Evidence of entropically driven C60 fullerene aggregation in aqueous solution.

In the present work, we report the first experimental evidence of entropically driven C60 fullerene aggregation in aqueous solution, occurring with nearly zero enthalpy change.

C60 fullerene aggregation in aqueous solution.

In the present work we develop a novel approach for quantification of the energetics of C60 fullerene aggregation in aqueous media in terms of equilibrium aggregation constant KF. In particular, it is shown that the experimental determination of the magnitude of KF is possible only within the framework of the 'up-scaled aggregation model', considering the C60 fullerene water solution as a solution of fullerene clusters. Using dynamic light scattering (DLS) data we report the value, K(F) = 56,000 M(-1), which is in good agreement with existing theoretical estimates and the results of energetic analyses. It is suggested that the proposed 'up-scaled model' may be used in any instances of non-specific aggregation resulting in formation of large spherical particles. The measurement of the translational diffusion coefficient and the dimensions of the light scattering particles using a DLS approach with respect to C60 fullerene aggregates is found to contain significant systematic errors originating from the interaction effect that is well-known for micellar solutions. As a result, corrections to the equations associated with DLS data are proposed.

Energetics of ligand binding to the DNA minor groove.

In the present work the decomposition of the total Gibbs free energy of ligand-DNA binding onto various physical terms was accomplished for the group of nine DNA minor groove binders (MGB ligands) differing in both structure and charge state. The decomposition protocol includes the analysis of the most complete set of physical factors known to contribute to the complexation process, viz. the net change in the number of degrees of freedom (translational, rotational, vibrations of the chemical bonds and vibrations of the ligand as a whole within the binding site), the conformational entropy, van der Waals, electrostatic and hydrophobic interactions, the polyelectrolyte contribution and the net effect of changes in the number of hydrogen bonds. All of these processes are further decomposed into the interaction with the solvent and the interaction of the ligand with DNA. The principal outcome of the decomposition is the possibility of performing a comparative analysis of the energetic contribution of various physical terms and provide an answer to the question concerning what physical factors stabilize or destabilize the complexes of MGB ligands with DNA.

Descriptions of three new species of Dzhanokmenia (Hymenoptera: Eulophidae) from China.

Three new species of Dzhanokmenia Kostjukov (Hymenoptera: Eulophidae: Tetrastichinae), D. muleica Li, Wang & Hu sp. n., D. karamayica Li, Wang & Zhu sp. n. and D. gobica Li, Wang & Zhu sp. n. from Xinjiang Uyghur Autonomous Region of China are described and illustrated. A key to all known species of the genus is provided.

Spectroscopic study of proflavine adsorption on the carbon nanotube surface.

Despite the fact that non-covalent interactions between various aromatic compounds and carbon nanotubes are being extensively investigated now, there is still a lack of understanding about the nature of such interactions. The present paper sheds light on one of the possible mechanisms of interaction between the typical aromatic dye proflavine and the carbon nanotube surface, namely, π-stacking between aromatic rings of these compounds. To investigate such a complexation, a qualitative analysis was performed by means of ultraviolet visible, infrared, and nuclear magnetic resonance spectroscopy. The data obtained suggest that π-stacking brings the major contribution to the stabilization of the complex between proflavine and the carbon nanotube.

Relation between the change in DNA elasticity on ligand binding and the binding energetics.

The widespread use of tweezers for measurement of ligand-DNA binding parameters is based on the McGhee-von Hippel treatment of the DNA contour and persistence length as a function of concentration. The McGhee-von Hippel approach contains the basic assumption that the binding constant K is independent of the number of already bound ligands. However, the change in elasticity of DNA on binding affects the entropic part of the Gibbs free energy and, hence, the K value in a concentration-dependent manner, making the whole approach inconsistent. In the present work we show that the energetic effect of DNA stiffening on noncovalent binding of small ligands is negligible with respect to the net energy of reaction, whereas the DNA stiffening on binding of large ligands must always be considered in each particular case.

Hexamer oligonucleotide topology and assembly under solution phase NMR and theoretical modeling scrutiny.

The entire family of noncomplementary hexamer oligodeoxyribonucleotides d(GCXYGC) (X and Y = A, G, C, or T) were assessed for topological indicators and equilibrium thermodynamics using a priori molecular modeling and solution phase NMR spectroscopy. Feasible modeled hairpin structures formed a basis from which solution structure and equilibria for each oligonucleotide were considered. ¹H and ³¹P variable temperature-dependent (VT) and concentration-dependent NMR data, NMR signal assignments, and diffusion parameters led to d(GCGAGC) and d(GCGGGC) being understood as exceptions within the family in terms of self-association and topological character. A mean diffusion coefficient D(298 K) = (2.0 ± 0.07) × 10⁻¹° m² s⁻¹ was evaluated across all hexamers except for d(GCGAGC) (D(298 K) = 1.7 × 10⁻¹° m² s⁻¹) and d(GCGGGC) (D(298 K) = 1.2 × 10⁻¹° m² s⁻¹). Melting under VT analysis (T(m) = 323 K) combined with supporting NMR evidence confirmed d(GCGAGC) as the shortest tandem sheared GA mismatched duplex. Diffusion measurements were used to conclude that d(GCGGGC) preferentially exists as the shortest stable quadruplex structure. Thermodynamic analysis of all data led to the assertion that, with the exception of XY = GA and GG, the remaining noncomplementary oligonucleotides adopt equilibria between monomer and duplex, contributed largely by monomer random-coil forms. Contrastingly, d(GCGAGC) showed preference for tandem sheared GA mismatch duplex formation with an association constant K = 3.9 × 105M⁻¹. No direct evidence was acquired for hairpin formation in any instance although its potential existence is considered possible for d(GCGAGC) on the basis of molecular modeling studies.