Aquacobalamin binds hydrogen peroxide
reversibly to form a cobalt(III)
hydroperoxo adduct with a 0.25 mM dissociation constant, as evidenced
by UV–vis absorption spectroscopy and corroborated by NMR,
Raman spectroscopy, stopped-flow UV–vis measurements, and density
functional theory calculations.
The nature of the blue color in the iodine-starch reaction is still a matter of debate. Some textbooks still invoke charge-transfer bands within a chain of neutral I2 molecules inside the hydrophobic channel defined by the interior of the amylose helical structure. However, the consensus is that the interior of the helix is not altogether hydrophobic—and that a mixture of I2 molecules and iodide anions reside there and are responsible for the intense charge-transfer bands that yield the blue color of the “iodine-starch complex”. Indeed, iodide is a prerequisite of the reaction. However, some debate still exists regarding the nature of the iodine-iodine units inside the amylose helix. Species such as I3-, I5-, I7- etc. have been invoked. Here, we report UV-vis titration data and computational simulations using density functional theory (DFT) for the iodine/iodide chains as well as semiempirical (AM1, PM3) calculations of the amylose-iodine/iodide complexes, that (1) confirm that iodide is a pre-requisite for blue color formation in the iodine-starch system, (2) propose the nature of the complex to involve alternating sets of I2 and Ix- units, and (3) identify the nature of the charge-transfer bands as involving transfer from the Ix- σ* orbitals (HOMO) to I2 σ* LUMO orbitals. The best candidate for the “blue complex”, based on DFT geometry optimizations and TD-DFT spectral simulations, is an I2-I5-I2 unit, which is expected to occur in a repetitive manner inside the amylose helix.
The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp 2 M 2 B 4 H 8 (Cp = η 5 -C 5 H 5 ; M = Ir, Ru/Os, Re, Mo/W, and Ta), including the experimentally known Ir, Ru, and Re derivatives synthesized by Fehlner and co-workers, have now been examined by density functional theory. The nature of the central M 2 B 4 polyhedra in the lowest energy Cp 2 M 2 B 4 H 8 structures relates to the skeletal electron count as determined by the Wade−Mingos rules. Thus, the lowest energy Cp 2 Ir 2 B 4 H 8 structures with 16 Wadean skeletal electrons have central pentagonalpyramidal Ir 2 B 4 units similar to that of the known pentagonal-pyramidal B 6 H 10 . The lowest energy Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M 2 B 4 units. However, isomeric Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with central M 2 B 4 octahedra are found at energies starting at ∼15 kcal/ mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The lowest energy electron poorer Cp 2 M 2 B 4 H 8 structures (M = Re, Mo, W, Ta) have central M 2 B 4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. Higher energy Cp 2 Re 2 B 4 H 8 structures include capped-tetragonal-pyramidal structures with surface ReRe double bonds and a pentagonal-pyramidal structure with a surface ReRe triple bond. The lowest energy Cp 2 M 2 B 4 H 8 (M = Mo, W) structures appear to have surface MM double bonds and thus also the 12 skeletal electrons for their bicapped-tetrahedral structures. However, the lowest energy likewise bicapped-tetrahedral Cp 2 Ta 2 B 4 H 8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta−Ta single bond.
The lowest energy structures of Au9+ and Au113+ are aromatic with relatively spherical polyhedra, like Au102+. However, the lowest energy Au124+ structure has a central planar unit with four Au4 tetrahedra. The surfaces of these clusters have σ-holes.
Sulfite reductase (SiR) catalyzes a six electron and six proton reduction of sulfite to sulfide. Similarly to the cytochrome P450 (cytP450) family, the active site in SiR contains a (partially reduced) heme bound axially to a cysteinate ligand-though with an extra Fe 4 S 4 cluster. Fe(III) SO 2− , Fe(III) SOH − , and Fe(III) SO(H 2 ) intermediates have been proposed for the catalytic cycle of SiR, leading to a formally Fe(V) S species-akin to the widely accepted reaction mechanism in cytP450. Here, density functional theory (DFT) data is reported for of such FeSO(H 2 ) intermediates. The Fe(III) SO 2− models display relatively high energies for homolytic bond breaking compared to their isomeric oxygen-bound Fe(III) OS 2− models, and thus offer a better alternative in terms of avoiding radical side products able to induce enzyme suicide. This could be due to the fact that the (iron-bound) sulfur is more active from a redox standpoint compared to oxygen, thus permitting the departing oxygen to maintain a redox-inert state. Di-protonation of the oxygen is computed to lead to a compound I type Fe(IV) S coupled to a porphyrin radical anion-consistent with an intermediate previously observed by x-ray crystallography. K E Y W O R D S cytP450, heme, peroxide, sulfite reductase, sulfur oxide
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