The pH-dependent role of superoxide in riboflavin-catalyzed photooxidation of 8-oxo-7,8-dihydroguanosine

[reaction: see text]. The riboflavin-catalyzed photooxidation of 2',3',5'-tri-O-acetyl-8-oxo-7,8-dihydroguanosine generates a radical intermediate that is competitively trapped by H(2)O, O2(-)(*), or O(2). The products of H(2)O trapping have been previously described as the spiroiminodihydantoin (pH >or= 7) and iminoallantoin/guanidinohydantoin (pH < 7) nucleosides. Trapping by O2(-)(*) leads to the oxaluric acid (pH or= 8.6) pathways (R' ', R' ' = H or 2,3,5-tri-O-Ac-ribofuranosyl). The pH-dependent role of superoxide was probed using Mn-SOD and compared to guanosine and 8-methoxyguanosine photooxidation.     

From oscillations to excitability: A case study in spatially extended systems

This volume is devoted to the presentation of the main contributions to the workshop "From oscillations to excitability: A case study in spatially extended systems," organized by the authors in Nice in June 1993. It gives an overview of the current research on spatiotemporal patterns in a wide range of systems that display self-oscillatory or excitable behavior. It tries to give a better understanding of the transition from the oscillatory to the excitable regime and of its effect on the properties of spiral waves, and to fill the gap between the theories and concepts used to describe both regimes in the so-called "active media."     

Tertiary phosphine induced migratory carbonyl insertion in cyclopentadienyl complexes of iron(II)

Cyclopentadienyldicarbonylmethyliron, [CpFe(CO)₂Me] (1), undergoes migratory carbonyl insertion under the influence of isosteric phosphine ligands P(4-FC₆H₄)₃ and P(4-MeC₆H₄)₃. The products of the reaction, [CpFe(CO)(COMe)P(4-FC₆H₄)₃] (2a) and [CpFe(CO)(COMe)P(4-MeC₆H₄)₃] (2b), were characterised by X-ray crystallography. In both structures, the iron atom adopts a pseudo octahedral coordination geometry. Fe-P bond distances are the same at 2.1932(8) Å in 2a and 2b, respectively. Thus, contrary to what was expected, X-ray data could not be used to quantitatively differentiate between the two phosphine ligands in 2a and 2b. Therefore, additional spectroscopic techniques such as IR and NMR were employed. Similarly, the Fe-C bond lengths of the carbonyl (Fe-CO) and acetyl (Fe-COMe) are 1.748(3) and 1.955(3) in 2a, and 1.744(3) and 1.951(3) Å in 2b, respectively.The migratory carbonyl insertion was studied by NMR, IR, and UV-vis spectroscopies to determine the mechanism and the rate law. Results from NMR spectroscopy show that the formation of the product is accompanied by oxidation of the corresponding phosphine ligand. An increase in the reactivity of migratory carbonyl insertion for P(4-MeC₆H₄)₃ was observed when the solvent was changed from CH₂Cl₂ to MeCN. The kinetic data showed that P(4-MeC₆H₄)₃ reacts faster than P(4-FC₆H₄)₃.