Second-Coordination Sphere Effects on Selectivity and Specificity of Heme and Nonheme Iron Enzymes

Mononuclear iron-containing enzymes are highly versatile oxidants that often react stereospecifically and/or regioselectively with substrates. Combined experimental and computational studies on heme monooxygenases, nonheme iron dioxygenases and halogenases have revealed the intricate details of the second-coordination sphere, which determine this specificity and selectivity. These second-coordination sphere effects originate from the positioning of the substrate and oxidant, which involve the binding of the co-factors and substrate into the active site of the protein. In addition, some enzymes affect the selectivity and reactivity through charge-stabilization from nearby bound cations/anions, an induced electric field or through the positioning of salt bridges and hydrogen-bonding interactions to first-coordination sphere iron ligands and/or the substrate. Examples of all of these second-coordination sphere effects in iron-containing enzymes and how these influence structure and reactivity are given.     

Experimental investigation of a random repeated impact process

A simple random repeated impact process which has previously been investigated theoretically has now been investigated experimentally. The process, which consists of a ball bouncing on a randomly vibrating surface, is analogous to loss of contact situations which can occur in machinery and transportation systems where a hard rolling element separates from the rolling surface. Experimental data was acquired and processed by using a digital data acquisition system and associated software. The results were obtained in the form of histograms which could be directly compared with the predicted probability density functions. The agreement between the predicted behaviour and the measured results was excellent, and confirmed the dependence of the behaviour of the random process on the coefficient of restitution and the excitation level. Also, it was verified that the probability density function of table velocities at impact was distinct from the Gaussian density function of the excitation. Consequently the theoretical analysis previously developed has been validated with sufficient confidence to enable it to be applied to predictions of acoustic noise generated by the impact process.     

Absolute fixed point sets for continuum-valued maps

The notion of an absolute fixed point set in the setting of continuum-valued maps will be defined and characterized.

     

A fluidized bed process for electron sterilization of powders

A small capacity (100 g.s⁻¹) pilot system is described for presentation of powders and fine aggregates at high velocity, to an electron beam. Electron beam dose rate is continuously monitored in real time, while the thickness of the fluidized bed used to pneumatically transport the product can be monitored and controlled using beta-gauge techniques. Using electron paramagnetic resonance (EPR) techniques, alanine power mixed with the product is used for precise determination of dose delivered to the powder stream. Thin film dosimeters transported in the bed are also used for dose determination. Results with a variety of products are presented using both dose rate and velocity as the independent variables. Lethality data for the bioburdens present in several powdered foodstuffs are discussed.     

Characterization and properties of low-linear-density polyethylene/Typha latifolia composites

The characterization of the effects of different sizes and loadings of Typha latifolia on the tensile, thermal, and morphological properties of linear-low-density polyethylene (LLDPE)/T. latifolia composites were evaluated using tensile test, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy analyses. Results indicated the tensile strength and elongation at break decreased as T. latifolia loading increased. However, T. latifolia fine size (fs) exhibited better tensile properties than coarse size at the same loading in the composite. T. latifolia (15%, fs) recorded higher thermal stability than LLDPE control and other T. latifolia loadings and sizes due to the strong interaction of T. latifolia (15%, fs) in LLDPE matrix as shown in morphology.     

High electrical conductivity for the partially oxidised d8 Ru–Ru bonded chain [Ru(bpy)(CO)2]n (bpy = 2,2′-bipyridine) and its wide range redox control

High, electrochemically controlled electrical conductivity (0.2–0.3 S cm^?1) was measured for the first time for the metal–metal bonded polymeric compound [Ru(bpy)(CO)₂]_n (bpy = 2,2′-bipyridine). The conductivity depended on the degree of partial oxidation of the initially zero-valent Ru to Ru^I in the chains and was found to increase by four orders of magnitude after 5% oxidative doping. Linear conductance vs. doping level dependence was found for low levels and the conductivity of films could repeatedly be switched on and off. The conduction process is ascribed to charge delocalisation on the Ru chains in analogy to that of other similar chains formed by stacking of planar transition metal d⁸ complexes.     

NMR method for the determination of solute hydrogen bond acidity

It is shown that the difference in the 1H NMR chemical shift of a protic hydrogen in DMSO and CDCl3 solvents is directly related to the overall, or summation, hydrogen bond acidity for a wide range of solutes. This provides a new and direct method of measuring the hydrogen bond acidity. For 54 compounds, the observed shifts for 72 protic hydrogens could be correlated to the Abraham solute hydrogen bond acidity parameter, A, with a correlation coefficient squared, R2, of 0.938 and a standard deviation, SD, of 0.054 units in A. A training equation that used half the data could predict A values for the remaining data with an average error of 0.001 and a standard deviation, SD, of 0.053 units, thus demonstrating the predictive power of the method. Unlike any previous method for the determination of solute hydrogen bond acidities, the NMR method allows the determination of A values for individual protic hydrogens in multifunctional solutes.     

Determination of palladium in biological samples by neutron-activation analysis

Summary A neutron-activation method for determination of nanogram levels of palladium in biological samples is described. It is based on radiochemical separation of 13.5-hr¹⁰⁹Pd by solvent extraction of its dimethylglyoximate complex into chloroform from the acid solution obtained by wet ashing of the irradiated sample. The effect of the acidity of the aqueous phase on the extraction coefficient was also investigated by radiotracer experiments. Results are reported for biological standard reference materials and a few environmental samples.

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Bestimmung von Palladium in biologischen Proben durch Neutronen-Aktivierung

Zusammenfassung Eine Neutronen-Aktivierungsmethode zur Bestimmung von Nano-grammengen Palladium in biologischen Proben wurde beschrieben. Sie beruht auf der radiochemischen Trennung des¹⁰⁹Pd durch Extraktion seines Dimethylglyoxim-Komplexes mit Chloroform aus der sauren, durch nasse Veraschung gewonnenen Lösung der bestrahlten Probe. Der Einfluß der Acidität der wäßrigen Phase auf den Extraktionskoeffizienten wurde ebenfalls durch Radiotracer-Versuche geprüft. Untersuchungsergebnisse für biologische Standardproben und einige Milieuproben wurden mitgeteilt.

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Presented at the 8th International Microchemical Symposium, Graz, August 25–30, 1980.     

Mutagenesis of morphinone reductase induces multiple reactive configurations and identifies potential ambiguity in kinetic analysis of enzyme tunneling mechanisms

We have identified multiple reactive configurations (MRCs) of an enzyme-coenzyme complex that have measurably different kinetic properties. In the complex formed between morphinone reductase (MR) and the NADH analogue 1,4,5,6-tetrahydro-NADH (NADH4) the nicotinamide moiety is restrained close to the FMN isoalloxazine ring by hydrogen bonds from Asn-189 and His-186 as determined from the X-ray crystal structure. Molecular dynamic simulations indicate that removal of one of these hydrogen bonds in the N189A MR mutant allows the nicotinamide moiety to occupy a region of configurational space not accessible in wild-type enzyme. Using stopped-flow spectroscopy, we show that reduction of the FMN cofactor by NADH in N189A MR is multiphasic, identifying at least four different reactive configurations of the MR-NADH complex. This contrasts with wild-type MR in which hydride transfer occurs by environmentally coupled tunneling in a single kinetic phase [Pudney et al. J. Am. Chem. Soc. 2006, 128, 14053-14058]. Values for primary and alpha-secondary kinetic isotope effects, and their temperature dependence, for three of the kinetic phases in the N189A MR are consistent with hydride transfer by tunneling. Our analysis enables derivation of mechanistic information concerning different reactive configurations of the same enzyme-coenzyme complex using ensemble stopped-flow methods. Implications for the interpretation from kinetic data of tunneling mechanisms in enzymes are discussed.