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The variations of the electronic properties of the [2Fe-2S*] ferredoxins, from the cyanobacaterium Anabaena are investigated by density functional theory, depending on the redox-linked structural changes observed by X-ray diffraction at atomic resolution. Calculation indicates that the Glu94 carboxylate negative charge regulates the electron charge delocalization between the Ser47 CO group and the Phe65 aromatic ring, depending on the redox state.

An analytical expression of varDeltaP2-(Cu) for two bridged Cu(II) dimers is presented.

A (4Fe-4S)+ center with properties similar to reduced states in some ferredoxins, has been studied using proton ENDOR spectroscopy on gamma-irradiated single crystals of the synthetic model compound (Etsub4N)sub2-(Fesub4Ssub4(SCsub6Hsub4-o-OH)sub4). One iron atom was found to be five-coordinate, having an extra phenolic oxygen attached to its thiolate ligand. This represents an asymmetrical (4Fe-4S) cluster in the solid crystalline state.

An experimental EPR study on (4Fe-4S)super3+ and (4Fe-4S)+ centers with single crystals of a model compound with asymmetrical cubane-type clusters is presented. A theoretical model to interpret the g-tensors of such states observed in synthetic complexes and protein active sites is also reported. The analysis enables understanding of the reasons leading to the diversity of paramagnetic centres, and proposes a useful set of localizations.

Evaluation of redox potentials for 1Fe, 2Fe and 4Fe iron-sulfur clusters by density functional/Poisson-Boltzmann calculations reveals the significant contribution of both spin coupling and resonance delocalization to higher redox potential of 4Fe iron-sulfur clusters. Redox potential shift due to Heisenberg spin coupling is more negative for 2Fe clusters than for one iron clusters. Examination of the contribution of solvents to redox potential indicates the significant contribution of solvation for all redox couples and in all clusters.

EPR studies of the (Et4N)2(FeS4(SBenz)4) model compound reveal the presence of gamma irradiation-created (Fe4S4)3+ and (Fe4S4)+ paramagnetic species in single crystals. Analysis of the 9-tensors yields principal values that agree well with the corresponding geometric and electronic structure of model compounds. The EPR spectra for (Fe4S4)3+ species allow good interpretation of a high-potential protein's oxidized state.

The structure of a spore photoproduct of the dinucleoside TpT (SPTpT) is reexamined to determine the absolute configuration of the natural SP by combining NMR measurements and density functional theory calculations. The results have shown that this natural SPTpT compound is the dinucleotide 5'->3' with an R configuration at the carbon [C.sub.5a].

Studies the exchange interaction in azido-bridged copper dimers using spin population analysis. Quantification of the possible spin polarization effect; Calculation of the exchange coupling constants.

Examines the role of plant-type Anabaena ferredoxin in photosynthesis. Biophysical properties of the ferredoxin; Implications of hydrogen bond for hydroxyl-containing side chain; Use of X-ray diffraction in determining the structural changes of ferredoxin.

By irradiation of bacterial spores under UV radiation, a photoproduct (SP) bearing a covalent methylene link between two adjacent thymines is formed in DNA. Because of the presence of an asymmetric carbon on the aglycone and of two possible orientations for the formation of the cross-link, four isomers could in principle be obtained. Currently, no conclusive structural information of this photoproduct is available. The structure of the isolated SPTpT dinucleotide was revisited in order to determine the type of cross-link and the absolute configuration of the C[sub5a] carbon. For this purpose, a study combining NMR spectroscopy and DFT calculations was pursued on the spore photoproduct of the dinucleoside TpT since its structure was previously shown to be identical to the one produced in DNA. A full characterization of SPTpT by NMR analyses was performed in D[sub2]O and DMSO. 2D NMR measurements (¹H-[sup13]C, ¹H-[sup31]P, COSY, NOESY, and ROESY) and DFT calculations (geometries optimization of R and S isomers and theoretical chemical shifts) lead us to conclude without ambiguity that the absolute configuration of the C[sub5a] carbon is Rand that the methylene bridge of the photoproduct corresponds to the methyl group of the thymine located on the 3'-end of the dinucleoside monophosphate. [ABSTRACT FROM AUTHOR]

PylB is a radical S -adenosyl-l-methionine (SAM) enzyme predicted to convert l-lysine into (3 R )-3-methyl-d-ornithine, a precursor in the biosynthesis of the 22nd proteogenic amino acid pyrrolysine. This protein highly resembles that of the radical SAM tyrosine and tryptophan lyases, which activate their substrate by abstracting a H atom from the amino-nitrogen position. Here, combining in vitro assays, analytical methods, electron paramagnetic resonance spectroscopy, and theoretical methods, we demonstrated that instead, PylB activates its substrate by abstracting a H atom from the Cγ position of l-lysine to afford the radical-based β-scission. Strikingly, we also showed that PylB catalyzes the reverse reaction, converting (3 R )-3-methyl-d-ornithine into l-lysine and using catalytic amounts of the 5'-deoxyadenosyl radical. Finally, we identified significant in vitro production of 5'-thioadenosine, an unexpected shunt product that we propose to result from the quenching of the 5'-deoxyadenosyl radical species by the nearby [Fe 4 S 4 ] cluster.

The radical S-adenosyl-l-methionine tryptophan lyase uses radical-based chemistry to convert l-tryptophan into 3-methyl-2-indolic acid, a fragment in the biosynthesis of the thiopeptide antibiotic nosiheptide. This complex reaction involves several successive steps corresponding to (i) the activation by a specific hydrogen-atom abstraction, (ii) an unprecedented •CO 2 - radical migration, (iii) a cyanide fragment release, and (iv) the termination of the radical-based reaction. In vitro study of this reaction is made more difficult because the enzyme produces a significant amount of a shunt product instead of the natural product. Here, using a combination of X-ray crystallography, electron paramagnetic resonance spectroscopy, and quantum and hybrid quantum mechanical/molecular mechanical calculations, we have deciphered the fine mechanism of the key •CO 2 - radical migration, highlighting how the preorganized active site of the protein tightly controls this reaction.