Cross-catalytic replication of an RNA ligase ribozyme

A self-replicating RNA ligase ribozyme was converted to a cross-catalytic format whereby two ribozymes direct each other's synthesis from a total of four component substrates. Each ribozyme binds two RNA substrates and catalyzes their ligation to form the opposing ribozyme. The two ribozymes are not perfectly complementary, as is the case for replicating nucleic acid genomes in biology. Rather, the ribozymes contain both template elements, which are complementary, and catalytic elements, which are identical. The specificity of the template interactions allows the cross-catalytic pathway to dominate over all other reaction pathways. As the concentration of the two ribozymes increases, the rate of formation of additional ribozyme molecules increases, consistent with the overall autocatalytic behavior of the reaction system.     

Additivity and the physical basis of multivalency effects: a thermodynamic investigation of the calcium EDTA interaction

To better understand the origin of multivalency effects in ligand binding, the binding of a series of mono-, bi-, tri- and tetravalent carboxylate ligands to Ca(II) was examined by isothermal titration calorimetry (ITC). The data are inconsistent with an entropic origin of enhanced affinity, but rather show that at least in this instance the multivalency effect is enthalpic in origin. Analysis of binding data using the Jencks model shows the addition of incremental carboxylate "ligands" produces an unfavorable interaction entropy that is more than offset by a strongly favorable interaction enthalpy. The most likely source of this interaction enthalpy is the relief of repulsive Coulombic interactions in the unbound state. The conformational entropy penalty arising from the restriction of flexible dihedrals is negligible, within experimental error. On the other hand, an enthalpic contribution from linker restriction contributes strongly to the overall thermodynamics of ligand binding. Together, these data suggest that enthalpic effects dominate ligand binding, and design strategies should seek to optimize these interactions. The incorporation of unfavorable interactions in the unbound ligand that are relieved during binding provides an important mechanism by which to enhance ligand affinities.     

Molecular calculations with the nonempirical ab initioMODPOT,VRDDO, and MODPOT/VRDDO procedures. XI. Theoretical study of the [C6H5OH ⃛OC6H5]− molecular complex: Ab initioMODPOT/VRDDO calculations and electrostatic molecular potential contour maps

The transport of C₆H₅O^? (or similarly charged moieties) through a lipoidal membrane may possibly be facilitated by forming complexes with the neutral compound. Thus, theoretical studies were performed on the model [C₆H₅OH ?OC₆H₅]^? molecular complex to obtain some information concerning the possible molecular and electronic structure of such complexes. Ab initio MODPOT /VRDDO SCF calculations were carried out on the neutral-anion dimer [C₆H₅OH ?OC₆H₅] to optimize the equilibrium geometry. Electrostatic molecular potential contour maps have been generated from the ab initio MODPOT /VRDDO results in the molecular plane and in the plane perpendicular to the molecular plane and intersecting the hydrogen bond O ?H? O. Difference maps have also been generated showing the change of potential on complex formation. There is a decrease of electrostatic interactions of the phenoxide anion upon complex formation with the neutral phenol. Counterpoise corrections for basis set size could not be made since calculation of the phenoxide anions in the basis set of the phenol plus the phenoxide anion led to an excited state for the phenoxide anion. This behavior is somewhat similar to that occurring in the stabilization method for excited states of negative ions as the size of the basis set is increased.     

Procedure supplementing SCF interaction energies by dispersion term evaluated in dimer basis set within variation-perturbation approach

A variation-perturbation procedure for the evaluation of dispersion interaction, originally proposed by Jeziorski and van Hemert, has been reformulated to include basis set extension effects on an equal footing with the SCF interaction energy, corrected for basis set superposition error (BSSE). This approach has been tested for He₂, (H₂)₂, (H₂O)₂, and (C₂H₄)₂ complexes.The research was supported by ONR under contract N00014-80-C-0003 and in part by PAN within MR-I-9 project.     

RHEED studies of MOMBE growth using TMGa or TEGa with As2

MOMBE and CBE growth has until recently been based on largely empirical studies of the epitaxial process. We have used reflection high energy electron diffraction (RHEED), previously applied to the study of MBE, to study the growth GaAs using TMGa and As₂. In this work we have extended our previous studies to include a detailed study of the effect of As₂ flux on growth rate and to compare data on singular and vicinal plane surfaces cut off orientation in two orthogonal {110} directions. Clear evidence for site blocking mechanisms is observed together with an indication that the concentration of elemental Ga present on the surface during growth is negligible even under conditions where the arrival rate of TMGa exceeds that As₂. We have compared this behaviour with that observed using TEGa and As₂ under identical conditions. Using TEGa a conversion from a (2×4) to (4×2) reconstructed surface is observed under As deficient conditions indicating the presence of elemental Ga on the surface. This is accompanied by an abrupt change in growth rate similar to that secn in MBE.     

XeCl excimer laser assisted CBE growth of GaAs

The effect of 12 ns, 308 nm (XeCl) excimer laser pulses on the CBE growth rate of GaAs, at temperatures below the maximum non-laser assisted growth rate, G_max, has been studied as a function of laser fluence and repetition frequency. There is a threshold fluence for growth rate enhancement, above which the growth rate is dependent on repetition frequency, being restored to G_max at 20 Hz. The growth rate in the laser spot is measured by dynamic optical reflectivity (DOR).     

Effect of acetic acid on xylose conversion to ethanol by genetically engineeredE. coli

Applied Biochemistry and Biotechnology - Efficient utilization of the pentosan fraction of hemicellulose from lignocellulosic feedstocks offers an opportunity to increase the yield and to reduce...     

Spectrophotometric determination of baygon and carbaryl in air

A spectrophotometric method for the determination of baygon and carbaryl in air is based on the coupling of their hydrolysation products with diazotised p-amino-acetophenone. The dyes formed are measured at 580 nm and 555 nm, respectively. Beer's law is obeyed in the range of 0.9–5.6 mg/m³ and 0.4–3.6 mg/m³.