Amido binding to ReO core: Synthesis, structure and intermolecular interactions

The light green coloured complexes of general formula [Re^VO(L)Cl(OH₂)]Cl have been synthesised in good yields by reacting [Re^VOCl₃(AsPh₃)₂] with HL in dichloromethane in dinitrogen atmosphere. Here, L⁻ is the deprotonated form of N,N-bis(2-pyridylmethyl)amine (HL¹); N-(2-pyridylmethyl)-N′,N′-dimethylethylenediamine (HL²) and N-(2-pyridylmethyl)-N′,N′-diethylethylenediamine (HL³). Single crystal X-ray structure determination of [Re^VO(L¹)Cl(OH₂)]Cl confirms the amido binding of ReO³⁺ species. In the solid state of [Re^VO(L¹)Cl(OH₂)]Cl, the coordinated and counter chloride ions are engaged in Re-Cl…H-C(ring), Cl…H-C(ring) and Re-(OH₂)…Cl hydrogen bonding and forming of a supramolecular network in the solid state. The subunit of the supramolecular network consists of one eight-membered and two nine-membered hydrogen bonded rings. The average diameters of eight-membered and nine-membered rings are ∼3.70 and ∼5.26 Å, respectively.     

Metal ion coordination to 2′ functionality of guanosine mediates substrate-guanosine coupling in group I ribozymes: implications for conserved role of metal ions and for variability in RNA folding in ribozyme catalysis

Divalent metal ions are necessary in the self splicing reaction of group I introns, and we report that metal interaction to the 2′ position of guanosine for the Azoarcus ribozyme is required for catalysis. Moreover, this metal coordination promotes the guanosine-substrate coupled binding to the ribozyme, which is another conserved feature seen across phylogenetic boundaries. Typically there is a 4-9-fold difference in binding of G to E_free versus E · S. In the Tetrahymena ribozyme’s case this substrate-guanosine communication was attributed to conformational change(s) that lead to cooperative binding of the two cofactors which is almost nonexistent at low temperatures (4 °C). In the prokaryotic Azoarcus ribozyme we also see a 4-5-fold difference in binding of the guanosine/substrate to E_free versus E · G or E · S at 10 °C that is attributed to guanosine-substrate coupling. This coupling is diminished when the metal (Mg²⁺) coordination to the 2′ is disrupted with use of 2′-amino-2′-deoxyguanosine. The coupling is restored when softer Mn²⁺ ions are added to the buffer. This evidence generalizes a model for group I ribozyme catalysis that involves metal coordination to the 2′ position of guanosine. However, we see one striking difference in that the guanosine-substrate coupling is reversed. In the Azoarcus system (10 °C) the guanosine/substrate binds 5-fold more tightly to E_free than to E · S or E · G, which is the opposite for Tetrahymena even when the later is run at 4 °C. One implication for this difference in coupling is that the Azoarcus is in a folded state well accommodated for guanosine or substrate binding. This initial binding actually causes a conformational change that retards the subsequent binding of the second cofactor, which contrasts what was found for the Tetrahymena ribozyme. These results indicate that while the role for the metal ions in the chemical catalysis is conserved across phylogenetic boundaries, there is variability in the folding pattern of the ribozyme that leads to phosphoryl transfer.     

The Sequence Dependence of Human Nucleotide Excision Repair Efficiencies of Benzo[a]pyrene-derived DNA Lesions: Insights into the Structural Factors that Favor Dual Incisions

Nucleotide excision repair (NER) is a vital cellular defense system against carcinogen-DNA adducts, which, if not repaired, can initiate cancer development. The structural features of bulky DNA lesions that account for differences in NER efficiencies in mammalian cells are not well understood. In vivo, the predominant DNA adduct derived from metabolically activated benzo[a]pyrene (BP), a prominent environmental carcinogen, is the 10S (+)-trans-anti-[BP]-N²-dG adduct (G*), which resides in the B-DNA minor groove 5′-oriented along the modified strand. We have compared the structural distortions in double-stranded DNA, imposed by this adduct, in the different sequence contexts 5′-…CGG*C…, 5′-…CG*GC…, 5′-…CIG*C… (I is 2′-deoxyinosine), and 5′-…CG*C…. On the basis of electrophoretic mobilities, all duplexes manifest moderate bends, except the 5′-…CGG*C…duplex, which exhibits an anomalous, slow mobility attributed to a pronounced flexible kink at the site of the lesion. This kink, resulting from steric hindrance between the 5′-flanking guanine amino group and the BP aromatic rings, both positioned in the minor groove, is abolished in the 5′-…CIG*C…duplex (the 2′-deoxyinosine group, I, lacks this amino group). In contrast, the sequence-isomeric 5′-…CG*GC…duplex exhibits only a moderate bend, but displays a remarkably increased opening rate at the 5′-flanking base pair of G*, indicating a significant destabilization of Watson-Crick hydrogen bonding. The NER dual incision product yields were compared for these different sequences embedded in otherwise identical 135-mer duplexes in cell-free human HeLa extracts. The yields of excision products varied by a factor of as much as ∼ 4 in the order 5′-...CG*GC…> 5′...CGG*C…≥ 5′...CIG*C…≥ 5′-…CG*C…. Overall, destabilized Watson-Crick hydrogen bonding, manifested in the 5′-...CG*GC...duplex, elicits the most significant NER response, while the flexible kink displayed in the sequence-isomeric 5′-...CGG*C...duplex represents a less significant signal in this series of substrates. These results demonstrate that the identical lesion can be repaired with markedly variable efficiency in different local sequence contexts that differentially alter the structural features of the DNA duplex around the lesion site.     

Structures of Intermediates along the Catalytic Cycle of Terminal Deoxynucleotidyltransferase: Dynamical Aspects of the Two-Metal Ion Mechanism

Terminal deoxynucleotidyltransferase (Tdt) is a non-templated eukaryotic DNA polymerase of the polX family that is responsible for the random addition of nucleotides at the V(D)J junctions of immunoglobulins and T-cell receptors. Here we describe a series of high-resolution X-ray structures that mimic the pre-catalytic state, the post-catalytic state and a competent state that can be transformed into the two other ones in crystallo via the addition of dAMPcPP and Zn^2 +, respectively. We examined the effect of Mn^2 +, Co^2 + and Zn^2 + because they all have a marked influence on the kinetics of the reaction. We demonstrate a dynamic role of divalent transition metal ions bound to site A: (i) Zn^2 + (or Co^2 +) in Metal A site changes coordination from octahedral to tetrahedral after the chemical step, which explains the known higher affinity of Tdt for the primer strand when these ions are present, and (ii) metal A has to leave to allow the translocation of the primer strand and to clear the active site, a typical feature for a ratchet-like mechanism. Except for Zn^2 +, the sugar puckering of the primer strand 3′ terminus changes from C2′-endo to C3′-endo during catalysis. In addition, our data are compatible with a scheme where metal A is the last component that binds to the active site to complete its productive assembly, as already inferred in human pol beta. The new structures have potential implications for modeling pol mu, a closely related polX implicated in the repair of DNA double-strand breaks, in a complex with a DNA synapsis.     

The structural definition of adducts of stoichiometry AgX:dppf (1:1)(n), X = simple anion, dppf = bis(diphenylphosphino)ferrocene

Single crystal X-ray structural characterizations are recorded for an array of adducts of the form AgX:dppf (1:1)_(n), X = simple (pseudo-)halide or oxy-anion, ‘dppf’ = bis(diphenyl phosphino)ferrocene, for adducts X = Cl (new phase), Br, I, SCN, OCN, CN, NO₃ (new phase), O₂CCH₃, n = 2, the form being dimeric [(dppf-P,P′)Ag(μ-X)₂Ag(P,P′-dppf)], for X = I, SCN, [Ag(μ-X)₂(P-dppf-P′)₂Ag′]; for X = O₂CCF₃, n = ∞, the form is an extended polymer: ?Ag(O · CO · CF₃)(P-dppf-P′)Ag′(O?. A dichloromethane solvate phase of CuI:dppf (1:1)₂ (also centrosymmetric) is also recorded. Synthetic procedures for all adducts have been reported. All compounds have been characterized both in solution (¹H, ¹³C, ³¹P NMR, ESI MS) and in the solid state (IR). The topology of the structures in the solid state was found to depend on the nature of the counterion.     

The synthesis and characterization of mixed ligand Ru(II) complexes with dipyrido(2,3-a:3′,2′-j)phenazine (dpop′) and 2,3,5,6-tetra(2-pyridyl)-pyrazine (tppz)

The synthesis of the mixed ligand mono metallic [Ru(dpop′)(tppz)]²⁺ and bimetallic [(dpop′)Ru(tppz)Ru(dpop′)]⁴⁺ (dpop′ = dipyrido(2,3-a:3′,2′-j)phenazine; tppz = 2,3,5,6 tetra-(2-pyridyl)pyrazine) complexes is described. The [Ru(dpop′)(tppz)]²⁺ complex display an intense absorption at 518 nm which is assigned to a Ru(dπ) → dpop′ (π∗) MLCT transition, and at 447 nm which is assigned to a Ru(dπ) → tppz(π∗) MLCT transition. It undergoes emission at RT in CH₃CN with λ_em = 722 nm. The bimetallic [(dpop′)Ru(tppz)Ru(dpop′)]⁴⁺ complex shows a low energy absorption shoulder near 635 nm assigned to a Ru(dπ) → tppz(π∗) MLCT transition and an intense peak at 542 nm due to Ru(dπ) → dpop′ (π∗) MLCT transition. The bimetallic complex also emits at RT in CH₃CN with λ_em = 785 nm. Cyclic voltammetry shows reversible Ru^+2/+3 oxidations at 1.68 V for the monometallic complex and Ru^+2/+3 oxidation couples at +1.94 and +1.70 V for the bimetallic complex.     

Reaction behavior of molybdocene dichloride towards ribonucleosides and ribonucleoside monophosphates: Rare example of sugar coordination

The coordination behavior of Cp₂Mo²⁺ towards the ribonucleosides and ribonucleoside monophosphates uridine, adenosine, cytidine, guanosine, 5′-UMP, 5′-AMP, 5′-CMP and 5′-GMP has been studied in solution in the range 4 ? pD ? 9 using NMR spectroscopy. The ribonucleosides were found to bind Cp₂Mo²⁺ exclusively through the ribose moiety giving rise to the chelate complexes [Cp₂Mo(urd-O2′,O3′)], [Cp₂Mo(ade-O2′,O3′)], [Cp₂Mo(cyd-O2′,O3′)], and [Cp₂Mo(gua-O2′,O3′)]. The ribonucleotides form three types of complex with Cp₂Mo²⁺ in neutral solution, namely N,PO-macrochelates, PO,O3′-coordinated species as well as O2′,O3′-chelates, while at pD 9 only sugar coordination is observed.     

Synthesis, structure and magnetic studies of two-dimensional network of Cu(II) polymer using malonate and 4,4′-azobispyridine ligands

The malonato-bridged copper(II) complex [Cu(mal)(H₂O)(azpy)_1/2] · H₂O (1) (mal = malonate, azpy = 4,4′-azobispyridine) has been synthesized and characterized by X-ray diffraction. The structure of 1 consists of malonato-bridged uniform copper(II) chains which are covalent connected through azpy to form two-dimensional wavelike network. The magnetic pathway of complex 1 is through a single syn-anti carboxylate bridge connecting equatorial and equatorial positions of adjacent copper(II) atoms, and have the value of the intrachain ferromagnetic coupling (J = 8.73(3) cm⁻¹) and interchain antiferromagnetic coupling (zJ′ = − 1.31(1) cm⁻¹) through a numerical expression for a ferromagnetic uniform chain.     

Molecular mechanism of ADP-ribose hydrolysis by human NUDT5 from structural and kinetic studies

Human NUDT5 (hNUDT5) is an ADP-ribose (ADPR) pyrophosphatase (ADPRase) that plays important roles in controlling the intracellular levels of ADPR and preventing non-enzymatic ADP-ribosylation of proteins by hydrolyzing ADPR to AMP and ribose 5′-phosphate. We report the crystal structure of hNUDT5 in complex with a non-hydrolyzable ADPR analogue, α,β-methyleneadenosine diphosphoribose, and three Mg^2 + ions representing the transition state of the enzyme during catalysis. Analysis of this structure and comparison with previously reported hNUDT5 structures identify key residues involved in substrate binding and catalysis. In the transition-state structure, three metal ions are bound at the active site and are coordinated by surrounding residues and water molecules. A conserved water molecule is at an ideal position for nucleophilic attack on the α-phosphate of ADPR. The side chain of Glu166 on loop L9 changes its conformation to interact with the conserved water molecule compared with that in the substrate-bound structure and appears to function as a catalytic base. Mutagenesis and kinetic studies show that Trp28 and Trp46 are important for the substrate binding; Arg51 is involved in both the substrate binding and the catalysis; and Glu112 and Glu116 of the Nudix motif, Glu166 on loop L9, and Arg111 are critical for the catalysis. The structural and biochemical data together reveal the molecular basis of the catalytic mechanism of ADPR hydrolysis by hNUDT5. Specifically, Glu166 functions as a catalytic base to deprotonate a conserved water molecule that acts as a nucleophile to attack the α-phosphate of ADPR, and three Mg^2 + ions are involved in the activation of the nucleophile and the binding of the substrate. Structural comparison of different ADPRases also suggests that most dimeric ADPRases may share a similar catalytic mechanism of ADPR hydrolysis.     

Synthesis, characterization and anti-diabetic effect of bis[(1-R-imidazolinyl)phenolato]oxovanadium(IV) complexes

The five-coordinate oxovanadium(IV) complexes; [VO(pimin)₂] (1a), [VO(Etpimin)₂] (2) and [VO(EtOHpimin)₂] (3), were prepared by reacting the ligands; 2-(2′-hydroxyphenyl)-1H-imidazoline (piminH), 2-(2′-hydroxyphenyl)-1-ethylimidazoline (EtpiminH) and 2-(2′-hydroxyphenyl)-1-ethanolimidazoline (EtOHpiminH), with VOSO₄. The complexes were characterized by elemental analysis, IR, UV-Vis and cyclic voltammetry. All complexes show VO stretching vibrations between 932 and 987 cm⁻¹. The presence of three d-d transition occurring between 400 and 625 nm and the irreversible oxidation (V^IV → V^V) between 400 and 490 mV confirm the d¹ electronic configuration of the complexes. The solid state structures of [VO(pimin)₂] (1a) and its autoxidation hydrolysis product [VO₂(pimin)(piminH′)] (1b) were determined by single crystal X-ray diffraction. The geometry of [VO(pimin)₂] was found to be intermediate between trigonal bipyramidal and square pyramidal and sits on a crystallographic twofold axis, while the geometry of [VO₂(pimin)(piminH′)] was distorted trigonal bipyramidal. Potentiometric titrations were used to determine the protonation and stability constants for the ligands and oxovanadium(IV) complexes, respectively. The species existing over a biological pH range were also investigated. The in vitro studies indicated that the oxovanadium(IV) complexes were effective in enhancing glucose uptake in the 3T3-L1 adipocytes, C2C12 muscle cells and Chang liver cell lines. In these cell lines, the anti-hyperglycemic effect was equivalent to or surpassed the effect of metformin.     

The FG loop of a heme-based gas sensor enzyme, Ec DOS, functions in heme binding, autoxidation and catalysis

Ec DOS is a heme-based gas sensor enzyme that catalyzes conversion from cyclic-di-GMP to linear-di-GMP in response to gas molecules, such as oxygen, CO and NO. Ec DOS contains an N-terminal heme-binding PAS domain and C-terminal phosphodiesterase domain. Based on crystal structures of the isolated heme-binding domain, it is suggested that the FG loop is involved in intra-molecular signal transduction to the catalytic domain. We generated nine full-length proteins mutated at ionic and non-ionic polar residues between positions 83 and 96 corresponding to the F-helix and FG loop, and examined the heme binding properties, autoxidation rates, and catalytic activities of mutant proteins. N84A and R85A mutant proteins displayed lower heme binding affinities, consistent with the finding that Asn84 interacts with propionate of protoporphyrin IX, and Arg85 with Asp40 on the heme proximal side. Autoxidation rates (0.058-0.54 min⁻¹) of R91A, S96A and K89A/R91A/E93A mutant proteins were significantly higher than that (0.0053 min⁻¹) of wild-type protein, suggesting that these residues in the FG loop form heme distal architecture conferring stability to the Fe(II)-O₂ complex. Catalytic activities of N84A and R85A mutant proteins with low heme affinity were significantly higher than those of wild-type protein in the absence of gas molecules. Accordingly, we propose that loss of heme binding enhances basal catalysis without the gas molecule, consistent with previous reports on heme inhibition of Ec DOS catalysis.     

Synthesis and isomerisation of two metallated N,O-complexes of ruthenium: Models for the Murai reaction

Two isomers of the N,O-coordinated acetylpyrrolyl complex [Ru(PPh₃)₂(CO)(NC₄H₃C(O)CH₃)H] {cis-N,H (1) and trans-N,H (2)} have been prepared as models for catalytic intermediates in the Murai reaction. Complex 2 isomerises to 1 upon heating via a dissociative pathway (ΔH^‡ = 195 ± 41 kJ mol⁻¹; ΔS^‡ = 232 ± 62 J mol⁻¹ K⁻¹); the mechanism of this process has been modeled using density functional calculations. Complex 2 displays moderate catalytic activity for the Murai coupling of 2′-methylacetophenone with trimethylvinylsilane, but 1 proved to be catalytically inactive under the same conditions.     

Aminocarboxylate complexes of vanadium(III): Electronic structure investigation by high-frequency and -field electron paramagnetic resonance spectroscopy

Aminocarboxylate complexes of vanadium(III) are of interest as models for biologically and medicinally relevant forms of this interesting and somewhat neglected ion. The V(III) ion is paramagnetic, but not readily suited to conventional EPR, due to its integer-spin ground state (S = 1) and associated large zero-field splitting (zfs). High-frequency and -field EPR (HFEPR), however, has the ability to study such systems effectively. Three complexes, all previously structurally characterized: Na[V(trdta)] · 3H₂O, Na[V(edta)(H₂O)] · 3H₂O, and [V(nta)(H₂O)₃] · 4H₂O (where trdta stands for trimethylenediamine-N,N,N′,N′-tetraacetate and nta stands for nitrilotriacetate) were studied by HFEPR. All the investigated complexes produced HFEPR responses both in the solid state, and in aqueous solution, but those of [V(nta)(H₂O)₃] · 4H₂O were poorly interpretable. Analysis of multi-frequency HFEPR spectra yielded a set of spin Hamiltonian parameters (including axial and rhombic zfs parameters: D and E, respectively) for these first two complexes as solids: Na[V(trdta)] · 3H₂O: D = 5.60 cm⁻¹, E = 0.85 cm⁻¹, g = 1.95; Na[V(edta)(H₂O)] · 3H₂O: D = 1.4 cm⁻¹, E = 0.14 cm⁻¹, g = 1.97. Spectra in frozen solution yielded similar parameters and showed multiple species in the case of the trdta complex, which are the consequence of the flexibility of this ligand. The EPR spectra obtained in frozen aqueous solution are the first, to our knowledge, of V(III) in solution in general and show the applicability of HFEPR to these systems. In combination with very insightful previous studies of the electronic absorption of these complexes which provided ligand-field parameters, it has been possible to describe the electronic structure of V(III) in [V(trdta)]⁻ and [V(edta)(H₂O)]⁻; the quality of data for [V(nta)(H₂O)₃] does not permit analysis. Qualitatively, six-coordinate V(III) complexes with O,N donor atoms show no electronic absorption band in the NIR region, and exhibit relatively large magnitude zfs (D ? 5 cm⁻¹), while analogous seven-coordinate complexes do have a NIR absorption band and show relatively small magnitude zfs (D < 2 cm⁻¹).     

In situ hydrothermal syntheses, crystal structures and luminescent properties of two novel zinc(II) coordination polymers based on tetrapyridyl ligand

Two novel Zn(II) coordination polymers, [Zn(2-pytpy)(fum)]_n·nH₂O (1) and [Zn₆(4-pytpy)₃(mal)₄]_n·5n(H₂O) (2), (2-pytpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine, 4-pytpy = 4′-(4-pyridyl)-4,2′:6′,4″-terpyridine, H₂fum = fumaric acid and H₂mal = malic acid) have been hydrothermally synthesized and structurally characterized. Notably, in situ ligand reactions occur in the formation of complexes 1 and 2, in which maleic acid is converted into fumaric acid and malic acid, respectively. Complex 1 is a 1D infinite chain structure, which is extended into a supramolecular layer by intermolecular π…π stacking interactions. Complex 2 is a 3D network structure, in which the bidentate-bridging 4-pytpy ligands link the layers based on the tetranuclear Zn(II) subunits to form the (4,10)-connected network. The luminescent properties of 1 and 2 have been investigated with emission spectra and UV-Vis diffuse reflectance spectra in the solid state. Additionally, these two complexes possess great thermal stabilities.     

Optimization of the in-silico-designed kemp eliminase KE70 by computational design and directed evolution

Although de novo computational enzyme design has been shown to be feasible, the field is still in its infancy: the kinetic parameters of designed enzymes are still orders of magnitude lower than those of naturally occurring ones. Nonetheless, designed enzymes can be improved by directed evolution, as recently exemplified for the designed Kemp eliminase KE07. Random mutagenesis and screening resulted in variants with > 200-fold higher catalytic efficiency and provided insights about features missing in the designed enzyme. Here we describe the optimization of KE70, another designed Kemp eliminase. Amino acid substitutions predicted to improve catalysis in design calculations involving extensive backbone sampling were individually tested. Those proven beneficial were combinatorially incorporated into the originally designed KE70 along with random mutations, and the resulting libraries were screened for improved eliminase activity. Nine rounds of mutation and selection resulted in > 400-fold improvement in the catalytic efficiency of the original KE70 design, reflected in both higher k_cat values and lower K_m values, with the best variants exhibiting k_cat/K_m values of > 5 × 10⁴ s^− 1 M^− 1. The optimized KE70 variants were characterized structurally and biochemically, providing insights into the origins of the improvements in catalysis. Three primary contributions were identified: first, the reshaping of the active-site cavity to achieve tighter substrate binding; second, the fine-tuning of electrostatics around the catalytic His-Asp dyad; and, third, the stabilization of the active-site dyad in a conformation optimal for catalysis.     

On the elusive, non-emissive yet reactive, upper excited states of [(4,4′-bpy)Re(CO)3(dppz)]PF6 (dppz = dipyridil[3,2-a:2′3′-c]phenazine)

Gradual solvation of [(4,4′-bpy)Re^I(CO)₃(dppz)]⁺ (dppz = dipyridil[3,2-a:2′3′-c]phenazine) by water molecules causes a quenching of the emission in accordance with Perrin’s model of spheres. The calculated radius of the sphere, r = 2.6 ± 0.2 ?, is therefore very close to the distance from the Re center to the oxygen atom of the CO ligands, i.e., l = 2.73 ?. In addition, excited state reactions with TEA produce [(4,4′-bpy⁻)Re^I(CO)₃(dppz)] and [(4,4′-bpy)Re^I(CO)₃(dppz⁻)]. This experimental observation is inconsistent with the formation of the products in the lowest lying and emissive dppz-centered ³ππ* excited state. Jablonski schemes based on the participation of excited states other than the lowest ³ππ* excited state are proposed.     

Binding of C5 Protein to P RNA Enhances the Rate Constant for Catalysis for P RNA Processing of Pre-tRNAs Lacking a Consensus G(+ 1)/C(+ 72) Pair

The RNA subunit of the ribonucleoprotein enzyme ribonuclease P (RNase P (P RNA) contains the active site, but binding of Escherichia coli RNase P protein (C5) to P RNA increases the rate constant for catalysis for certain pre-tRNA substrates up to 1000-fold. Structure-swapping experiments between a substrate that is cleaved slowly by P RNA alone (pre-tRNA^f-met605) and one that is cleaved quickly (pre-tRNA^met608) pinpoint the characteristic C(+ 1)/A(+ 72) base pair of initiator tRNA^f-met as the sole determinant of slow RNA-alone catalysis. Unlike other substrate modifications that slow RNA-alone catalysis, the presence of a C(+ 1)/A(+ 72) base pair reduces the rate constant for processing at both correct and miscleavage sites, indicating an indirect but nonetheless important role in catalysis. Analysis of the Mg²⁺ dependence of apparent catalytic rate constants for pre-tRNA^met608 and a pre-tRNA^met608 (+ 1)C/(+ 72)A mutant provides evidence that C5 promotes rate enhancement primarily by compensating for the decrease in the affinity of metal ions important for catalysis engendered by the presence of the CA pair. Together, these results support and extend current models for RNase P substrate recognition in which contacts involving the conserved (+ 1)G/C(+ 72) pair of tRNA stabilize functional metal ion binding. Additionally, these observations suggest that C5 protein has evolved to compensate for tRNA variation at positions important for binding to P RNA, allowing for tRNA specialization.     

Electrochemical formation of bi- versus tetranuclear μ-oxo terpyridine manganese complexes in CH3CN. Influence of the terpyridine substituents

To complete the elucidation of the electrochemical properties of Mn^II-bis(terpyridine) complexes in CH₃CN and evaluate the influence of the bulkiness of the terpy substituents, the oxidation processes of [Mn^II(L)₂]²⁺ (L = terpy for 2,2′:6′,2″-terpyridine, pTol-terpy for 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine and tBu₃-terpy for 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) have been investigated in aqueous (1 M) CH₃CN solution. In this medium, exhaustive oxidations at 1.10-1.20 V versus Ag/Ag⁺ release two electrons per molecule of initial complex and lead to clean dimerization processes with the quantitative formation of the oxo-bridged binuclear [Mn₂^IVO₂(L)₂(H₂O)₂]⁴⁺ complex for L = tBu₃-terpy and of the tetranuclear [Mn₄^IVO₅(L)₄(H₂O)₂]⁶⁺ complexes for L = terpy and pTol-terpy. The formation of the tetranuclear complex with the tBu₃-terpy derivative is prevented by the steric hindrance induced by the bulkiness of the tert-butyl groups, as confirmed by molecular mechanics calculations, as well as by their strong electron-donating properties. All these electrogenerated multinuclear complexes have been fully characterized in solution by UV-vis and electron paramagnetic resonance (EPR) spectroscopy. A markedly improved chemical synthesis of [Mn₄^IVO₅(terpy)₄(H₂O)₂]⁶⁺ is also reported.     

Interaction of the DNA bases and their mononucleotides with pyridine-2-carbaldehyde thiosemicarbazonecopper(II) complexes. Structure of the cytosine derivative

Experimental studies of the binding interactions of [CuL(NO₃)] and [{CuL′(NO₃)}₂] (HL = pyridine-2-carbaldehyde thiosemicarbazone, and HL′ = pyridine-2-carbaldehyde 4N-methylthiosemicarbazone) with adenine, guanine, cytosine, thymine and their mononucleotides (dNMP), 2-deoxyadenosine-5′-monophosphate, (dAMP), 2′-deoxyguanosine-5′-monophosphate, (dGMP), 2′-deoxycytidine-5′-monophpsphate (dCMP), and thymidine-5′-monophosphate (dTMP) have been carried out in aqueous solution at pH 6.0, I = 0.1 M (NaClO₄) and T = 25 °C. The complexation constants of these compounds, calculated by Hildebrand-Benesi plots for the dye binding, D, ([CuL] or [CuL′]) to the nucleobases or nucleotides (P), have shown two linear stretches in adenine, guanine, dAMP and dGMP. The data were analyzed in terms of formation of 1:1 DP and 1:2 DP₂ complexes with increasing purine base or nucleotide content. For cytosine and dCMP only 1:1 complexes have been observed, whereas for thymine and dTMP such complex structures were not observed. The [CuL(Hcyt)](ClO₄) cytosine derivative has been isolated and characterized. The crystal structure consists of perchlorate ions and [CuL(Hcyt)]⁺ monomers attached by hydrogen bond, chelate π−ring and anion-π interactions. The Cu²⁺ ions bind to the NNS chelating moiety of the thiosemicarbazone ligand and the cytosine N13 site (N3, most common notation) yielding a square-planar geometry. A pseudocoordination to the cytosine O12 site (=O2) can also be considered.     

A new type of electrochemical oxidation of alcohols mediated with a ruthenium-dioxolene-amine complex in neutral water

Electrochemical oxidation of [Ru^II(terpy)(sq)(NH₃)]⁺ in neutral water (pH 8.0) at +0.8 V (versus SCE) generated [Ru^II(terpy)(q)(NH₂)]²⁺ and/or [Ru^III(terpy)(sq)(NH₂)]²⁺ (terpy = 2,2′:6′,2′′-terpyridine, sq = 3,5-di-tert-butyl-1,2-semiquinonate, q = 3,5-di-tert-butyl-1,2-benzoquinone), which played roles in hydrogen abstraction and one-electron acceptor in the catalytic oxidation of methanol, ethanol, and 2-propanol affording formaldehyde, acetoaldehyde, and acetone, respectively, under the electrolysis conditions.