A pentaammineruthenium mixed-valence complex with 1,4-dicyanamido-2,3,5,6-tetrachlorobenzene dianion as bridging ligand: Symmetry-attenuated electronic coupling between redox sites

The dinuclear complex [(NH3)5RuLRu(NH3)5][Cl]4, where L2−= 1,4-dicyanamido-2,3,5,6-tetrachlorobenzene dianion, has been prepared. UV-vis-near-IR spectroscopy and electrochemical data strongly indicate that its mixed-valence derivative is a weakly coupled case, with Habestimated to be 185 cm−1. It is suggested that coupling between the metal centers via the low-energy LMCT system is symmetry forbidden, while coupling via the high-energy LMCT system is energetically unfavorable

Effect of point mutations on Herbaspirillum seropedicae NifA activity

NifA is the transcriptional activator of the nif genes in Proteobacteria. It is usually regulated by nitrogen and oxygen, allowing biological nitrogen fixation to occur under appropriate conditions. NifA proteins have a typical three-domain structure, including a regulatory N-terminal GAF domain, which is involved in control by fixed nitrogen and not strictly required for activity, a catalytic AAA+ central domain, which catalyzes open complex formation, and a C-terminal domain involved in DNA-binding. In Herbaspirillum seropedicae, a β-proteobacterium capable of colonizing Graminae of agricultural importance, NifA regulation by ammonium involves its N-terminal GAF domain and the signal transduction protein GlnK. When the GAF domain is removed, the protein can still activate nif genes transcription; however, ammonium regulation is lost. In this work, we generated eight constructs resulting in point mutations in H. seropedicae NifA and analyzed their effect on nifH transcription in Escherichia coli and H. seropedicae. Mutations K22V, T160E, M161V, L172R, and A215D resulted in inactive proteins. Mutations Q216I and S220I produced partially active proteins with activity control similar to wild-type NifA. However, mutation G25E, located in the GAF domain, resulted in an active protein that did not require GlnK for activity and was partially sensitive to ammonium. This suggested that G25E may affect the negative interaction between the N-terminal GAF domain and the catalytic central domain under high ammonium concentrations, thus rendering the protein constitutively active, or that G25E could lead to a conformational change comparable with that when GlnK interacts with the GAF domain

A pentaammineruthenium(III) dimer with the novel bridging ligand 4,4′-dicyanamidobiphenyl dianion

The novel ligand 4,4′-dicyanamidobiphenyl dianion (bp2-) has been synthesized and characterized by 13C NMR spectroscopy, cyclic voltammetry, and crystallography. The crystal structure of [Ph4As]2[bp]·H2O showed that bp2- is approximately planar with a dihedral angle of 8.2° between phenyl ring planes and the cyanamide groups in an ami conformation. The water of crystallization is asymmetrically hydrogen bonded between cyanamide groups of adjacent bp2- ions. The crystal data for C62H48N4As2+H2O are monoclinic crystal system and space group P21/c with a = 12.998(5) Å, b = 13.465(4) Å, c = 28.703(13) Å, β = 98.94(3)°, V = 4963(3) Å3, and Z = 4. The structure was refined by using 4555 reflections with I > 2.5σ(I) to an R factor of 0.058. The complex, [{(NH3}5Ru}2(μ-bp)][X]4, where X = tosylate or PF6 - ions, was also synthesized and characterized by 1H NMR spectroscopy, cyclic voltammetry, spectroelectrochemistry, and temperature-dependent magnetic susceptibility measurements. From cyclic voltammetry measurements, the comproportionation constants to form the mixed-valence complex [{(NH3}5Ru)2(μ-bp)]3+ were estimated to be 4.1, 16, and 22 in water, acetonitrile, and nitromethane, respectively. The trend and magnitude of Kc suggests solvent valence trapping of a weakly coupled Class II ion. The MMCT band of the mixed-valence complex had to be deconvoluted from the low-energy LMCT band and had the following properties in acetonitrile, νmax = 8400 cm-1, εmax = 3300 M-1 cm-1, and Δν1/2 = 3300 cm-1. The weak superexchange mediating properties of bp2- compared to 1,4-dicyanamidobenzene dianion were suggested to arise from the