Partha Dutta

Solvation dynamics in a protein-surfactant complex

Solvation dynamics in the denatured state of a protein, lysozyme (denatured by sodium dodecyl sulfate, SDS) is markedly slower than that in the native state. For coumarin 153 bound to lysozyme, the average solvation time, hssi is 330 ps. In the lysozyme–SDS complex, the solvation dynamics is markedly slower with hss i¼ 7250 ps. On addition of dithiothreitol (DTT) to the lysozyme–SDS complex, when the di-sulfide bonds are destroyed, hssi is found to be 1140 ps. The slow dynamics in the denatured protein is attributed to the polymer chain dynamics and the exchange of bound and free water molecules. 2003 Elsevier B.V. All rights reserved.

Obtaining low-frequency spectra of acetone dissolved in cyclohexane by terahertz time-domain spectroscopy.

We have performed measurements of the low-frequency spectra of the refractive index and the extinction coefficient of solutions of acetone in cyclohexane at concentrations ranging between 27 and 135 mM by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. We have verified Beers law for the concentration range of our experiment, which establishes the fact that the solute-solvent interaction has a significant contribution to the observed spectra, whereas the interaction between the solute molecules is negligible in the present experimental conditions. Combining the data reported by Vij and Hufnagel (J. Phys. Chem., 1991, 95, 6142) with the results of the present experiments, an analytical model for the time correlation function for the total dipole moment of the system was examined in order to reproduce the low-frequency spectra. The molecular origins of the components in the TCF are discussed in light of the results of our simulation.

Solvation dynamics of TNS in polymer (PEG)–surfactant (SDS) aggregate

Abstract Solvation dynamics of 2,6- p -toluidinonaphthalene sulfonate (TNS) is studied using picosecond time resolved emission spectroscopy in a polymer–surfactant aggregate consisting of polyethylene glycol (PEG) and sodium dodecyl sulfate (SDS). The critical association concentration (CAC) of SDS for PEG, is found to be 4.5±0.5 mM. Solvation dynamics of TNS in PEG–SDS aggregate is described by two components, 90±10 ps (31%) and 570±50 ps (69%). Solvation dynamics in PEG–SDS aggregate is slow compared to that in bulk water or in PEG alone or in SDS alone indicating restricted movement of the water molecules squeezed in between the polymer chains and the micellar (SDS) surface.

Dependence of low frequency spectra on solute and solvent in solutions studied by terahertz time-domain spectroscopy

This paper presents the results of measuring the frequency-dependent refractive index, the extinction coefficient, and the absorption coefficient of polar solutes (nitrobenzene, benzonitrile, and chlorobenzene) in non-polar solvents by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The intensities of the difference absorption spectra of the solution and the pure solvent show a specific peak for each particular solution. This finding demonstrates the potential application of THz-TDS in studying the microscopic details of a particular solute in solution. The low frequency spectra were analysed by using a model function for the time correlation function of the total dipole moment in order to understand the dynamics and the interactions in the systems. The picosecond exponential component was found to be less sensitive to the viscosity of the bulk solvent, and it is proposed that this component can be regarded as the result of structural relaxation in the solution. It was also found that the spectra of the benzonitrile solutions receive a large contribution from the Gaussian component, which is attributed to the librational motion. This suggests that the formation of dimers or oligomers might take place in the case of the benzonitrile solution.

Ultrafast chemistry in complex and confined systems

Self-organized molecular assemblies play a crucial role in many natural and biological processes. Recent applications of ultrafast laser spectroscopy and computer simulations revealed that chemistry in a confined environment is fundamentally different from that in ordinary solutions. Many recent examples of slow dynamics in constrained environments and their biological implications are discussed

Solvation dynamics of 4-aminophthalimide in a polymer (PVP)–surfactant (SDS) aggregate

Solvation dynamics of 4-aminophthalimide (4-AP) in a polymer–surfactant aggregate comprising poly(vinylpyrrolidone) (PVP) and sodium dodecyl sulfate (SDS) is reported. In an aqueous solution even at a polymer concentration much higher than the cross over value (C*), a significant portion (30%) of 4-AP molecules do not bind to PVP. In the presence of a surfactant, SDS, 4-AP binds to the PVP–SDS aggregate completely at a polymer concentration greater than C*. The critical aggregation concentration (c.a.c.) of SDS for PVP is found to be 2 mM. The solvation dynamics of 4-AP in a PVP–SDS aggregate is observed to be slower compared to that in bulk water or in SDS micelles. The slow solvation dynamics is attributed to restrictions imposed on the water molecules squeezed between the polymer chain and micellar aggregates.

Solvation dynamics in a worm-like CTAB micelle

The solvation dynamics of 4-(dicyanomethylene)-2-methyl-(p-dimethyl-aminostyryl) 4Hpyran (DCM) is studied in a worm-like micelle consisting of cetyl trimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Solvation dynamics in the worm-like micelle is found to be about 4-times slower than that in ordinary CTAB micelle. This indicates that the fluorescent probe molecule (DCM) experience a different microenvironment inside the worm-like micellar aggregates compared to that in ordinary spherical CTAB micelle. The spectral width (Γ) of the time resolved emission spectra of DCM in the micellar aggregates is found to be time dependent. This is ascribed to the diffusion of the probe (DCM).

Photoisomerization of merocyanine 540 in polymer-surfactant aggregate

Photoisomerization of merocyanine 540 (MC540) in a polymer-surfactant aggregate is studied using picosecond time resolved emission spectroscopy. The aggregate consists of the polymer, poly(vinylpyrrolidone) (PVP) and the surfactant, sodium dodecyl sulphate (SDS). With increase in the concentration of SDS in an aqueous solution of MC540 containing PVP, the emission quantum yield and lifetime of MC540 increase markedly. This indicates marked retardation in the nonradiative photoisomerization process of MC540, when it binds to the polymer-surfactant aggregate. The critical association concentration of SDS for binding to PVP has been found to be 0.5 mM. This is about 16 times lower than the CMC of SDS in pure water (8 mM).

Isomerization and fluorescence depolarization of merocyanine 540 in polyacrylic acid. Effect ofpH

Dynamics of isomerization and fluorescence depolarization of merocyanine 540 (MC540) in an aqueous solution of polyacrylic acid (PAA) have been studied using picosecond time resolved fluorescence spectroscopy. It is observed that the dynamics of isomerization and depolarization are sensitive enough to monitor the uncoiling of PAA at high pH (> 6). At low pH (< 3), when the polymer remains in a hypercoiled form, polymer bound MC540 experiences very high microscopic friction and, hence, the isomerization and depolarization processes are very slow. At high pH (> 6) a polyanion is formed and the polymer assumes an extended configuration due to electrostatic repulsion. At high pH (> 6), the anionic probe MC540 is expelled from the polyanion to bulk water and the dynamics of isomerization and fluorescence depolarization become faster by 12 and 5 times respectively, compared to those at low pH.

Effect of an anionic surfactant (SDS) on the photoluminescence of graphene oxide (GO) in acidic and alkaline medium

An anionic surfactant (SDS) modulates the photoluminescence of graphene oxide (GO) in both acidic and alkaline medium. In the acidic medium (pH ≈ 2), formation of hemi spherical surface micelles on the GO sheets creates a non-polar environment around the flourophoric moiety of GO and hinders the solvent relaxation. This leads to a significant 36 nm blue shift of the photoluminescence band, whereas in alkaline medium (pH ≈ 10), SDS interacts with GO sheets in a different way due to the presence of negatively charged carboxylate ions at the GO edges. The repulsion between the negatively charged GO sheets and the intercalation of SDS within the basal planes of GO may weaken π–π stacking interaction which produces largely separate layers of GO. The largely separated GO sheets due to very weak stacking interactions among successive layers may behave almost like isolated functionalized GO, resulting in an enhancement of the photoluminescence intensity at 303 nm.