Stabilization of the tertiary structure of yeast phenylalanine tRNA by [Co(NH3)6]3+: X-ray evidence for hydrogen bonding to pairs of guanine bases in the major groove

The sites of three [Co(NH₃)₆]³⁺ ions bound to the phenylalanine tRNA of yeast have been determined by X-ray diffraction analysis. [Co(NH₃)₆]³⁺ binds to purine-purine sequences in yeast tRNA^Phe. It is different from the binding of Co²⁺, which binds to the base and phosphate of residue G15. There are no direct metal-nucleotide bonds, although hydrogen bonding of the coordinated ammines to double-helical guanylguanosine sequences in the major groove and to phosphate oxygens in neighboring polynucleotide strands increases the stability of the structure. Hydrogen-bonding appears to be via cis ammine ligands to N(7) and O(6) positions of adjacent purine bases.     

Stability of metal ion complexes formed with methyl phosphate and hydrogen phosphate

 The acidity constants of methyl phosphoric acid, CH₃OPO(OH)₂, and orthophosphoric acid, HOPO(OH)₂, and the stability constants of the 1 : 1 complexes formed between Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, or Cd²⁺ and methyl phosphate, CH₃OPO₃ ^2–, or hydrogen phosphate, HOPO₃ ^2–, were determined by potentiometric pH titration in aqueous solution (25  °C;I = 0.1 M, NaNO₃). On the basis of previously established log K versus pK _a straight-line plots for the complexes of simple phosphate monoesters and phosphonate derivatives, R-PO₃ ^2–, where R is a noncoordinating residue, it is shown that the stability of the M(CH₃OPO₃) complexes is solely determined (as one might expect) by the basicity of the –PO₃ ^2– residue. It is emphasized that the mentioned reference lines may also be used to reveal increased complex stabilities, for example, for certain complexes formed with 8-quinolyl phosphate the occurrence of 7-membered chelates can be proven in this way; the same procedure is also applicable to complexes of nucleotides, etc. The M(HOPO₃) complexes are slightly more stable (on average by 0.08 log unit) than it is expected from the basicity of HPO₄ ^2–; this observation is attributed to a more effective solvation, including hydrogen bonding, than is possible with CH₃OPO₃ ^2– species. Received: 9 November 1995 / Accepted: 5 February 1996     

Kinetics and mechanism for reduction of anticancer-active tetrachloroam(m)ine platinum(IV) compounds by glutathione

trans -[PtCl₄(NH₃)(thiazole)] (1), trans-[PtCl₄(cha)(NH₃)] (2), cis-[PtCl₄(cha)(NH₃)] (3) (cha =cyclohexylamine), and cis-[PtCl₄(NH₃)₂] (4) has been investigatedat 25 °C in a 1.0 M aqueous medium at pH 2.0–5.0 (1) and 4.5–6.8 (2–4) using stopped-flow spectrophotometry. The redox reactions follow the second-order rate law , where k is a pH-dependent rate constant and [GSH]_tot the total concentration of glutathione. The reduction takes place via parallel reactions between the platinum(IV) complexes and the various protolytic species of glutathione. The pH dependence of the redox kinetics is ascribed to displacement of these protolytic equilibria. The thiolate species GS⁻ is the major reductant under the reaction conditions used. The second-order rate constants for reduction of compounds 1–4 by GS⁻ are (1.43±0.01)×10⁷, (3.86±0.03)×10⁶, (1.83±0.01)×10⁶, and (1.18±0.01)×10⁶M⁻¹s⁻¹, respectively. Rate constants for reduction of 1 by the protonated species GSH are more than five orders of magnitude smaller. The mechanism for the reductive elimination reactions of the Pt(IV) compounds is proposed to involve an attack by glutathione on one of the mutually trans coordinated chloride ligands, leading to two-electron transfer via a chloride-bridged activated complex. The kinetics results together with literature data indicate that platinum(IV) complexes with a trans Cl-Pt-Cl axis are reduced rapidly by glutathione as well as by ascorbate. In agreement with this observation, cytotoxicity profiles for such complexes are very similar to those for the corresponding platinum(II) product complexes. The rapid reduction within 1 s of the platinum(IV) compounds with a trans Cl-Pt-Cl axis to their platinum(II) analogs does not seem to support the strategy of using kinetic inertness as a parameter to increase anticancer activity, at least for this class of compounds. Received: 8 December 1999 / Accepted: 15 February 2000     

FTIR and Raman Spectra Compared with Ab Initio Calculated Frequency Modes for 5-Aminouracil

Infrared (IR) and Raman spectra of 5-aminouracil were recorded in the region 200–4,000 cm^− 1. Assuming under the C_s point group that the distribution of the normal mode of vibrations between the two species are planar (a′) and non-planar (a″), given by 25a′ + 11a″ of which 30 modes (21a′ + 9a″) correspond to the uracil moiety and six modes (4a′ + 2a″) correspond to the NH₂ group, with a comparison of theoretically ab initio calculated frequencies, the results are in reasonably good agreement with the experimental IR and Raman spectra. Consistent assignments have been made for the internal modes of the NH₂ group, especially for the anti-symmetric NH₂ stretching and bending modes. The non-equivalence of the two NH bonds of the NH₂ group suggests a difference in the strength of the two hydrogen bonds on the pyrimidine ring. Symmetry and anti-symmetry NH stretching modes of the NH₂ group show the invalidity of the empirical relationship. These two NH₂ stretching frequencies are distinctly separated from the CH/NH ring stretching frequencies. A strong and sharp IR band that acts at 3,380 cm^− 1 could be identified as the anti-symmetric NH₂ mode whereas the band at 3,290 cm^− 1 smaller density could be identified as the symmetric NH₂ stretching mode.     

Theoretical study of crown ethers with incorporated azobenzene moiety

A series of crown ethers containing the azobenzene moiety incorporated into crowns of various sizes [Cr(O₆)_, Cr(O₇) and Cr(O₈)] and their corresponding alkali metal cation (Li⁺, Na⁺, K⁺, Rb⁺) complexes have been studied theoretically. The density functional theory (DFT) method was employed to elucidate the stereochemical structural natures and thermodynamic properties of all of the target molecules at the B3LYP/6-31 G(d) and LANL2DZ level for the cation Rb⁺. The fully optimized geometries had real frequencies, thus indicating their minimum-energy status. In addition, the bond lengths between the metal cation and oxygen atoms, atomic torsion angles and thermodynamic energies for complexes were studied. Natural bond orbital (NBO) analysis was used to explore the origin of the internal forces and the intermolecular interactions for the metal complexes. The calculated results show that the most significant interaction is that between the lone pair electrons of electron-donating oxygens in the cis-forms of azobenzene crown ethers (cis-ACEs) and the LP* (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Li⁺, Na⁺, K⁺ and Rb⁺). The electronic spectra for the cis-ACEs [cis-Cr(O₆), cis-Cr(O₇) and cis-Cr(O₈)] are obtained by the time-dependent density functional theory (TDDFT) at the B3LYP/6-31 G(d) level. The spectra of the cis-isomers show broad π → π* (S₀ → S₂) absorption bands at 310–340 nm but weaker n → π* (S₀ → S₁) bands at 480–490 nm. The calculated results are in good agreement with the experimental results.     

Unusual silver coordination by mono-carboxylate utilization of a poly-carboxylate anion, solid-state structures of silver-tetrachlorophthalate with phthalazine and amine ligands

Silver carboxylate coordination with the tetrachlorophthalate anion, in combination with neutral donor ligands, has been found to deviate from other known poly-carboxylate complexes. Both complexes reported here, bis-[tetrachlorophthalato-silver phthalazine] and bis-[ammino-tetrachlorophthato-silver di-ammino-silver], utilize mixed carboxylate bonding types for silver coordination. In the case of the phthalazine ligand, both chelating and monodentate carboxylates form the framework for the oligomeric structure. In the case of the ammine ligand, one carboxylate forms a monodentate connection to a silver-ammine group, while the other is simply involved with hydrogen bonding to lock in a [(Ag-NH₃)₂-Ag-NH₃] substructure with an adjacent tetrachlorophthalato-silver unit. Both structures exhibit supramolecular connections via hydrogen bonding and π-π interactions.     

Trans-pyridine tetraammine complexes of RuII and RuIII with N7-coordninated purine nucleosides

 The synthesis, spectroscopic, and electrochemical properties of trans-[L(Pyr)(NH₃)₄Ru^II/III] (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, or 4-bnpy; L=H₂O, Guo, dGuo, 1MeGuo, Gua, Ino, or G⁷-DNA) are reported. As expected, the Pyr ligand slows DNA binding by trans-[(H₂O)(Pyr)(NH₃)₄Ru^II]²⁺ relative to [(H₂O)(NH₃)₅Ru^II]²⁺ and favors reduction of Ru^III by about 150 mV. The pyridine ligand also promotes the disproportionation of Ru^III to afford the corresponding complexes of Ru^II and, presumably, Ru^IV. For L=Ino, disproportionation follows the rate law: d[Ru^II]/dt=k ₀[Ru^III]+k ₁[OH^–][Ru^III], k ₀=(2.7±0.7)×10^–4s^–1 and k ₁=70±1 M^–1s^–1. Received: 11 May 1998 / Accepted: 3 March 1999     

The platinum-olefin binding energy in series of (PH<Subscript>3</Subscript>)<Subscript>2</Subscript>Pt(olefin) complexes - a theoretical study

Theoretical investigation of Pt(0)-olefin organometallic complexes containing tertiary phosphine ligands was focused on the strength of platinum-olefin electronic interaction. DFT theoretical study of electronic effects in a substantial number of ethylene derivatives was evaluated in terms of the Pt-olefin binding energy using MP2 correlation theory. Organometallics bearing coordinated olefins with general formula (R¹R²C = CR³R⁴)Pt(PH₃)₂ [R = various substituents] had been selected, including olefins containing both electron-donor substituents as well as electron-withdrawing groups. The stability of the corresponding complexes increases with a strengthening electron-withdrawal ability of the olefin substituents. Figure Representation of (CH₂ = CHR)Pt(PPh₃)₂ and the stability chart     

Metallopeptide-promoted inactivation of angiotensin-converting enzyme and endothelin-converting enzyme 1: toward dual-action therapeutics

A series of metallopeptides based on the amino terminal copper/nickel (ATCUN) binding motif have been evaluated as classical inhibitors and catalytic inactivators of both rabbit and human angiotensin-converting enzyme (hACE), and human endothelin-converting enzyme 1 (hECE-1). The cobalt complex [KGHK–Co(NH₃)₂]²⁺, where KGHK is lysylglycylhistidyllysine, displayed similar K _I and IC₅₀ values to those found for [KGHK–Cu]⁺, in spite of the enhanced charge, and so either the influence of charge is offset by the steric influence of the axially coordinated ammine ligands, or binding is dominated by contributions from the amino acid side chains, especially the C-terminal lysine that mimics the binding pattern observed for lisinopril. Moreover, the inhibition observed for [KGHK–Co(NH₃)₂]²⁺ contrasts with the activation of hACE by Co²⁺(aq), reflecting the stimulation of enzyme activity following replacement of the catalytic zinc cofactor by cobalt ion at each of the two active sites. Quantitative analysis of the dose-dependent stimulation of activity by Co²⁺(aq) yielded apparent affinities of 1.3 ± 0.2 and 56 ± 8 μM for the two sites in the presence of saturating Zn²⁺ (10 μM). Catalytic inactivation of hACE by [KGHK–Cu] ⁺ at subsaturating concentrations had previously been characterized, with k _obs = 2.9 ± 0.5 × 10⁻² min⁻¹. Under similar conditions, the same complex is found to catalytically inactivate hECE-1, with k _obs = 2.12 ± 0.16 × 10⁻² min⁻¹, demonstrating the potential for dual-action activity against two key drug targets in cardiovascular disease. Irreversible inactivation of a drug target represents a novel mechanism of drug action that complements existing classical inhibitor strategies that underlie current drug discovery efforts.Electronic Supplementary Material Supplementary material is available to authorized users in the online version of this article at .     

The TpG chelate of cis(diammineplatinum) forms two head-to-head rotamers in H2O solution

 d(TpG)^– reacts with cis-[Pt(NH₃)₂(H₂O)₂]²⁺ in two steps to yield the platinum chelate cis-[Pt(NH₃)₂{d(TpG)-N3(1),N7(2)}]. In the latter, hindered rotation of the bases leads to an equilibrium between two rotamers interconverting slowly on the NMR time scale. The structure of the two rotameric chelates was studied by means of ¹H NMR and molecular modeling techniques. The major and minor rotamers could be assigned unambiguously to the two head-to-head conformational domains which are characterized by syn/anti and anti/anti sugar-base orientations, respectively. Molecular models derived for both rotamers show that the orientations of the bases are mutually quasi-enantiomeric. The interconversion between the two rotamers (k ≈ 1 s^–1 at 293 K) is approximately 10⁴ times faster than the analogous rotamer interconversion observed in cis-[Pt(NH₃)₂{r(CpG)-N3(1),N7(2)}]⁺ [Girault J-P, Chottard G, Lallemand J-Y, Huguenin F, Chottard J-C (1984) J Am Chem Soc 106 : 7227–7232], suggesting that the steric clash of the exocyclic amino group of the platinum-bound cytosine with the ligands in cis position is more severe than that of the two thymine oxo groups. Received: 23 June 1997 / Accepted: 30 September 1997     

Active excretion of ammonia across the gills of the shore crab Carcinus maenas and its relation to osmoregulatory ion uptake

The mechanism of transbranchial excretion of total ammonia of brackish-water acclimated shore crabs, Carcinus maenas was examined using isolated, perfused gills. Applying physiological gradients of NH₄Cl (100–200 μmol · l⁻¹) directed from the haemolymph space to the bath showed that the efflux of total ammonia consisted of two components. The saturable component (excretion of NH₄ ⁺) greatly exceeded the linear component (diffusion of NH₃). When an outwardly directed gradient (200 μmol · l⁻¹) was applied, total ammonia in the perfusate was reduced by more than 50% during a single passage of saline through the gill. Effluxes of ammonia along the gradient were sensitive to basolateral dinitrophenol, ouabain, and Cs⁺ and to apical amiloride. Acetazolamide (1 mmol · l⁻¹ basolateral) or Cl⁻-free conditions had no substantial effects on ammonia flux, which was thus independent of both carbonic anhydrase mediated pH regulation and osmoregulatory NaCl uptake. When an inwardly directed gradient (200 μmol · l⁻¹) was employed, influx rates were about 10-fold smaller and unaffected by basolateral ouabain (5 mmol · l⁻¹) or dinitrophenol (0.5 mmol · l⁻¹). Under symmetrical conditions (100 μmol · l⁻¹ NH₄Cl on both sides) ammonia was actively excreted against the gradient of total ammonia, which increased strongly during the experiment and against the gradient of the partial pressure of NH₃. The active excretion rate was reduced to 7% of controls by basolateral dinitrophenol (0.5 mmol · l⁻¹), to 44% by basolateral ouabain (5 mmol · l⁻¹), to 46% by Na⁺-free conditions and to 42% by basolateral Cs⁺ (10 mmol · l⁻¹), indicating basolateral membrane transport of NH₄ ⁺ via the Na⁺/K⁺-ATPase and K⁺-channels and a second active, apically located, Na⁺ independent transport mechanism of NH₄ ⁺. Anterior gills, which are less capable of active ion uptake than posterior gills, exhibited even increased rates of active excretion of ammonia. We conclude that, under physiological conditions, branchial excretion of ammonia is a directed process with a high degree of effectiveness. It even allows active extrusion against an inwardly directed gradient, if necessary. Accepted: 11 March 1998     

Reduction and Subsequent Binding of Ruthenium Ions Catalyzed by Subcellular Components

The reduction of Cl(NH₃)₅Ru(III) and subsequent binding of heterocyclic ligands by the resultant (H₂O)(NH₃)₅Ru(II) ion is shown to be catalyzed by components of rat-liver cells. The presence of air significantly decreases the rate of heterocyclic ligand binding. In the case of microsome and soluble component catalysis, this is probably due to oxidation of the Ru(II) ion prior to complexation. Various inhibitors of electron-transfer proteins were employed in an effort to determine the preferred reducing species. These results lend support to the hypothesis that the antitumor activity of acido ruthenium(III) ammine complexes involves activation by reduction in vivo prior to metal coordination to nucleic acids. Anticancer drugs functioning by this mechanism may be preferentially toxic to or may localize in hypoxic areas of tumors.     

Yeast inorganic pyrophosphatase substrate recognition

Monodentate Co(NH₃)₅PP_i was determined not to be a substrate for yeast inorganic pyrophosphatase while P¹,P²-bidentate Co(NH₃)₄PP_i was turned over by the enzyme at a rate of 7.5 min^?1. A kinetic analysis of the substrate activities of the P¹,P²-bidentate complexes, Co(en)₂PP_i, Cr(NH₃)₄PP_i, Cr(H₂O)(NH₃)₃PP_i, Cr(H₂O)₂(NH₃)₂PP_i, and Cr(H₂O)₄PP_i was carried out in order to access the potential role of the metal-water ligands in productive binding. While substitution of the H₂O ligands with NH₃ ligands had a minimal affect on the K_m for Mg²⁺, the binding affinity of the complexes decreased with an increasing $\text{NH}{}_3\text{H}{}_2\text{O}$ ligand ratio as did the turnover number of the corresponding central complexes. The Co(en)₂PP_i complex was hydrolyzed at a rate approximately 0.6% of that for the Co(NH₃)₄PP_i complex. The substrate activities of β,γ-bidentate Co(NH₃)₄PPP_i and α,β,γ-tridentate Co(NH₃)₃PPP with pyrophosphatase were also tested. While both complexes were shown to bind tightly to the Mg²⁺-activated enzyme neither was hydrolyzed. On the other hand, in the presence of the Zn²⁺-activated enzyme the tridentate complex was turned over at a rate of 0.17 min^?1 while the bidentate complex remained inert to hydrolysis.     

Analysis of metallonucleoside hydrolysis by high-pressure liquid chromatography

A number of ammine complexes of transition metals in the platinum group exhibit antitumor and mutagenic activities, which probably result from in vivo metal ion coordination to nucleic acids. Coordination at N-7 purine sites on nucleic acids may induce depurination through a general acid-catalyzed cleavage of the sugar-purine bond. This possibility was tested by observing the hydrolysis of [dGuo)(NH₃)₅Ru]³⁺ under physiological conditions. Since multiple products result from this reaction, a high-pressure liquid chromatography technique was developed for separating various ammineruthenium(III) complexes with purine, pyrimidine, and nucleoside ligands. Using this technique it was possible to identify and follow the relative concentrations of several of the hydrolysis products. These data were used to develop a preliminary reaction scheme for the decomposition of (dGuo)(NH₃)₅Ru(III). The net rate constant (1.8 × 10^?6 s^?1) for the disappearance of this complex yields an upper limit for the rate of metal-induced sugar hydrolysis. While the half-life ($\text{5}\text{days}\text{< t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 28}\text{days}$) for the sugar hydrolysis reaction is substantially shorter than that for the free nucleoside under the same conditions, it cannot be concluded that this represents a major contribution to the mutagenicity of Ru(III) complexes.     

Hydrogen bonding: further evidence for the cause of the color change of nitrosylpentaamminechromium (III) compounds. Crystal structures of [Cr(NO)(NH3)5](PF6)2 and [Cr(NO)(NH3)5]Cl(PF6)

The crystal structures of [Cr(NO)(NH₃)₅](PF₆)₂ (red) and [Cr(NO)(NH₃)₅]Cl(PF₆) (brown) have been determined. The [Cr(NO)(NH₃)₅]²⁺(A) complex cations in these compounds have a slightly distorted octahedral geometry with a strictly linear Cr-N-O arrangement (from symmetry). The short interatomic distances (2.399 Å × 4) between the O (nitrosyl) and H (ammonia in adjacent complex cations) atoms in A(PF₆)₂ indicate the existence of hydrogen bonds, while the interatomic distances (3.258 Å × 8) between those in ACl(PF₆) are much longer, and the hydrogen bonds should be weak in spite of the presence of the smaller counter anion of chloride ion in ACl(PF₆). Comparisons of the five crystal structures of A(PF₆)₂, ACl₂, ACl(ClO₄), ACl(PF₆), and A(ClO₄)₂ have led to the conclusion that the existence of the strong hydrogen bonds gives red crystals of A(PF₆)₂, while the absence of hydrogen bonds results in the formation of green crystals of A(ClO₄)₂ (O ? H, 3.595 Å × 2). The color change of the crystals (from red to green) with the change of outer sphere anions is attributed to the change of the strength of the hydrogen bonding between the complex cations.     

Role of Na+/ H+ exchange and HCO3 − transport in pHi recovery from intracellular acid load in cultured epithelial cells of sheep rumen

This study sought to investigate effects of short-chain fatty acids and CO₂ on intracellular pH (pH_i) and mechanisms that mediate pH_i recovery from intracellular acidification in cultured ruminal epithelial cells of sheep. pH_i was studied by spectrofluorometry using the pH-sensitive fluorescent indicator 2′,7′-bis (carboxyethyl)-5(6′)-carboxyfluorescein acetoxymethyl ester (BCECF/AM). The resting pH_i in N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES)-buffered solution was 7.37 ± 0.03. In HEPES-buffered solution, a NH₄ ⁺/NH₃-prepulse (20 mM) or addition of butyrate (20 mM) led to a rapid intracellular acidification (P < 0.05). Addition of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; 10 μM) or HOE-694 (200 μM) inhibited pH_i recovery from an NH₄ ⁺/NH₃-induced acid load by 58% and 70%, respectively. pH_i recovery from acidification by butyrate was reduced by 62% and 69% in the presence of EIPA (10 μM) and HOE-694 (200 μM), respectively. Changing from HEPES- (20 mM) to CO₂/HCO₃ ⁻-buffered (5%/20 mM) solution caused a rapid decrease of pH_i (P < 0.01), followed by an effective counter-regulation. 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS; 100 μM) blocked the pH_i recovery by 88%. The results indicate that intracellular acidification by butyrate and CO₂ is effectively counter-regulated by an Na⁺/H⁺ exchanger and by DIDS-sensitive, HCO₃ ⁻-dependent mechanism(s). Considering the large amount of intraruminal weak acids in vivo, both mechanisms are of major importance for maintaining the pH_i homeostasis of ruminal epithelial cells. Accepted: 8 March 2000     

Cis-diammineplatinum(II) forms a macrochelate with 2′-deoxycytidine 5′-monophosphate (dCMP2–)! Reactivity and acid-base properties of cis-Pt(NH3)2(dCMP)

 The synthesis of cis-Pt(NH₃)₂(dCMP) is reported and by various physico-chemical methods it is demonstrated that it is a macrochelate in which Pt(II) is bound simultaneously to the N3 site of cytosine in dCMP^2– and to a phosphate-oxygen atom. According to the NOESY spectra (cross-peaks between cytosine H6 and H2′ and H3′) the cytosine ring adopts an anti orientation. Highly unusual is the significant (1 ppm) downfield shift of the sugar proton H5″ in the ¹H-NMR spectrum and the sensitivity of the cytosine H6 resonance on the protonation state of the phosphate group. Based on these three features a geometry for the macrochelate is proposed. The compound is a major product of the reaction of cis-[Pt(NH₃)₂(H₂O)₂]²⁺ with dCMP^2– at neutral pH, but it even forms at pH 5. By applying pD-dependent NMR spectroscopy (¹H, ³¹P) and potentiometric pH titration, it is demonstrated that the Pt-coordinated phosphate group can be protonated (pK _a/1=3.21±0.10 and 3.31±0.05, respectively), and ¹H- and ³¹P-NMR spectra also indicate deprotonation (pK _a/2=13.35±0.25) of the exocyclic amino group of the cytosine moiety. The metal ion binding affinity of cis-Pt(NH₃)₂(dCMP) is very small, as shown for Cu²⁺ (log K<0.6). The cis-Pt(NH₃)₂(dCMP) complex reacts with nucleosides and nucleotides (L′) by losing its chelate structure and forming mixed ligand complexes, cis-Pt(NH₃)₂(dCMP)(L′); this means that the phosphate group is released from the coordination sphere of Pt(II), indicating that the Pt(II)-O(phosphate) bond is not very strong. Received: 23 October 1997 / Accepted: 17 February 1998     

Effects of trans-pyridine ligands on the interactions of RuII and RuIII ammine complexes N7-coordinated to purine nucleosides and DNA

 DNA binding by trans-[(H₂O)(Pyr)(NH₃)₄Ru^II]²⁺ (Pyr=py, 3-phpy, 4-phpy, 3-bnpy, 4-bnpy) is highly selective for G⁷ with K _G=1.1×10⁴ to 2.8×10⁴, with the more hydrophobic Pyr ligands exhibiting slightly higher binding. A strong dependence on ionic strength indicates that ion-pairing with DNA occurs prior to binding. At μ=0.05, d[Ru^II-DNA]/dt=k[Ru^II][DNA], where k=0.17–0.21 M^–1s^–1 with the various Pyr ligands. The air oxidation of [(py)(NH₃)₄Ru^II] _n -DNA to [(py)(NH₃)₄Ru^III] _n -DNA at pH 6 occurs with a pseudo-first-order rate constant of k _obs=5.6×10^–4s^–1 at μ=0.1, T=25  °C. Strand cleavage of plasmid DNA appears to occur by both Fenton/Haber-Weiss chemistry and by base-catalyzed routes, some of which are independent of oxygen. Base-catalyzed cleavage is more efficient than O₂ activation at neutral pH and involves the disproportionation of covalently bound Ru^III and, in the presence of O₂, Ru-facilitated autoxidation to 8-oxoguanine. Disproportionation of [py(NH₃)₄Ru^III] _n -DNA occurs according to the rate law: d[Ru^II–G_DNA]/dt=k ₀[Ru^III–G_DNA]+k ₁[Ru^III–G_DNA][OH^–], where k ₀=5.4×10^–4s^–1 and k ₁=8.8 M^–1s^–1 at 25  °C, μ=0.1. The appearance of [(Gua)(py)(NH₃)₄Ru^III] under argon, which occurs according to the rate law: d[Ru^III–G]/dt=k ₀[Ru^III–G_DNA]+k ₁[OH^–][Ru^III–G_DNA] (k ₀=5.74×10^–5 s^–1, k ₁=1.93×10^–2M^–1s^–1 at T=25  °C, μ=0.1), is consistent with lysis of the N-glycosidic bond by Ru^IV-induced general acid hydrolysis. In air, the ratio of [Ru-8-OG]/[Ru-G] and their net rates of appearance are 1.7 at pH 11, 25  °C. Small amounts of phosphate glycolate indicate a minor oxidative pathway involving C4′ of the sugar. In air, a dynamic steady-state system arises in which reduction of Ru^IV produces additional Ru^II. Received: 11 November 1998 / Accepted: 3 March 1999     

Evaluation of the Acute Oral Toxicity Class of Trinuclear Chromium(III) Glycinate Complex in Rat

Chromium(III) is considered as an essential element playing a role in carbohydrate and lipid metabolism, and various chemical forms of this element are widely used in dietary supplements. A new trinuclear chromium(III) glycinate complex [Cr₃O(NH₂CH₂CO₂)₆(H₂O)₃]⁺NO₃⁻ (CrGly), an analogue of Cr3 (trinuclear Cr(III) propionate complex) has been synthesized as a potential source of supplementary Cr. In this study, we evaluated the acute toxicity class of CrGly in Wistar rats applying the OECD 423 procedure. Male and female Wistar rats (n = 12, 6 ♀ and 6 ♂) were given by gavage either a single dose of CrGly 2,000 mg/kg body mass (equals to 300 mg Cr(III)/kg body mass; in aqueous solution) or equivalent volumes of distilled water and fed ad libitum commercial Labofeed B diet, and observed carefully for 14 days, then sacrificed to collect blood and internal organs for biochemical and histologic examination. No death cases were detected. No abnormalities in animal behavior, body mass gains, gross organ histology, or blood morphology and biochemistry were observed. The results demonstrate that LD₅₀ of CrGly is greater than 2,000 mg/kg when administrated orally to rat; thus, this compound appears to belong to the fifth category in the GHS system or the fourth class (“unclassified”) in the EU classification system.     

Quantum chemical study of silanediols as metal binding groups for metalloprotease inhibitors

DFT (B3LYP and M06L) as well as ab initio (MP2) methods with Dunning cc-pVnZ (n?=?2,3) basis sets are employed for the study of the binding ability of the new class of protease inhibitors, i.e., silanediols, in comparison to the well-known and well-studied class of inhibitors with hydroxamic functionality (HAM). Active sites of metalloproteases are modeled by [R₃M-OH₂]²⁺ complexes, where R stands for ammonia or imidazole molecules and M is a divalent cation, namely zinc, iron or nickel (in their different spin states). The inhibiting activity is estimated by calculating Gibbs free energies of the water displacement by metal binding groups (MBGs) according to: [R₃M-OH₂]²⁺ + MBG → [R₃M-MBG]²⁺ + H₂O. The binding energy of silanediol is only a few kcal mol^?1 inferior to that of HAM for zinc and iron complexes and is even slightly higher for the triplet state of the (NH₃)₃Ni²⁺ complex. For both MBGs studied in the ammonia model the binding ability is nearly the same, i.e., Fe²⁺(t) > Ni²⁺(t) > Fe²⁺(q) > Ni²⁺(s) > Zn²⁺. However, for the imidazole model the order is slightly different, i.e., Ni²⁺(t) > Fe²⁺(t) > Fe²⁺(q) > Ni²⁺(s) ≥ Zn²⁺. Equilibrium structures of the R₃Zn ²⁺ complexes with both HAM and silanediol are characterized by the monodentate binding, but the bidentate character of binding increases on going to iron and nickel complexes. Two types of intermediates of the water displacement reactions for [(NH₃)₃M-OH₂]²⁺ complexes were found which differ by the direction of the attack of the MBG. Hexacoordinated complexes exhibit bidentate bonding of MBGs and are lower in energy for M=Ni and Fe. For Zn penta- and hexacoordinated complexes have nearly the same energy. Intermediate complexes with imidazole ligands have only octahedral structures with bidentate bonding of both HAM and dimethylsilanediol molecules.