Thermal decomposition of di-tertiarybutyl selenide and dimethylzinc in a metalorganic vapour phase epitaxy reactor

The thermal decomposition of di-tertiarybutyl selenide (DtBSe), both alone and in the presence of dimethylzinc (DMZn) was investigated using ''ex-situ'' Fourier transform infrared (FTIR) absorption spectroscopy in a low-pressure metalorganic vapour phase epitaxy (LP-MOVPE) reactor. The decomposition of DtBSe alone, yields isobutene as the major product, with a much smaller proportion of isobutane detected. Pyrolysis of DMZn in dihydrogen in the presence of DtBSe is very similar to pyrolysis of DMZn alone in dihydrogen with methane the exclusive product. This indicates that co-pyrolysis of the DMZn/DtBSe mixture occurs via radical attack by H on DMZn and largely independent pyrolysis of DtBSe via a beta-hydrogen elimination reaction. Traces of the intermediate tertiarybutyl selenol ((t)BuSeH) were also detected, The small difference observed in the decomposition behaviour of the DtBSe-DMZn mixture in a dihydrogen compared to a helium ambient further indicate that the pyrolysis processes are independent. These conclusions are supported by PM3 semi-empirical molecular orbital calculations, which indicate that the most likely pathway for unimolecular dissociation of DtBSe is via beta-hydrogen elimination with C-Se bond homolysis only likely to be an effective competing mechanism at higher growth temperatures and reactor pressures.</p

Intraprotein radical transfer during photoactivation of DNA photolyase

International audienceAmino-acid radicals play key roles in many enzymatic reactions. Catalysis often involves transfer of a radical character within the protein, as in class I ribonucleotide reductase where radical transfer occurs over 35 Å, from a tyrosyl radical to a cystein. It is currently debated whether this kind of long-range transfer occurs by electron transfer, followed by proton release to create a neutral radical, or by H-atom transfer, that is, simultaneous transfer of electrons and protons. The latter mechanism avoids the energetic cost of charge formation in the low dielectric protein, but it is less robust to structural changes than is electron transfer. Available experimental data do not clearly discriminate between these proposals. We have studied the mechanism of photoactivation (light-induced reduction of the flavin adenine dinucleotide cofactor) of Escherichia coli DNA photolyase using time-resolved absorption spectroscopy. Here we show that the excited flavin adenine dinucleotide radical abstracts an electron from a nearby tryptophan in 30 ps. After subsequent electron transfer along a chain of three tryptophans, the most remote tryptophan (as a cation radical) releases a proton to the solvent in about 300 ns, showing that electron transfer occurs before proton dissociation. A similar process may take place in photolyase-like blue-light receptors