Geometric and Electronic Structure Contributions to O-O Cleavage and the Resultant Intermediate Generated in Heme-Copper Oxidases.

This study investigates the mechanism of O-O bond cleavage in heme-copper oxidase (HCO) enzymes, combining experimental and computational insights from enzyme intermediates and synthetic models. It is determined that HCOs undergo a proton-initiated O-O cleavage mechanism where a single water molecule in the active site enables proton transfer (PT) from the cross-linked tyrosine to a peroxo ligand bridging the heme FeIII and CuII, and multiple H-bonding interactions lower the tyrosine p Ka. Due to sterics within the active site, the proton must either transfer initially to the O(Fe) (a high-energy intermediate), or from another residue over a ∼10 Å distance to reach the O(Cu) atom directly. While the distance between the H+ donor (Tyr) and acceptor (O(Cu)) results in a barrier to PT, this separation is critical for the low barrier to O-O cleavage as it enhances backbonding from Fe into the O22- σ* orbital. Thus, PT from Tyr precedes O-O elongation and is rate-limiting, consistent with available kinetic data. The electron transfers from tyrosinate after the barrier via a superexchange pathway provided by the cross-link, generating intermediate PM. PM is evaluated using available experimental data. The geometric structure contains an FeIV═O that is H-bonded to the CuII-OH. The electronic structure is a singlet, where the FeIV and CuII are antiferromagnetically coupled through the H-bond between the oxo(Fe) and hydroxo(Cu) ligands, while the CuII and Tyr• are ferromagnetically coupled due their delocalization into orthogonal magnetic orbitals on the cross-linked His residue. These findings provide critical insights into the mechanism of efficient O2 reduction in HCOs, and the nature of the PM intermediate that couples this reaction to proton pumping.

The three-spin intermediate at the O-O cleavage and proton-pumping junction in heme-Cu oxidases.

Understanding the mechanistic coupling of molecular oxygen reduction and proton pumping for adenosine triphosphate synthesis during cellular respiration is the primary goal of research on heme-copper oxidases­the terminal complex in the membrane-bound electron transport chain. Cleavage of the oxygen-oxygen bond by the heme-copper oxidases forms the key intermediate PM, which initiates proton pumping. This intermediate is now experimentally defined by variable-temperature, variable-field magnetic circular dichroism spectroscopy on a previously unobserved excited state feature associated with its heme iron(IV)-oxo center. These data provide evidence that the iron(IV)-oxo in PM is magnetically coupled to both a copper(II) and a cross-linked tyrosyl radical in the active site. These results provide new insight into the oxygen-oxygen bond cleavage and proton-pumping mechanisms of heme-copper oxidases.

Light-activated generation of nitric oxide (NO) and sulfite anion radicals (SO3˙-) from a ruthenium(ii) nitrosylsulphito complex.

This manuscript describes the preparation of a new Ru(ii) nitrosylsulphito complex, trans-[Ru(NH3)4(isn)(N(O)SO3)]+ (complex 1), its spectroscopic and structural characterization, photochemistry, and thermal reactivity. Complex 1 was obtained by the reaction of sulfite ions (SO32-) with the nitrosyl complex trans-[Ru(NH3)4(isn)(NO)]3+ (complex 2) in aqueous solution resulting in the formation of the N-bonded nitrosylsulphito (N(O)SO3) ligand. To the best of our knowledge, only four nitrosylsulphito metal complexes have been described so far (J. Chem. Soc., Dalton Trans., 1983, 2465-2472), and there is no information about the photochemistry of such complexes. Complex 1 was characterized by spectroscopic means (UV-Vis, EPR, FT-IR, 1H- and 15N-NMR), elemental analysis and single-crystal X-ray diffraction. The X-ray structure of the precursor complex 2 is also discussed in the manuscript and is used as a reference for comparisons with the structure of 1. Complex 1 is water-soluble and kinetically stable at pH 7.4, with a first-order rate constant of 3.1 × 10-5 s-1 for isn labilization at 298 K (t1/2∼ 373 min). Under acidic conditions (1.0 M trifluoroacetic acid), 1 is stoichiometrically converted into the precursor complex 2. The reaction of hydroxide ions (OH-) with 1 and with 2 yields the Ru(ii) nitro complex trans-[Ru(NH3)4(isn)(NO2)]+ with second-order rate constants of 2.1 and 10.5 M-1 s-1 (at 288 K), respectively, showing the nucleophilic attack of OH- at the nitrosyl in 2 (Ru-NO) and at the nitrosylsulphito in 1 (Ru-N(O)SO3). The pKa value of the -SO3 moiety of the N(O)SO3 ligand in 1 was determined to be 5.08 ± 0.06 (at 298 K). The unprecedented photochemistry of a nitrosylsulphito complex is investigated in detail with 1. The proposed mechanism is based on experimental (UV-Vis, EPR, NMR and Transient Absorption Laser Flash Photolysis) and theoretical data (DFT) and involves photorelease of the N(O)SO3- ligand followed by formation of nitric oxide (NO˙) and sulfite radicals (SO3˙-, sulfur trioxide anion radical).

Anti-inflammatory and Anti-nociceptive Activity of Ruthenium Complexes with Isonicotinic and Nicotinic Acids (Niacin) as Ligands.

This work evaluated the analgesic and anti-inflammatory activity of ruthenium(II) complexes trans-[Ru(NO(+))(NH3)4(L)](BF4)3 and [Ru(NH3)5(L)](BF4)3 containing the nonsteroidal anti-inflammatory drugs nicotinic acid (Hnic) and its isomer isonicotinic acid (ina) as ligands (L). The anti-nociceptive potential of these complexes and the free ligands (noncoordinated to ruthenium) was tested in different models with doses ranging from 1 to 100 µmol/kg. The ligands themselves were inactive; however, the ruthenium complexes containing Hnic and ina inhibited mechanical hyperalgesia induced by prostaglandin E2, carrageenan-induced hyperalgesia, and antigen-induced arthritis. Moreover, the ruthenium complexes inhibited overt nociception induced by formalin, acetic acid, capsaicin, and cinnamaldehyde. The mechanism involved in the anti-nociceptive effects of the ruthenium complexes suggested that ATP-sensitive K(+) channel pathways were not involved because glibenclamide did not affect their anti-nociceptive activities. However, the anti-nociceptive effect appears to be a consequence of the reduction in neutrophil migration and inhibition of the protein kinase C pathway.

Antitubercular activity of Ru (II) isoniazid complexes.

Despite the resistance developed by the Mycobacterium tuberculosis (MTb) strains, isoniazid (INH) has been recognized as one of the best drug for treatment of Tuberculosis (Tb). The coordination of INH to ruthenium metal centers was investigated as a strategy to enhance the activity of this drug against the sensitive and resistant strains of MTb. The complexes trans-[Ru(NH3)4(L)(INH)](2+) (L=SO2 or NH3) were isolated and their chemical and antituberculosis properties studied. The minimal inhibitory concentration (MIC) data show that [Ru(NH3)5(INH)](2+) was active in both resistant and sensitive strains, whereas free INH (non-coordinated) showed to be active only against the sensitive strain. The coordination of INH to the metal center in both [Ru(NH3)5(INH)](2+) and trans-[Ru(NH3)4(SO2)(INH)](2+) complexes led to a shift in the INH oxidation potential to less positive values compared to free INH. Despite, the ease of oxidation of INH did not lead to an increase in the in vitro INH activity against MTb, it might have provided sensitivity toward resistant strains. Furthermore, ruthenium complexes with chemical structures analogous to those described above were synthesized using the oxidation products of INH as ligands (namely, isonicotinic acid and isonicotinamide). These last compounds were not active against any strains of MTb. Moreover, according to DFT calculations the formation of the acyl radical, a proposed intermediate in the INH oxidation, is favored in the [Ru(NH3)5(INH)](2+) complex by 50.7kcalmol(-1) with respect to the free INH. This result suggests that the stabilization of the acyl radical promoted by the metal center would be a more important feature than the oxidation potential of the INH for the antituberculosis activity against resistant strains.