Sulfur, the Versatile Non-metal.

The non-metallic chemical element sulfur, 3216S , referred to in Genesis as brimstone and identified as element by Lavoisier, is the tenth most abundant element in the universe and the fifth most common element on Earth. Important inorganic forms of sulfur in the biosphere are elemental sulfur (S8), sulfate (SO2-4), and sulfide (S2-), sulfite (SO2-3), thiosulfate, (S2O23), and polythionates (S3O62-; S4O62-). Because of its wide range of stable oxidation states, from +6to -2, sulfur plays important roles in central biochemistry as a structural and redoxactive element and is intimately related to life on Earth. Unusual reaction pathways involving sulfur compounds become possible by the specific properties of this element. Sulfur occurs in all the major classes of biomolecules, including enzymes, proteins, sugars, nucleic acids, vitamin cofactors, and metabolites. The flexibility of these biomolecules follows from its versatile chemistry. The best known sulfur mineral is perhaps pyrite (Fool's gold), with the chemical formula, FeS2. Sulfur radical anions, such as [S3].-, are responsible for the intense blue color of lapis lazuli, one of the most desired and expensive artists' materials. In the microbial world, inorganic sulfur compounds, e.g., elemental sulfur and sulfate, belong to the most important electron acceptors. Studies on microbial sulfur metabolism revealed many novel enzymes and pathways and advanced the understanding on metabolic processes used for energy conservation, not only of the microbes, but of biology in general. Transition metal sulfur complexes display intriguing catalytic activities, they provide surfaces and complex cavities in metalloenzymes that activate inert molecules such as H2, CO, N2 or N2O, and they catalyze the transformations of numerous organic molecules. Both thiamine diphosphate- (ThDP) and S-adenosyl- L-methionine- (SAM) dependent enzymes belong to Nature's most powerful catalysts with a remarkable spectrum of catalytic activities. In conclusion, given sulfur's diverse properties, evolution made an excellent choice in selecting sulfur as one the basic elements of life.

Structure, kinetics, molecular and redox properties of a cytosolic and developmentally regulated fungal catalase-peroxidase.

CAT-2, a cytosolic catalase-peroxidase (CP) from Neurospora crassa, which is induced during asexual spore formation, was heterologously expressed and characterized. CAT-2 had the Met-Tyr-Trp (M-Y-W) adduct required for catalase activity. Its KM for H2O2 was micromolar for peroxidase and millimolar for catalase activity. A Em = -158 mV reduction potential value was obtained and the Soret band shift suggested a mixture of low and high spin ferric iron. CAT-2 EPR spectrum at 10 K indicated an axial and a rhombic component. With peroxyacetic acid (PAA), formation of Compound I* was observed with EPR. CAT-2 homodimer crystallographic structure contained two K+ ions; Glu107 residues were displaced to bind them. CAT-2 showed the essential amino acid residues for activity in similar positions to other CPs. CAT-2 Arg426 is oriented towards the M-Y-W adduct, interacting with the deprotonated Tyr238 hydroxyl group. A perhydroxy modification of the indole nitrogen of Trp90 was oriented toward the catalytic His91. In contrast to cytochrome c peroxidase and ascorbate peroxidase, the catalase-peroxidase heme propionates are not exposed to the solvent. Together with other N. crassa enzymes that utilize H2O2 as a substrate, CAT-2 has many tryptophan and proline residues at its surface, probably related to H2O2 selection in water.

The nitric oxide production in the moss Physcomitrella patens is mediated by nitrate reductase.

During the last 20 years multiple roles of the nitric oxide gas (•NO) have been uncovered in plant growth, development and many physiological processes. In seed plants the enzymatic synthesis of •NO is mediated by a nitric oxide synthase (NOS)-like activity performed by a still unknown enzyme(s) and nitrate reductase (NR). In green algae the •NO production has been linked only to NR activity, although a NOS gene was reported for Ostreococcus tauri and O. lucimarinus, no other Viridiplantae species has such gene. As there is no information about •NO synthesis neither for non-vascular plants nor for non-seed vascular plants, the interesting question regarding the evolution of the enzymatic •NO production systems during land plant natural history remains open. To address this issue the endogenous •NO production by protonema was demonstrated using Electron Paramagnetic Resonance (EPR). The •NO signal was almost eliminated in plants treated with sodium tungstate, which also reduced the NR activity, demonstrating that in P. patens NR activity is the main source for •NO production. The analysis with confocal laser scanning microscopy (CLSM) confirmed endogenous NO production and showed that •NO signal is accumulated in the cytoplasm of protonema cells. The results presented here show for the first time the •NO production in a non-vascular plant and demonstrate that the NR-dependent enzymatic synthesis of •NO is common for embryophytes and green algae.

Magnetic and high-frequency EPR studies of an octahedral Fe(III) compound with unusual zero-field splitting parameters.

Temperature-dependent magnetic susceptibility and multi-frequency EPR (9.4, 34.5, 94 and 188 GHz) spectroscopic measurements have been carried out together with an X-ray study at 100 K to study [Fe(DMSO)(6)](NO(3))(3). The iron(III) ion remains high-spin (S = 5/2) in the temperature range studied, therefore, the EPR data were interpreted using the conventional S = 5/2 spin Hamiltonian. A full analysis of EPR spectra at 95 GHz of a powdered sample at 290 K revealed that they are extremely sensitive to D and E values. The zfs parameters were precisely determined: D = + 0.1730 cm(-1), E = 0.00 cm(-1) and lambda = |E/D| = 0.00. A sequence of the spectra neatly shows that the compound has a clear magnetic dependence on temperature. The study at 5 K, showed that the zfs parameters increase: D = + 0.1970 cm(-1), E = 0.017 cm(-1) and lambda = |E/D| = 0.086. These data indicate that as the temperature decreases the D tensor increases slightly showing an increase in the rhombicity. These results confirm that |2D| is congruent to h nu at X-band in this case. Additionally, it has been shown by X-ray crystal analysis of [Fe(DMSO)(6)](NO(3))(3) at 100 K that this is involved in a hydrogen bonding network, consisting of C-H...O interactions between the nitrate anions and the methyl groups of the coordinated DMSO molecules, thus suggesting that the differences found in the spectroscopic parameters D and E at different temperatures must be due to these supramolecular interactions.

High-field EPR study and crystal and molecular structure of trans-RSSR-[CrCl2(cyclam)]nX (X=ZnCl4 2-, Cl- and Cl-.4H2O.0.5HCl).

For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-[CrCl(2)(cyclam)](2)ZnCl(4), 1, the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl, 2, and the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl.4H(2)O.0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis of EPR spectra at 94.5 GHz of diluted complexes 1, 2 and 3 at 300 K revealed that they are extremely sensitive to D and E values. The rhombic distortion was precisely determined for each compound. For 1, g= 2.01, D=-0.305 cm(-1), E= 0.041 cm(-1) and lambda=|E/D|= 0.1396; for 2, g= 2.01; D=-0.348 cm(-1), E= 0.042 cm(-1) and lambda=|E/D|= 0.1206 and for 3, g= 1.99, D=-0.320 cm(-1), E= 0.041 cm(-1) and ambda=|E/D|= 0.1281. The EPR study at 94.5 GHz at 10 K allowed us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid state due to different hydrogen bonding networks in their crystalline arrangement.