Helpful correlations to estimate the pKa of coordinated HNO: a potential-pH exploration in a pendant-arm cyclam-based ruthenium nitroxyl.

The acid-base speciation of coordinated azanone (HNO) remains a highly relevant topic in bioinorganic chemistry. Ruthenium nitroxyl complexes with sufficient robustness towards ligand loss have gained significance as operating platforms to delve into such studies. In this work, we revisit an octahedral {RuNO}6 complex containing the cyclam-based pentadentate ligand Lpy = 1-(pyridine-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane and explore the thermodynamic and spectroscopic aspects of its reduced states in aqueous media. Upon in situ electro-generation of the bound HNO moiety, we have undertaken different strategies to determine both its acidity and electrochemical properties. This robust HNO complex does not undergo deprotonation in a wide pH range. We have found pKa ([Ru(Lpy)(HNO)]2+) = 13.0 ± 0.1 and . There are indications that pKa (HNO) values in several ruthenium-based species correlate with the redox potential associated with the {RuNO}6,7 and {RuNO}7,8 couples. The present pKa extends the range of acidity of bound HNO to more than five pH units, confirming a remarkable sensitivity to the nature of the coordination sphere. This result lays new foundations to continue rational ligand design that may contribute to a better understanding of the different biological roles of both HNO and NO- by investigating key chemical aspects of model complexes.

Four Ni(II) complexes with the new cyclam-methylimidazole ligand 1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane.

Although it has not proved possible to crystallize the newly prepared cyclam-methylimidazole ligand 1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane (L(Im1)), the trans and cis isomers of an Ni(II) complex, namely trans-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) monohydrate, [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2)·H(2)O, (1), and cis-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate), [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2), (2), have been prepared and structurally characterized. At different stages of the crystallization and thermal treatment from which (1) and (2) were obtained, a further two compounds were isolated in crystalline form and their structures also analysed, namely trans-{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}(perchlorato)nickel(II) perchlorate, [Ni(ClO(4))(C(15)H(30)N(6))]ClO(4), (3), and cis-{1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) 0.24-hydrate, [Ni(C(20)H(36)N(6))](ClO(4))(2)·0.24H(2)O, (4); the 1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane ligand is a minor side product, probably formed in trace amounts in the synthesis of L(Im1). The configurations of the cyclam macrocycles in the complexes have been analysed and the structures are compared with analogues from the literature.

Chlorido(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) perchlorate acetonitrile disolvate and aqua(4,4'-dimethoxy-2,2'-bipyridine)(1,4,7-trimethyl-1,4,7-triazacyclononane)ruthenium(II) bis(perchlorate) dihydrate.

The title complexes, [RuCl(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))]ClO(4)·2C(2)H(3)N, (I), and [Ru(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))(H(2)O)](ClO(4))(2)·2H(2)O, (II), display similar structures with the Ru atom in a distorted octahedral environment. In the crystal packing of the chloride complex, (I), the Ru complex molecules are held together in pairs through C-H···Cl interactions of the 4,4'-dimethoxy-2,2'-bipyridine and chloride ligands. In the case of the aqua complex, (II), hydrogen bonding affords a tetrameric hydrogen-bonded network. These two structures are the first examples of complexes with the {Ru(1,4,7-trimethyl-1,4,7-triazacyclononane)} motif and an electron-rich substituted 2,2'-bipyridine ligand.

Electron transfer dynamics of Rhodothermus marinus caa3 cytochrome c domains on biomimetic films.

The subunit II of the caa(3) oxygen reductase from Rhodothermus marinus contains, in addition to the Cu(A) center, a c-type heme group in the cytochrome c domain (Cyt-D) that is the putative primary electron acceptor of the enzyme. In this work we have combined surface-enhanced resonance Raman (SERR) spectroelectrochemistry, molecular dynamics (MD) simulations and electron pathway calculations to assess the most likely interaction domains and electron entry/exit points of the truncated Cyt-D of subunit II in the reactions with its electron donor, HiPIP and electron acceptor, Cu(A). The results indicate that the transient interaction between Cyt-D and HiPIP relies upon a delicate balance of hydrophobic and polar contacts for establishing an optimized electron transfer pathway that involves the exposed edge of the heme group and guaranties efficient inter-protein electron transfer on the nanosecond time scale. The reorganization energy of ca. 0.7 eV was determined by time-resolved SERR spectroelectrochemistry. The intramolecular electron transfer pathway in integral subunit II from Cyt-D to the Cu(A) redox center most likely involves the iron ligand histidine 20 as an electron exit point in Cyt-D.

Probing the chemotaxis periplasmic sensor domains from Geobacter sulfurreducens by combined resonance Raman and molecular dynamic approaches: NO and CO sensing.

The periplasmic sensor domains encoded by genes gsu0582 and gsu0935 are part of methyl accepting chemotaxis proteins in the bacterium Geobacter sulfurreducens (Gs). The sensor domains of these proteins contain a heme-c prosthetic group and a PAS-like fold as revealed by their crystal structures. Biophysical studies of the two domains showed that nitric oxide (NO) binds to the heme in both the ferric and ferrous forms, whereas carbon monoxide (CO) binds only to the reduced form. In order to address these exogenous molecules as possible physiological ligands, binding studies and resonance Raman (RR) spectroscopic characterization of the respective CO and NO adducts were performed in this work. In the absence of exogenous ligands, typical RR frequencies of five-coordinated (5c) high-spin and six-coordinated (6c) low-spin species were observed in the oxidized form. In the reduced state, only frequencies corresponding to the latter were detected. In both sensors, CO binding yields 6c low-spin adducts by replacing the endogenous distal ligand. The binding of NO by the two proteins causes partial disruption of the proximal Fe-His bond, as revealed by the RR fingerprint features of 5cFe-NO and 6cNO-Fe-His species. The measured CO and NO dissociation constants of ferrous GSU0582 and GSU0935 sensors reveal that both proteins have high and similar affinity toward these molecules (K(d) approximately = 0.04-0.08 microM). On the contrary, in the ferric form, sensor GSU0582 showed a much higher affinity for NO (K(d) approximately = 0.3 microM for GSU0582 versus 17 microM for GSU0935). Molecular dynamics calculations revealed a more open heme pocket in GSU0935, which could account for the different affinities for NO. Taken together, spectroscopic data and MD calculations revealed subtle differences in the binding properties and structural features of formed CO and NO adducts, but also indicated a possibility that a (5c) high-spin/(6c) low-spin redox-linked equilibrium could drive the physiological sensing of Gs cells.

Widely differing photochemical behavior in related octahedral {Ru-NO}6 compounds: intramolecular redox isomerism of the excited state controlling the photodelivery of NO.

trans-[(NC)Ru(py)(4)(mu-CN)Ru(py)(4)(NO)](3+) (py = pyridine) is a stable species in aqueous solution. It displays an intense absorption in the visible region of the spectrum (lambda(max) = 518 nm; epsilon(max) = 6100 M(-1) cm(-1)), which turns this compound into a promising agent for the photodelivery of NO. The quantum yield for the photodelivery process resulting from irradiation with 455 nm visible light was found experimentally to be (0.06 +/- 0.01) x 10(-3) mol einstein(-1), almost 3 orders of magnitude smaller than that in the closely related cis-[RuL(NH(3))(4)(mu-pz)Ru(bpy)(2)(NO)](5+) species (L = NH(3) or pyridine, pz = pyrazine, bpy = 2,2'-bipyridine; phi(NO) = 0.02-0.04 mol einstein(-1) depending on L) and also much smaller than the one in the mononuclear compound trans-[ClRu(py)(4)(NO)](2+) (phi(NO) = (1.63 +/- 0.04) x 10(-3) mol einstein(-1)). DFT computations provide an electronic structure picture of the photoactive excited states that helps to understand this apparently abnormal behavior.

All-trans-[ClRu(II)(py)4(NC)Ru(II)(py)4(CN)Ru(II)(py)4(NO)](PF6)4: a redox-active 2-donor/1-acceptor system based on the electrophilic {RuNO}6 motif.

The new linear homotrinuclear compound trans-[ClRu(II)(py)(4)(NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO)](PF(6))(4) was prepared by reaction between the nitro complex trans-[(NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO(2))](+) and the solvento complex obtained by reaction between [ClRu(II)(py)(4)(NO)](3+) and N(3)(-) in acetone. The trans-[ClRu(II)(py)(4) (NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO)](4+) ion (I) has been characterized by (1)H NMR and IR spectroscopy (nu(NO) = 1919 cm(-1)). This species displays intense electronic absorptions in the visible region which can be assigned to donor-acceptor charge-transfer transitions (DACT) involving {RuNO}(6)-centered acceptor orbitals and donor orbitals located on the two different neighboring metal centers at ca. 6.7 and 12.6 A distance from the metal in the {RuNO}(6) fragment. Addition of OH(-) to I generated the nitro complex with a second-order rate constant of (12.5 +/- 0.2) x 10(3) M(-1) s(-1) (25 degrees C). Cyclic voltammetry experiments complemented by spectroelectrochemistry in the UV-vis-NIR region reveal that I can be reversibly reduced at 0.49 or 0.20 V vs AgCl/Ag for acetonitrile and water, respectively, and oxidized at 0.71 or 0.57 V vs AgCl/Ag. The spectroscopic and spectroelectrochemical information (UV-vis-NIR, X-band EPR) supplemented with electronic structure computation (DFT) reveals that the one-electron reduction is centered on the nitrosyl moiety to yield a {RuNO}(7) species, while oxidation occurs on the chlororuthenium side of the molecule. Both processes yield significant changes of the electronic spectra which are discussed in parallel with the electronic structure picture as obtained by DFT.

Reduction thermodynamics of the T1 Cu site in plant and fungal laccases.

The thermodynamic parameters for reduction of the type-1 (T1) copper site in Rhus vernicifera and Trametes versicolor laccases and for the derivative of the former protein from which the type-2 copper has been selectively removed (T2D) have been determined with UV-vis spectroelectrochemistry. In all cases, the enthalpic term turns out to be the main determinant of the Eo' of the T1 site. Also the difference between the reduction potentials of the two laccases is enthalpy-based and reflects differences in the coordination features of the T1 sites and their protein environment. The T1 sites in native R. vernicifera laccase and its T2D derivative show the same Eo', as a result of compensatory differences in the reduction thermodynamics. This suggests that removal of the type-2 (T2) copper results in modification of the reduction-induced solvent reorganization effects, with no influence in the structure of the multicopper protein site. This conclusion is supported by NMR data recorded on the native, the T2D, and Hg-substituted T1 derivatives of R. vernicifera laccase, which show that the T1 and T2/T3 sites are largely noninteracting.