Structural and spectroscopic characterization of exogenous ligand binding to isolated factor F430 and its configurational isomers

Deconstructing F(430): quantum chemical perspectives of biological methanogenesis

What stabilizes the unique Ni(I) state of the active form of coenzyme F(430) and of methylcoenzyme M reductase, the enzyme responsible for the last methane-evolving step of biological methanogenesis? A survey of F(430) model compounds suggests that the monoanionic nature of the F(430) ligand goes a long way toward explaining the stability of Ni(I) F(430). Second, nature appears to have manipulated the stereochemistry of the macrocycle, particularly that of the 12- and 13- substituents, so that the cofactor is sterically constrained against ruffling and forced to adopt a relatively planar conformation with long Ni--N distances. Third, the carbonyl substituent at the 15-meso position electronically stabilizes the Ni(I) state of the cofactor. With regard to the mechanism of methylcoenzyme M reductase, the most reasonable mechanism, in our opinion, involves a Ni(I)-mediated homolytic cleavage of the S--CH(3) bond in methylcoenzyme M, followed immediately by the quenching of the methyl radical by coenzyme B (a thiol) to produce methane.

Four-Coordinate, Low-Spin (S = 0) and Six-Coordinate, High-Spin (S = 1) Nickel(II) Complexes of Tetraphenylporphyrins with beta-Pyrrole Electron-Withdrawing Substituents: Porphyrin-Core Expansion and Conformation

To examine the porphyrin-core expansion and the conformational variations induced by a change in the coordination sphere of nickel(II) from four-coordinate, low-spin (S = 0) to six-coordinate, high-spin (S = 1), several nickel(II) derivatives of tetraphenylporphyrins, substituted in their beta-pyrrole positions with electron-withdrawing groups, were isolated and studied by X-ray crystallography. The four-coordinate nickel(II) complex studied, Ni(TPP(Br)(4)(CN)(4)) (1), is a derivative of the antipodal beta-pyrrole brominated and cyanated macrocycle, 2,3,12,13-tetrabromo-7,8,17,18-tetracyano-5,10,15,20-tetraphenylporphyrin. One of the six-coordinate species is the bis-pyridine nickel(II) adduct, Ni(TPP(Br)(4)(CN)(4))(py)(2) (2), of this antipodal beta-pyrrole brominated and cyanated ring, whereas the second six-coordinate complex is the bis-1-methylimidazole nickel(II) adduct, Ni(TPP(CN)(4))(1-MeIm)(2) (3), of the antipodal tetracyanated macrocycle, 7,8,17,18-tetracyano-5,10,15,20-tetraphenylporphyrin. This study confirms that, upon conversion of a four-coordinate, low-spin nickel(II) derivative of a beta-pyrrole-substituted tetraphenylporphyrin into a six-coordinate, high-spin complex, a radial expansion of the porphyrin core also takes place. This radial expansion flattens the porphyrin core, inducing more planarity in the conformations of the six-coordinate nickel(II) porphyrin species. The X-ray structure of 1 reveals an average Ni-N(p) bond distance of 1.920(2) Å, similar to that present in tetragonal Ni(OEP) [(Ni-N(p))(av) = 1.929(3) Å], the ruffled low-spin nickel(II) (S = 0) porphyrin derivative, and many other low-spin nickel(II) complexes of distorted porphyrins. The conformation of the macrocycle present in 1 is severely saddle-shaped with a small ruffling and a minor doming. The structures of 2 and 3 indicate a clear expansion of the porphyrin cores with two sets of (Ni-N(p))(av) bond lengths of 2.040(2) and 2.073(2) Å in 2 and 2.037(2) and 2.068(2) Å in 3, which differ significantly from the average Ni-N(p) distance occurring in 1. Relative to the conformation of the porphyrin ring present in 1, the high-spin complex 2 is less saddle-shaped, but more ruffled. In 3, in which only two opposite pyrrole rings of the porphyrin are substituted by cyano groups, the macrocycle adopts a slightly waved conformation. Crystal data: Ni(TPP(Br)(4)(CN)(4)).2.5(1,2-Cl(2)C(2)H(4)), [1.2.5(1,2-Cl(2)C(2)H(4))], triclinic, space group P&onemacr;, a = 13.741(1) Å, b = 14.029(2) Å, c = 15.201(2) Å, alpha = 89.037(9) degrees, beta = 70.829(8) degrees, gamma = 67.607(8) degrees, V = 2539.4(5) Å(3), T = 20 degrees C, Z = 2, R(F)() = 0.047 and R(wF) = 0.067 based on 5341 reflections with I > 3sigma(I); Ni(TPP(Br)(4)(CN)(4))(py)(2).2.5(1,2-Cl(2)C(2)H(4)), [2.2.5(1,2-Cl(2)C(2)H(4))], triclinic, space group P&onemacr;, a = 11.9472(9) Å, b = 14.3651(7) Å, c = 19.556(1) Å, alpha = 87.049(5) degrees, beta = 73.181(6) degrees, gamma = 70.613(5) degrees, V = 3026.8(4) Å(3), T = 173 K, Z = 2, R(F)() = 0.036, R(wF) = 0.041 based on 6846 reflections with I > 3sigma(I); Ni(TPP(CN)(4))(1-MeIm)(2).2CH(2)Cl(2), [3.2(CH(2)Cl(2))], monoclinic, space group C2/c, a = 26.804(8) Å, b = 10.948(3) Å, c = 19.453(5) Å, beta = 113.13(2) degrees, V = 5251.5 Å(3), T = 20 degrees C, Z = 4, R(F)() = 0.047, R(wF) = 0.067 based on 4384 reflections with I > 3sigma(I).

Nickel-catalyzed N-terminal oxidative deamination in peptides containing histidine at position 2 coupled with sulfite oxidation

Peptides containing histidine at position 2 were observed to undergo spontaneous N-terminal oxidative deamination in aqueous solution in the presence of Ni(II), sulfite, and ambient oxygen. The reaction resulted in the formation of a free carbonyl on the N-terminal alpha-carbon (alpha-ketoamide) and was catalytic with respect to nickel. This oxidative deamination was confirmed by 13C NMR, 1H NMR, mass spectrometry, and chemical tests. No evidence of modification of histidine was found. It was demonstrated that the nickel-dependent N-terminal oxidative deamination also occurred in His-2 peptides using potassium peroxymonosulfate (oxone) as an oxidant. When oxone was used, oxygen was not required for the deamination to proceed. The results suggest that both nickel-catalyzed reactions (sulfite and oxygen, and oxone) produce an imine intermediate that spontaneously hydrolyzes to form the free carbonyl. These findings may provide a physiologically relevant model for oxidative carbonyl formation in vivo, as well as a useful method for producing a site-specific carbonyl on peptides and proteins.

Use of X-ray Absorption Spectroscopy and Esterification to Investigate Cr(III) and Ni(II) Ligands in Alfalf a Biomass

Previously performed studies have shown that alfalfa shoot biomass can bind an appreciable amount of nickel(II) and chromium(III) ions from aqueous solution. Direct and indirect approaches were applied to study the possible mechanis ms involved in metal binding by the alfalfa biomass. The direct approach involves investigations of the metal-bound alfal fa shoot biomass by X-ray absorption spectroscopic analysis (XANES and EXAFS). Results from these studies suggest that ni ckel(II) and chromium(III) binding mostly occurs through coordination with oxygen ligands. Indirect approaches consist of chemical modification of carboxylate groups that have been shown to play an important role in metal binding to the alfal fa biomass. An appreciable decrease in metal binding resulted after acidic methanol esterification of the biomass, indica ting that carboxyl groups are entailed in the metal binding by the alfalfa biomass. In addition, base hydrolysis of the a lfalfa biomass increased the binding of these metals, which further indicates that carboxyl groups play an important role in the binding of these metal ions from solution. Therefore, by combining two different techniques, our results indicate that carboxylate groups are the major ligands responsible for the binding of nickel(II) and chromium(III) by alfalfa bio mass.