Tyrosyl radical, diiron center and enzyme mechanism in ribonucleotide reductase

In this minireview we will describe work where electron paramagnetic resonance spectroscopy has been used as the major methodology to identify and study the function of an amino acidbased free radical in enzyme function. For many redox processes in biochemistry the interplay between redox-active metal ions and free radicals turns out to be a recurring theme. Seen in this perspective, the enzyme we will describe here, ribonucleotide reductase, is related to several other important enzyme systems, including photosystem II.     

Oxidation-reduction potential of soluble and membrane-bound rabbit liver microsomal cytochrome P-450 LM2

The redox midpoint potentials of rabbit liver microsomal cytochromes P-450 and of soluble and membrane-bound rabbit liver microsomal cytochrome P-450 LM2 were determined using EPR-spectroscopy and absorption difference spectrometry with NADPH or dithionite as reductants. Using EPR, a redox midpoint potential of -0.36 V was obtained both for the low spin and the high spin components of microsomal cytochrome P-450. Spectrophotometrical determinations yielded very similar values: -0.37 V and -0.34 V for the low and high spin signals, respectively. Soluble cytochrome P-450 LM2 had a midpoint potential of -0.32 V. This redox potential was not significantly affected by incorporation of the protein into an artificial membrane structure or, furthermore, by the presence of cytochrome b5 the same membrane.     

Intentions-to-Buy and Claimed Brand Usage

Consumers' expressed Intentions-to-buy different brands are frequently measured in marketing research. Study of several hundred surveys shows that a relationship between the percentage I of consumers expressing such an Intention-to-buy and the percentage U claiming current Usage of the brand can be represented by a single one-parameter relationship I = K √U ± 3. This holds for different brands (large and small) in over 20 product-fields, for different demographic subgroups, for certain different definitions of the Intentions and Usage variables, for some American as well as British data, and even for trends over time.Both the nature of this relationship between I and U and the paradoxical finding that relatively few people express an Intention-to-buy a new brand which is sub-sequently successful and that relatively many people do so for a brand which is slowly dying are explained by the effect of past Usage. Methodological conclusions of the findings are indicated.     

Li8Cu12+xAl6−x (x = 1.16): a new structure type related to Laves phases

The new ternary lithium copper aluminide Li₈Cu_12+xAl_6−x (x = 1.16) crystallizes in the P6₃/mmc space group with six independent atom positions of site symmetries m. (Al/Cu mixture), m2 (Li atoms), 3m. (Al/Cu mixture and Li atoms) and .m. (Cu atoms). The compound is a derivative of the K₇Cs₆ binary structure type and is related to the binary MgZn₂ Laves phase and the LiCuAl₂, MgCu_1.07Al_0.93 and Mg(Cu_1−xAl_x)₂ (x = 0.465) ternary Laves phases. The coordination polyhedra of the atoms in this structure are icosahedra (Cu atoms), slightly distorted icosahedra and bicapped hexagonal antiprisms (Al/Cu statistical mixture), and Frank–Kasper and distorted Frank–Kasper polyhedra (Li atoms). All interatomic distances indicate metallic type bonding.     

Proton transfer in hydrogen-bonded pyridine/acid systems: the role of higher aggregation

In this work the role of higher molecular aggregation in the proton transfer processes within hydrogen bond (H-bond) is investigated. The H-bonded complex consisting of 4-cyanopyridine (CyPy) with trichloroacetic acid (TCA) has been studied in the solutions of acetonitrile, carbon tetrachloride, chloroform and dichloroethane as solvent by FTIR spectroscopy and quantum chemical DFT calculations. In order to illustrate the effect of increasing H-bond strength FTIR investigations have also been performed on solutions of CyPy with H(2)O, acetic-, trifluoroacetic- and methanesulfonic acids. Proton states in the H-bond have been monitored using vibrational CyPy ring modes in FTIR spectra. The stabilization of the CyPy/TCA complex in its protonated form upon increasing polarity of the solvent has been evidenced. It was shown that formation of the CyPy/(TCA)(2) aggregates in the solutions favors the proton transfer process. An X-ray diffraction study has been performed on a single 1 : 2 co-crystal of pyridine/3,5-dinitrobenzoic acid. The H-bond motif found in this system exhibits the same connectivity by strong hydrogen bonds N-H(+)[dot dot dot]O(-) and O-H[dot dot dot]O as that in the CyPy/(TCA)(2) complex predicted by DFT calculation. Certain discrepancies are observed in C-H[dot dot dot]O connectivity only. The networks of H-bonds in both assemblies differ from those usually pictured for 1 : 2 base/carboxylic acid complexes in the literature.     

Evidence for a cubic-to-icosahedral transition of quasi-free Pd-H-clusters controlled by the hydrogen content

An in situ synchrotron radiation study of quasi-free five nanometer-sized palladium clusters during hydrogen absorption is combined with molecular dynamics simulations to investigate the structural development. In the diffraction patterns, strong intensity changes are found that provide evidence for a structural phase transformation that is significantly different from the α--Pd-H bulk phase transition. The structural transition is reversible and driven by the hydrogen concentration. The intensity changes are consistent with a cubic-to-icosahedral structural phase transition obtained in molecular dynamical simulations using embedded-atom-method potentials. Received 15 October 2001 and Received in final form 7 February 2002