On the chemistry of flavin-dependent oxygen activation III. synthesis of 1.10a-dihydroflavin and its 10.10-aring opened derivative as model chromophores for enzyme-bound intermediates

Model chromophores of 1.10-dihydroflavin and its 10.10a-ring opened derivative have been isolated. By comparison with enzymatic intermediates, reaction mechanisms of flavin-dependent oxygenase and luciferase are discussed in terms of chemical structure.     

Oxidative dealkylation of 4a-alkylated 4a,5-dihydroflavins: Properties of 4a-alkylated flavin radicals

Covalent coenzyme substrate adducts (“σ-complexes”) are probable intermediates in flavin-dependent biological dehydrogenations. As chemical model reaction for the σ-complex decay, oxidative dealkylation of stable 4a-alkyl-4a,5-dihydroflavins was studied as a function of alkyl mobility and nature of the oxidizing agent. The alkyl groups studied were n-propyl, allyl and benzyl, the oxidizing agents ³O₂, ¹O₂^*, nitroxide radical, ferricyanide and light-excited flavin.For all three alkyl residues, the primary reaction is formation of the 4a-alkyl-4a-hydroflavin radical by le^?-abstraction. ³O₂ and ferricyanide are too weak to initiate this step. If, however, the radical 4a-RFl is once formed, at least five decay modes can be observed depending on the nature of R:(1) For saturated R the exclusive decay is back transfer of the electron initially abstracted. In this case, dealkylation can only be obtained with ¹O₂*, albeit with the relatively slow rate of < 10⁶ M^?1s^?1.(2) For unsaturated R further 1e^?-oxidation leads to quantitative formation of oxidized flavin, while the fate of the alkyl group is still uncertain: In any case, ROH and the corresponding aldehydes as well as the dimers R₂ can be excluded as products.(3) Further oxidation by ³O₂ again leads to a quantitative yield of oxidized flavin while the alkyl residues are converted to peroxy radicals. In an autocatalytic reaction they form the corresponding hydroperoxides with starting 4a-R-Fl_redH, leading to acrolein (R = allyl) or benzaldehyde (R = benzyl) as the major products.(4) In the absence of further oxidant, slow intramolecular alkyl migration is observed leading to the stable 5-alkyl-l,5-dihydroflavin isomer.(5) Competitively, alkyl migration occurs intermolecularly with the starting material as carbenium acceptor, resulting in formation of the stable 4a,5-dialkyl-4a,5-dihydroflavin and unsubstituted radical HFl, which disproportionates.     

Excitation of a d-Density wave in an optical lattice with driven tunneling

Quantum phases with unusual symmetries may play a key role in the understanding of solid state systems at low temperatures. We propose a realistic scenario, well in reach of present experimental techniques, which should permit us to produce a stationary quantum state with d x2-y2 symmetry in a two-dimensional bosonic optical square lattice. This state, characterized by alternating rotational flux in each plaquette, arises from driven tunneling implemented by a stimulated Raman scattering process. We discuss bosons in a square lattice; however, more complex systems involving other lattice geometries appear possible.     

A tunable dual frequency Tm:YAG laser

A compact design for a single mode Tm:YAG laser is presented. With two thin infrasil etalons as the only intracavity tuning elements a tuning range from 1940 to 2030 nm with a gap between 2000 and 2020 nm can be realized. The laser emits in two longitudinal modes separated by 1 GHz. Pumped with 500 mW power from a Ti:sapphire laser at 786 nm the output is above 50 mW over the entire tuning range. Frequencies can be rapidly varied over a range of 4 GHz.