Protein complexation with DNA phosphates as a cause for DNA duplex destabilization : a thermodynamic model

Complexation of positively charged sites in a protein with the negative DNA phosphate groups shields the phosphate charges. This diminishes interstrand electrostatic repulsions, which stabilizes the duplex. When phosphate shidlding is present in one DNA strand only, the conformation of this strand changes due to a decrease of intrastrand phosphate-phosphate repulsions. This destabilizes the duplex since then the strands differ in conformation. A thermodynamic model is formulated to describe this stabilization/destabilization effect in terms of changed enthalpies and entropies of hybridization. It is found that protein complexation with one DNA strand can indeed lower the TM value of a duplex. The model is applied to the action of helicases (replication), RNA polymerases (transcription), and restriction endonucleases. Mechanisms with unilateral charge shielding are proposed for their duplex-destabilizing properties

Three coordination modes of the pentadentate ligand 2,6-diacetylpyridinedisemicarbazone

The pentadentate ligand 2,6-diacetylpyridinedisemicarbazone, DAPSC, reacts with Cr(NO3)3·9H2O and forms two kinds of complexes. At pH=3, the ligand is singly-deprotonated and crystals of [Cr- (DAPSC---H)(H2O)2](NO3)2·H2O (Ia) are obtained. Evaporation of a solution at pH=0, yields crystals of [Cr(DAPSC)(H2O)2](NO3)3·2H2O (II) in which the ligand is fully protonated. The reaction of DAPSC with UO2(O2CCH3)2 in methanol, followed by crystallization of the product from DMSO yields crystals of [UO2(DAPSC---2H)(H2O)]·2DMSO (III) in which the ligand is fully deprotonated. Compound Ia is monoclinic, space group P21/n with a=11.746(1), b=14.752(2), c=11.866(1) Å,ß=105.53(2)°, V= 1981(1) Å3 and Z=4. Compound II is monoclinic, space group, P21/n with a=38.000(3), b= 14.939(2), c=8.233(1) Å, ß=96.12(2)°, V= 4647(1) Å and Z=8. Compound III is monoclinic, space group P21/n with a=18.048(2), b=15.207(2), c=8.842(1) Å,ß=97.72(2)°, V=2405(1) Å3 and Z=4. The structures were refined using 2084, 4169 and 2516 reflections to R values of 4.4%, 7.8% and 4.8% respectively

Chirality in pyridoxal

Pyridoxal and its iminium derivatives form cyclic compounds via an intramolecular nucleophilic attack. This results in a configurational chirality on C4' which can be related to out-of-plane orientations around the C4C4' bond in the open forms. 1H- and 13C-NMR structural studies of the cyclic compounds were performed in solution and in the solid state (13C NMR only). It was found that the presence of an additional chiral centre in the iminium moiety gives rise to an excess of one of the diastereomers for the cyclic aminal. This indicates a preference for one of the axially chiral C4-C4' conformers prior to ring closure. The present results may be of relevance with respect to the function of the coenzyme pyridoxal phosphate, which forms iminium compounds with l or d amino acids. Axial chirality in these systems has been proposed by us as a mechanism for the stereospecificity of enzymatic amino acid transformations

Star-shaped poly(2,6-dimethyl-1,4-phenylene ether)

The properties of star-shaped poly(2,6-dimethyl-1,4-phenylene ether) (PPE) as prepared by the redistribution of PPE and tyrosine-modified poly(propylene imine) dendrimers, are studied in solution and in 50/50 wt% blends with linear polystyrene. Star polymers with constant armlength but increasing number of arms show the same hydrodynamic volume as measured by Size Exclusion Chromatography (SEC), but decreasing hydrodynamic radius as measured by Dynamic Light Scattering (DLS). This is caused by the restricted mobility of the more densely packed chains at high numbers of arms, also leading to a decrease in intrinsic viscosities. These solution properties are also reflected in the miscibility behaviour in polymer blends. Star-shaped polymers with a high number of PPE arms (16,32 or 64 respectively) give inhomogeneous blends with linear polystyrene, in contrast to the miscible combination of linear polystyrene with linear PPE or starshaped polymers with a low number (4 or 8) of PPE arms

Compositional analysis of the leaf oils of Piper callosum Ruiz & Pav. from Peru and Michelia montana Blume from India

The leaf oils of Piper callosum from Peruvian Amazon and Michelia montana from Assam, India, were prepared by hydrodistillation and analyzed by a combination of GC and GC/MS. Twenty five and thirty components have been identified, representing 96.3 and 100.0% of the respective oils. The major constituents were found to be asaricin (syn. sarisan) (35.9 and 81.8%, respectively) and safrole (20.2 and 13.0%). The oil of P. callosum contained in addition eugenyl methyl ether (9.7%) and (E)-asarone (7.8%), compounds not detected in the M. montana oil. The identity of the principal compound, an isomer of myristicin, was unequivocally established by 13C-NMR spectrometric techniques, especially long-range 1H–13C correlation