Energetic studies on DNA-peptide interaction in relation to the enthalpy-entropy compensation paradox.

This study aims to interpret the energetic basis of complex DNA-peptide interactions according to a novel allosteric interaction network approach. In common with other designed peptides, five new conjugates incorporating the XPRK or XHypRK motif (Hyp = hydroxyproline) attached to a N-methylpyrrole (Py) tract with a basic tail have been found to display cooperative binding to DNA involving multiple monodentate as well as interstrand bidentate interactions. Using quantitative DNase I footprinting it appears that allosteric communication via cooperative binding to multiple sites on complementary DNA strands corresponds to two different types of DNA-peptide interaction network. Temperature variation experiments using a dodecapeptide RY-12 show that lower temperature (25 °C) favor a circuit type of allosteric interaction network, whereas higher temperatures (31 and 37 °C) afford only a partial-circuit type of network. Circular dichroism studies show that our five peptides induce significant local conformational changes in DNA via the minor groove, with apparently dimeric binding stoichiometry. Isothermal titration calorimetry reveals that these peptides, together with another seven for comparison, are strongly exothermic upon binding to a model 13-mer DNA duplex, characterized by ΔH ranging from -14.7 to -74.4 kcal mol(-1), and also high TΔS ranging from -6.5 to -65.9 kcal mol(-1). Multiple monodentate and bidentate interactions, as well as ionic forces that mediate positive cooperativity in sequence recognition, are consistent with a dramatic decrease in entropy and a 'tightening' effect of DNA conformation. Distinctive enthalpy-entropy compensation (EEC) relationships are demonstrated for the interaction of all twelve designed peptides with DNA, affording a straight line of slope close to unity when ΔH is plotted versus TΔS, with a y-axis intercept (average ΔG) corresponding to -8.5 kcal mol(-1), while the observed ΔG ranges from -8.2 to -9.1 kcal mol(-1) for the peptides. The EEC seen with peptide RY-12 binding to the model duplex persists throughout various incubation temperatures. The net compensation of energy between the favorable negative ΔH and unfavorable negative ΔS components thus constrains the value of net binding free energy ΔG within a remarkably constant range, as is clearly visible in a 3-dimensional energetic plot. We conclude that the preservation of a rather narrowly-defined ΔG value is central to the EEC in DNA-peptide interactions, illuminating the universal EEC paradox commonly found in diverse biochemical reactions.

Liquid crystals of gold(I) N-heterocyclic carbene complexes.

Three types of Au(I) N-heterocyclic carbene (NHC) and imidazole (Im) complexes, namely [Au(NHC)Cl], [Au(NHC)(Im)][NO(3)], and [Au(NHC)(2)][NO(3)], with two-, three-, and four-N-long alkyl chains, respectively, were synthesized and their mesomorphic properties investigated. Only the [Au(NHC)(Im)][NO(3)] series of compounds, adopting an m-shaped conformation in the solid state, showed liquid crystalline properties. For the molecules with three alkyl chains in particular, the Coulombic and hydrogen bonding interactions between the cationic head core and anionic NO(3)(-), and the hydrophobic chain-chain interactions facilitate in sustaining the partially ordered motion, which is not observed for those with two and four arms. These complexes were employed for preparation of stable nanomaterials in organic solvents.