Exploration of the Dynamical Evolution and the Associated Energetics of Water Nanoclusters Formed in a Hydrophobic Solvent

Sensing Bioavailable Water Content of Granulated Matrices: A Combined Experimental and Computational Study

This paper represents the synthesis, characterization and validation of a cobalt chloride functionalised nano-porous cellulose membrane, a unique sensor for non-contact measurement of water potential in various biomedical and environmentally important matrices. The developed nano sensor, along with associated electronic components, is assembled as a prototype device called "MEGH" (Measuring Essential Good Hydration) to measure essential hydration of matrices of both environmental and biomedical importance, including soil and human skin. The relative humidity above the soil surface in equilibrium with the soil moisture has been studied for both hydrophobic and hydrophilic soil types. Our studies confirm that the percentage of water available to plants is greater in hydrophobic soil rather than in hydrophilic soil, which has also been corroborated using simulation studies. Furthermore, the requirement of hydration in human skin has also been evaluated by measuring the water potential of both dry and moist skin.

Direct Observation of Coupling between Structural Fluctuation and Ultrafast Hydration Dynamics of Fluorescent Probes in Anionic Micelles

The coupling of structural fluctuation and the dynamics of associated water molecules of biological macromolecules is vital for various biological activities. Although a number of molecular dynamics (MD) studies on proteins/DNA predicted the importance of such coupling, experimental evidence of variation of hydration dynamics with controlled structural fluctuation even in model macromolecule is sparse and raised controversies in the contemporary literature. Here, we have investigated dynamics of hydration at the surfaces of two similar anionic micelles sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) as model macromolecules using coumarin 500 (C500) as spectroscopic probe with femtosecond to picosecond time resolution up to 20 ns time window. The constituting surfactants SDS and SDBS are structurally similar except one benzene moiety in the SDBS may offer additional rigidity to the SDBS micelles through π-stacking and added bulkiness. The structural integrity of the micelles in the aqueous medium is confirmed in dynamic light scattering (DLS) studies. A variety of studies including polarization gated fluorescence spectroscopy and quasielastic neutron scattering (QENS) have been used to confirm differential structural fluctuation of SDS and SDBS micelles. We have also employed femtosecond-resolved Förster resonance energy transfer (FRET) in order to study binding of a cationic organic ligand ethidium bromide (EtBr) salt at the micellar surfaces. The distance distribution of the donor (C500)-acceptor (EtBr) in the micellar media reveals the manifestation of the structural flexibility of the micelles. Our studies on dynamical coupling of the structural flexibility with surface hydration in the nanoscopic micellar media may find the relevance in the "master-slave" type water dynamics in biologically relevant macromolecules.

Effect of hydrophobic interaction on structure, dynamics, and reactivity of water

The effect of hydrophobic interaction on water is still controversial and requires more detailed experimental and theoretical investigation. The interaction between organic-water molecular complexes might be indicative of the perturbation of hydrogen-bond network in the tetrahedral structure of bulk waters, due to hydrophobic effect. In this contribution, femto/picosecond-resolved solvation dynamics techniques have been adopted to explore the dynamical modification of water clusters in hydrophobic solvent methyl tert-butyl ether (MTBE). The dynamical evolution of water molecules at the surface of micelle-like MTBE has also been studied. Dynamic light scattering techniques have been employed to determine the size of the molecular clusters being formed in respective solvents. Fourier transform infrared (FTIR) spectroscopy well measures the changes in O-H vibration frequency of water induced by MTBE. We have also monitored temperature dependent picosecond-resolved solvation dynamics in order to explore the energetics associated with water solvation in bulk MTBE. Using detailed ab initio calculations at the MP2 level, our study attempts to predict the possible structures, energies, and thermochemical parameters of corresponding MTBE-water molecular complexes in more detail. The chemical reactivity of water further confirms the effect of the hydrophobic interaction on water molecules. The results impart an understanding on hydrophobic interaction imposed by a biomolecule on the structure and reactivity of water, significant for the in vivo cellular condition.

A molecular picture of diffusion controlled reaction: role of microviscosity and hydration on hydrolysis of benzoyl chloride at a polymer hydration region

In this study, we have attempted to explore the molecular mechanism associated with a diffusion controlled reaction at a polymer hydration region by monitoring temperature-dependent solvolysis reaction of benzoyl chloride (BzCl) in water-poly(ethylene glycol) mixture at low water concentration. BzCl being highly hydrophobic resides in the vicinity of the PEG surface and the reaction takes place at the interface. Temperature-dependent solvolysis allows one to estimate the overall Arrhenius type activation energy barrier associated with the reaction. To understand the relative contribution of hydration and diffusive motion on the overall activation energy we studied the temperature-dependent picosecond-resolved solvation dynamics using a fluorescence probe Coumarin 500 (C500). The observed acceleration of solvation dynamics with temperature finds its origin in temperature-induced transition of bound to free type interfacial water molecules near the PEG surface. Temperature-dependent acoustic and densimetric studies also support this phenomenon. The temperature-induced enhancement of the local viscosity experienced by the probe, which is calculated from the rotational anisotropy studies, furnishes the activation barrier for microviscosity as applicable to the Kramers model. The activation energy barriers estimated from the temperature-dependent solvation dynamics and microviscosity studies are correlated with that obtained from the solvolysis reaction.