Elucidation of Binding Mechanism of Photodynamic Therapeutic Agent Toluidine Blue O with Chicken Egg White Lysozyme by Spectroscopic and Molecular Dynamics Studies.

The nature of binding mechanism of toluidine blue O (TBO) with chicken egg white lysozyme was studied comprehensively by various spectroscopic and computational methods. Both steady state and time-resolved fluorescence studies unambiguously point to the prevalence of static quenching mechanism in lysozyme-TBO system. Thermodynamic parameters revealed that the association of TBO with lysozyme was a spontaneous process in which hydrophobic and hydrogen bond interactions played a pivotal role in the binding process. The secondary and tertiary conformational changes of lysozyme in the presence of TBO were unraveled using absorption, Fourier transform infrared spectroscopy (FT-IR) and circular dichroism (CD) techniques. Molecular docking studies of lysozyme-TBO system substantiated the findings of site marker experiment and revealed TBO adjacent to Trp-63 and Trp-108 residues of lysozyme. Molecular dynamics (MD) simulation studies of lysozyme-TBO system indicate a stable and effective complexation of TBO with lysozyme. It is hoped that the results presented here will enable further understanding of TBO toxicity.

Multiwalled Carbon Nanotube Oxygen Sensor: Enhanced Oxygen Sensitivity at Room Temperature and Mechanism of Sensing.

A pyrolysis assisted method was applied for the synthesis of defect controlled carbon nanotubes (CNTs) by varying different growth temperatures. The fabricated resistive devices containing a random network of CNTs were tested for oxygen sensing under standard room-temperature and pressure conditions. Nanotubes grown at moderate growth temperatures (870 °C), when exposed to different concentrations of oxygen, displayed a higher sensitivity (3.6%), with fast response and recovery times of about 60 and 180 s, respectively, compared to nanotubes grown at higher and lower temperatures. A room-temperature oxygen detection concentration as low as 0.3% is achieved. The fast response and recovery of CNTs are explained in terms of physisorption of oxygen molecules at (i) carboxyl functional sites and (ii) graphitic carbon sites (pristine CNT) rather than chemisorption at (iii) defected sites. Interestingly, the density functional theory simulated interaction energies (Eads) of oxygen molecules with defected CNTs (-3.381 eV) and pristine CNTs (-0.753 eV) are higher than that of the carboxyl functional sites (-0.551 eV) and are well correlated with the observed sensing response and recovery times of CNT sensors. Our results show that the carboxyl sites provide lower activation energy or shorter time for desorption of oxygen molecules to yield higher response and fast recovery of the CNT sensors.

Density functional theory investigation of cocaine water complexes.

Twenty cocaine-water complexes were studied using density functional theory (DFT) B3LYP/6-311++G** level to understand their geometries, energies, vibrational frequencies, charge transfer and topological parameters. Among the 20 complexes, 12 are neutral and eight are protonated in the cocaine-water complexes. Based on the interaction energy, the protonated complexes are more stable than the neutral complexes. In both complexes, the most stable structure involves the hydrogen bond with water at nitrogen atom in the tropane ring and C=O groups in methyl ester. Carbonyl groups in benzoyl and methyl ester is the most reactive site in both forms and it is responsible for the stability order. The calculated topological results show that the interactions involved in the hydrogen bond are electrostatic dominant. Natural bond orbital (NBO) analysis confirms the presence of hydrogen bond and it supports the stability order. Atoms in molecules (AIM) and NBO analysis confirms the C-H···O hydrogen bonds formed between the cocaine-water complexes are blue shifted in nature.

Spectroscopic investigations and hydrogen bond interactions of 8-aza analogues of xanthine, theophylline and caffeine: a theoretical study.

The structure, spectral properties and the hydrogen bond interactions of 8-aza analogues of xanthine, theophylline and caffeine have been studied by using quantum chemical methods. The time-dependent density functional theory (TD-DFT) and the singly excited configuration interaction (CIS) methods are employed to optimize the excited state geometries of isolated 8-azaxanthine, 8-azatheophylline tautomers and 8-azacaffeine in both the gas and solvent phases. The solvent phase calculations are performed using the polarizable continuum model (PCM). The absorption and emission spectra are calculated using the time-dependent density functional theory (TD-DFT) method. The results from the TD-DFT calculations reveal that the excitation spectra are red shifted relative to absorption in aqueous medium. These changes in the transition energies are qualitatively comparable to the experimental data. The examination of molecular orbital reveals that the molecules with a small H→L energy gap possess maximum absorption and emission wavelength. The relative stability and hydrogen bonded interactions of mono and heptahydrated 8-azaxanthine, 8-azatheophylline tautomers and 8-azacaffeine have been studied using the density functional theory (DFT) and Møller Plesset perturbation theory (MP2) implementing the 6-311++G(d,p) basis set. The formation of strong N-H...O bond has resulted in the highest interaction energy among the monohydrates. Hydration does not show any significant impact on the stability of heptahydrated complexes. The atoms in molecule (AIM) and natural bonding orbital (NBO) analyses have been performed to elucidate the nature of the hydrogen bond interactions in these complexes.