• Biman Jana

      Articles written in Journal of Chemical Sciences

    • Orientational dynamics and energy landscape features of thermotropic liquid crystals: An analogy with supercooled liquids

      Biman Jana Biman Bagchi

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      Recent optical kerr effect (OKE) studies have revealed that orientational relaxation of rodlike nematogens near the isotropic-nematic (I-N) phase boundary and also in the nematic phase exhibit temporal power law decay at intermediate times. Such behaviour has drawn an intriguing analogy with supercooled liquids. Here, we have investigated the single-particle and collective orientational dynamics of a family of model system of thermotropic liquid crystals using extensive computer simulations. Several remarkable features of glassy dynamics are on display including non-exponential relaxation, dynamical heterogeneity, and non-Arrhenius temperature dependence of the orientational relaxation time. Over a temperature range near the I-N phase boundary, the system behaves like a fragile glass-forming liquid. Using proper scaling, we construct the usual relaxation time versus inverse temperature plot and explicitly demonstrate that one can successfully define a density dependent fragility of liquid crystals. The fragility of liquid crystals shows a temperature and density dependence which is remarkably similar to the fragility of glass forming supercooled liquids. Energy landscape analysis of inherent structures shows that the breakdown of the Arrhenius temperature dependence of relaxation rate occurs at a temperature that marks the onset of the growth of the depth of the potential energy minima explored by the system.

    • Hydration dynamics of protein molecules in aqueous solution: Unity among diversity

      Biman Jana Subrata Pal Biman Bagchi

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      Dielectric dispersion and NMRD experiments have revealed that a significant fraction of water molecules in the hydration shell of various proteins do not exhibit any slowing down of dynamics. This is usually attributed to the presence of the hydrophobic residues (HBR) on the surface, although HBRs alone cannot account for the large amplitude of the fast component. Solvation dynamics experiments and also computer simulation studies, on the other hand, repeatedly observed the presence of a non-negligible slow component. Here we show, by considering three well-known proteins (lysozyme, myoglobin and adelynate kinase), that the fast component arises partly from the response of those water molecules that are hydrogen bonded with the backbone oxygen (BBO) atoms. These are structurally and energetically less stable than those with the side chain oxygen (SCO) atoms. In addition, the electrostatic interaction energy distribution (EIED) of individual water molecules (hydrogen bonded to SCO) with side chain oxygen atoms shows a surprising two peak character with the lower energy peak almost coincident with the energy distribution of water hydrogen bonded to backbone oxygen atoms (BBO). This two peak contribution appears to be quite general as we find it for lysozyme, myoglobin and adenylate kinase (ADK). The sharp peak of EIED at small energy (at less than 2 $k_{B}T$) for the BBO atoms, together with the first peak of EIED of SCO and the HBRs on the protein surface, explain why a large fraction (∼ 80%) of water in the protein hydration layer remains almost as mobile as bulk water. Significant slowness arises only from the hydrogen bonds that populate the second peak of EIED at larger energy (at about 4 k$_B$T). Thus, if we consider hydrogen bond interaction alone, only 15-20% of water molecules in the protein hydration layer can exhibit slow dynamics, resulting in an average relaxation time of about 5-10 ps. The latter estimate assumes a time constant of 20-100 ps for the slow component. Interestingly, relaxation of water molecules hydrogen bonded to back bone oxygen exhibit an initial component faster than the bulk, suggesting that hydrogen bonding of these water molecules remains frustrated. This explanation of the heterogeneous and non-exponential dynamics of water in the hydration layer is quantitatively consistent with all the available experimental results, and provides unification among diverse features.

    • Size and Structure of Cytochrome-c bound to Gold nano-clusters: Effect of Ethanol

      CATHERINE GHOSH M D ASIF AMIN BIMAN JANA KANKAN BHATTACHARYYA

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      Size and structure of cytochrome c (Cyt C) bound to gold nano-clusters (AuNC) were studied using fluorescence correlation spectroscopy (FCS) and circular dichroism (CD) spectroscopy. The CD spectra of Cyt C indicate that the ellipticity is almost completely lost on binding to AuNC which indicates unfolding.Addition of ethanol causes partial restoration of ellipticity and hence, structure of Cyt C. FCS data indicate that size (hydrodynamic radius, rH) of free Cyt C is 17Å which increases to 24Å on binding to AuNC. This too suggests unfolding of Cyt C upon binding to AuNCs. Both the size and conformational relaxation time of Cyt C bound to AuNC vary non-monotonically with increase in ethanol content.

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