Articles written in Journal of Chemical Sciences

    • Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures


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      This paper reports results from temperature and composition dependent steady state UV-Visibleabsorption and fluorescence measurements and time-resolved fluorescence experiments, using a dipolar probecoumarin 153 (C153), in TMU/water binary mixtures.Q1Both steady state and time-resolved spectroscopic dataindicate, much like in water/alcohol mixtures, TMU-induced structural stiffening and transition of the tetrahedralH-bond network of water. The structural transition referred to above means cosolvent-induced transitionof three dimensional tetrahedral H-bond network to two dimensional zig-zag chain-like structure often foundin alcohols. A comparison to the results obtained for aqueous alcohol solutions suggests that the cosolvent concentrationat which the structural transition occurs depends both on the polarity of the cosolvent and the size ofthe cosolvent molecules. UV-Visible absorption measurements reveal aggregation among TMU molecules atlower TMU concentration which shows a temperature maximum. In addition, red edge excitation effects havebeen observed at very dilute TMU concentration suggesting distribution of C153 among heterogeneous environments.All these results indicate hydrophobic interaction-induced aggregation of TMU in dilute aqueoussolutions which corroborate well with the existing simulation observation.

    • Is dynamic heterogeneity of water in presence of a protein denaturing agent different from that in presence of a protein stabilizer? A molecular dynamics simulation study


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      Rotational and translational dynamic heterogeneities (DHs) of ambient aqueous solutions of trimethylamine-N-oxide (TMAO) and tetramethylurea (TMU) at several solute concentrations have been investigated and compared. Motional characteristics of water molecules at solute interfaces and in bulk solutionshave been thoroughly examined for the search of slow dynamics. Note, TMAO possesses zwitterionic structure and is a protein stabilizer whereas TMU is a neutral dipolar molecule and a strong denaturant. Results suggest that water-TMAO solutions possess stronger DH than water-TMU solutions with the solute concentration dependence being stronger for TMAO than for TMU. Diffusive dynamics slows down near the solute surface for both the solutes. Solvation structure shows TMAO-water interaction is stronger than TMU-waterinteraction, producing longer H-bond fluctuation timescale in TMAO solutions. In short, this paper presents, for the first time, a systematic and comparative study of motional features and inter-species interactions between aqueous solutions containing solutes that differ in their individual impacts on protein stability.

    • Nonpolar solvation dynamics for a nonpolar solute in room temperature ionic liquid: a nonequilibrium molecular dynamics simulation study


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      Nonpolar solvation dynamics of a nonpolar diatomic solute in a roomtemperature ionic liquid (RTIL) has been followed via nonequilibrium molecular dynamics (MD) simulation. Frank-Condon type excitation of the solute, previously in equilibrium inRTIL solvent, has been modelled by abruptly changing the Lennard-Jones(LJ) diameter of the solute atoms and thereby disrupting the equilibrium situation. The rearrangement of the RTIL solvent molecules,which has been seen to be mostly contributed by the solute’s first solvation shell, around the excited solute results overall spectral narrowing and biphasic decay of the solvation energy; a dominant and very rapid process having sub-100 fs relaxation time, followed by a slower one relaxing at a timescale of ∼5 ps. A mode-coupling theory based calculation is also used to obtain the nonpolar solvation relaxation function fora model nonpolar solute dissolved in model RTIL solvent. The theoretical relaxation decay is not in very good agreement with the simulated nonequilibrium solvation response function; the theory predicts the short timerelaxation component slower and the longtime component faster than those of the simulated nonequilibrium relaxation. We have also checked the validity of the linear response theory (LRT) for nonpolar solvation in RTIL by looking at the equilibrium solvation energy correlation in the RTIL solvent in presence of the ground state (GS) and the excited state (ES) solute. Apparent breakdown of the LRT in the present case elucidates the probable disagreement between the theoretical and simulated nonequilibrium nonpolar solvation responsefunctions

    • Dielectric relaxations of molten acetamide: dependence on the model interaction potentials and the effects of system size


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      Molecular dynamics simulations of dielectric relaxations (DRs) in neat molten acetamide(CH3CONH2) at ~358 K have been carried out by employing two different versions of the OPLS force fieldparameters, namely, the OPLS-UA (united-atom) and the OPLS-AA (all-atom) model interactions. Threesystems consisting of 250, 500, and 1000 molecules have been studied to examine the impact of system sizeon the simulated dielectric properties. A comparison between our simulation predictions and the experimentalDR data in the MHz-GHz frequency regime reveals that the OPLS-UA interaction parameters betterreproduce the experimental static dielectric constant, whereas the OPLS-AA interaction describes well themeasured DR time constants. Moreover, a weak system size dependence has been observed. A Cole-Cole plotof the simulated and experimental dielectric spectra reveal non-Debye nature of liquid acetamide and corroborateswell with the earlier observation on the collective single-particle reorientational relaxation of liquidacetamide. The simulated single dipole reorientation dynamics also reflects this weak non-Debye nature andreveals its contribution to the collective polarization relaxation. Simulation results obtained here set the rightground for investigating the colossal dielectric constant (~106) of ionic acetamide deep eutectics reportedearlier via DR measurements in the KHz-MHz regime.

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