoxybenzoic acids (MBCA:nABA), is synthesised by using liquid-crystalline p-n-alkoxybenzoic acids (nABA) (where n represents alkoxy carbon numbers, 3-10 and 12) and a nematogen, p-( p′-methoxybenzylidene)-cyanoaniline (MBCA). The thermal and phase behaviour of these compounds is studied by thermal microscopy (TM) and differential scanning calorimetry (DCS) techniques. These studies reveal the induction of smectic-G phase in all the compounds. The structural elucidation pertaining to the formation and stabilisation of intermolecular hydrogen bonding is carried out by a detailed IR spectral investigation. The impact of imino group of the nematogen, MBCA, on the liquid-crystallinity of the present series is realised from the comparative studies made on the reported analogous series, p-aminobenzonitrile:p-n-alkoxybenzoic acids (ABN:nABA).     

Induced crystal G phase through intermolecular hydrogen bonding II. Influence of alkyl chain length of n-alkyl p-hydroxybenzoates on thermal and phase behaviour

A new series of intermolecular hydrogen-bonded complexes have been synthesized using p-n-alkoxybenzoic acid (alkyl chain length varies from propyl- to decyl- and dodecyl-) and methyl p-hydroxybenzoate moieties. The thermal and phase behaviour of these complexes were studied by thermal microscopy and differential scanning calorimetry. Further, the stabilization of intermolecular hydrogen bonding in solution was studied by IR spectroscopy. A detailed IR spectral investigation in the solid and dissolved states suggests that the acid and phenol groups act as proton donor and proton acceptor, respectively. The thermal studies also reveal the inducement of a crystal G phase in the complexes.     

Crystal G phase induced through intermolecular hydrogen bonding IV: a study of crystallization kinetics

A systematic crystallization kinetic study using thermal microscopy and differential scanning calorimetry has been carried out on two novel liquid crystalline compounds, DBA:MHB and DBA:ACP. These involve intermolecular hydrogen bonding between 4-n-decyloxybenzoic acid (DBA) and methyl 4-hydroxybenzoate (MHB); and between DBA and 2-amino-5-chloropyridine (ACP). The kinetics experiments were performed from the crystal G phase, which is a common induced kinetophase in both the compounds. Further, the proton donor and acceptor capabilities of the -COOH group of DBA towards the -OH group of MHB and -N atom of ACP were studied in the light of mesomorphism and rate of crystallization. The dimensionality in the crystal growth and the sporadic nucleation were estimated from the Avrami exponent, n. A similar type of crystallization mechanism is predicted to operate for all the crystallization temperatures. The characteristic crystallization time (t?) at each crystallization temperature is deduced from the individual plots of log t vs. ΔH (change in enthalpy).     

Simultaneous Harvesting of Multiple Hot Holes via Visible-Light Excitation of Plasmonic Gold Nanospheres for Selective Oxidative Bond Scission of Olefins to Carbonyls

Using visible photoexcitation of gold nanospheres we successfully demonstrate the simultaneous harvesting of plasmon-induced multiple hot holes in the complete oxidative scission of the C=C bond in styrene at room temperature to selectively form benzaldehyde and formaldehyde, which is a reaction that requires activation of multiple substrates. Our results reveal that, while extraction of hot holes becomes efficient for interband excitation, harvesting of multiple hot holes from the excited Au nanospheres becomes prevalent only beyond a threshold light intensity. We show that the alkene oxidation proceeded via a sequence of two consecutive elementary steps; namely, a binding step and a cyclic oxometallate transition state as the rate-determining step. This demonstration of plasmon-excitation-mediated harvesting of multiple hot holes without the use of an extra hole transport media opens exciting possibilities, notably for difficult catalytic transformations involving multielectron oxidation processes.     

Modeling char surface area evolution during coal pyrolysis: Effect of swelling and gasification at high pressures

In this paper, a previous model for char surface area change during atmospheric coal pyrolysis was modified to include the effect of particle swelling and gasification to predict the N₂ adsorption specific surface area of high-volatile bituminous char generated at high pressures with or without gasification. A correlation between the change of char surface area and P₀ΔV_P/m_cm (the ratio of the expansion work for particle swelling to the mass of metaplast cross-linked with coal matrix) was developed and analyzed. The number of the defect regions generated by gasification was considered in calculating adsorption sites quantitatively. Particle swelling opens (at P₀ΔV_P/m_cm<1350 J/kg) and then compresses (at 1350 J/kg <P₀ΔV_P/m_cm<10,000 J/kg) the space between metaplast clusters, making the N₂ adsorption specific surface area of char increase first and then decrease. After the gaps between metaplast clusters are filled, the specific surface area changes are minimal. Gasification generates new defect regions in clusters and reduces the clusters in char, making the specific surface area of char first increase and then decrease.     

Label‐free Voltammetric Detection of Products of Terminal Deoxynucleotidyl Transferase Tailing Reaction

A label‐free approach that takes advantage of intrinsic electrochemical activity of nucleobases has been applied to study the products of terminal deoxynucleotidyl transferase (TdT) tailing reaction. DNA homooligonucleotides A₃₀, C₃₀ and T₃₀ were used as primers for the tailing reaction to which a dNTP – or a mixture of dNTPs – and TdT were added to form the tails. Electrochemical detection enabled study of the tailing reaction products created by various combinations of primers and dNTPs, with pyrolytic graphite electrode (PGE) being suitable for remarkably precise analysis of the length of tailing reaction products. Furthermore, the hanging mercury drop electrode (HMDE) was able to reveal formation of various DNA structures, such as DNA hairpins and G‐quadruplexes, which influence the behavior of DNA molecules at the negatively charged surface of HMDE. Thus, the described approach proves to be an excellent tool for studying the TdT tailing reactions and for exploring how various DNA structures affect both the tailing reactions and electrochemical behavior of DNA oligonucleotides at electrode surfaces.