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1.
3'-Exonuclease resistance of DNA oligodeoxynucleotides containing O6-[4-oxo-4-(3-pyridyl)butyl]guanine
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Tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is a chemical carcinogen thought to be involved in the initiation of lung cancer in smokers. NNK is metabolically activated to methylating and pyridyloxobutylating species that form promutagenic adducts with DNA nucleobases, e.g. O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-POB-dG). O6-POB-dG is a strongly mispairing DNA lesion capable of inducing both G→A and G→T base changes, suggesting its importance in NNK mutagenesis and carcinogenesis. Our earlier investigations have identified the ability of O6-POB-dG to hinder DNA digestion by snake venom phosphodiesterase (SVPDE), a 3′-exonuclease commonly used for DNA ladder sequencing and as a model enzyme to test nuclease sensitivity of anti-sense oligonucleotide drugs. We now extend our investigation to three other enzymes possessing 3′-exonuclease activity: bacteriophage T4 DNA polymerase, Escherichia coli DNA polymerase I, and E.coli exonuclease III. Our results indicate that, unlike SVPDE, 3′-exonuclease activities of these three enzymes are not blocked by O6-POB-dG lesion. Conformational analysis and molecular dynamics simulations of DNA containing O6-POB-dG suggest that the observed resistance of the O6-POB-dG lesion to SVPDE-catalyzed hydrolysis may result from the structural changes in the DNA strand induced by the O6-POB group, including C3′-endo sugar puckering and the loss of stacking interaction between the pyridyloxobutylated guanine and its flanking bases. In contrast, O6-methylguanine lesion used as a control does not induce similar structural changes in DNA and does not prevent its digestion by SVPDE. 相似文献
2.
Christopher L. Millington Amanda J. Watson Andrew S. Marriott Geoffrey P. Margison Andrew C. Povey David M. Williams 《Nucleosides, nucleotides & nucleic acids》2013,32(4):328-338
O 6-(carboxymethyl)guanine (O 6-CMG) and O 6-(4-oxo-4-(3-pyridyl)butyl)guanine (O 6-pobG) are toxic lesions formed in DNA following exposure to alkylating agents. O 6-CMG results from exposure to nitrosated glycine or nitrosated bile acid conjugates and may be associated with diets rich in red meat. O 6-pobG lesions are derived from alkylating agents found in tobacco smoke. Efficient syntheses of oligodeoxyribonucleotides (ODNs) containing O 6-CMG and O 6-pobG are described that involve nucleophilic displacement by the appropriate alcohol on a common synthetic ODN containing the reactive base 2-amino-6-methylsulfonylpurine. ODNs containing O 6-pobG and O 6-CMG were found to be good substrates for the S. pombe alkyltransferase-like protein Atl1. [Supplemental materials are available for this article. Go to the publisher's online edition of Nucleosides, Nucleotides & Nucleic Acids to view the free supplemental file.] 相似文献
3.
Covalent capture of a human O(6)-alkylguanine alkyltransferase-DNA complex using N(1),O(6)-ethanoxanthosine, a mechanism-based crosslinker
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The DNA repair protein O6-alkylguanine alkyltransferase (AGT) is responsible for removing promutagenic alkyl lesions from exocyclic oxygens located in the major groove of DNA, i.e. the O6 and O4 positions of guanine and thymine. The protein carries out this repair reaction by transferring the alkyl group to an active site cysteine and in doing so directly repairs the premutagenic lesion in a reaction that inactivates the protein. In order to trap a covalent AGT–DNA complex, oligodeoxyribonucleotides containing the novel nucleoside N1,O6-ethanoxanthosine (eX) have been prepared. The eX nucleoside was prepared by deamination of 3′,5′-protected O6-hydroxyethyl-2′-deoxyguanosine followed by cyclization to produce 3′,5′-protected N1,O6-ethano-2′-deoxyxanthosine, which was converted to the nucleoside phosphoramidite and used in the preparation of oligodeoxyribonucleotides. Incubation of human AGT with a DNA duplex containing eX resulted in the formation of a covalent protein–DNA complex. Formation of this complex was dependent on both active human AGT and eX and could be prevented by chemical inactivation of the AGT with O6-benzylguanine. The crosslinking of AGT to DNA using eX occurs with high yield and the resulting complex appears to be well suited for further biochemical and biophysical characterization. 相似文献
4.
Shibata T Glynn N McMurry TB McElhinney RS Margison GP Williams DM 《Nucleic acids research》2006,34(6):1884-1891
The human DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) dealkylates mutagenic O6-alkylguanine lesions within DNA in an irreversible reaction which results in inactivation of the protein. MGMT also provides resistance of tumours to alkylating agents used in cancer chemotherapy and its inactivation is therefore of particular clinical importance. We describe a post-DNA synthesis strategy which exploits the novel, modified base 2-amino-6-methylsulfonylpurine and allows access for the first time to a wide variety of oligodeoxyribonucleotides (ODNs) containing O6-alkylguanines. One such ODN containing O6-(4-bromothenyl)guanine is the most potent inactivator described to date with an IC50 of 0.1 nM. 相似文献
5.
Although it is known that (i) O6-alkylguanine-DNA alkyltransferase (AGT) confers tumor cell resistance to guanine O6-targeting drugs such as cloretazine, carmustine, and temozolomide and that (ii) AGT levels in tumors are highly variable, measurement of AGT activity in tumors before treatment is not a routine clinical practice. This derives in part from the lack of a reliable clinical AGT assay; therefore, a simple AGT assay was devised based on transfer of radioactive benzyl residues from [benzene-3H]O6-benzylguanine ([3H]BG) to AGT. The assay involves incubation of intact cells or cell homogenates with [3H]BG and measurement of radioactivity in a 70% methanol precipitable fraction. Approximately 85% of AGT in intact cells was recovered in cell homogenates. Accuracy of the AGT assay was confirmed by examination of AGT levels by Western blot analysis with the exception of false-positive results in melanin-containing cells due to [3H]BG binding to melanin. Second-order kinetic constants for human and murine AGT were 1100 and 380 M−1 s−1, respectively. AGT levels in various human cell lines ranged from less than 500 molecules/cell (detection limit) to 45,000 molecules/cell. Rodent cell lines frequently lacked AGT expression, and AGT levels in rodent cells were much lower than in human cells. 相似文献
6.
Carolyn Thatcher Miller Philip D. Lawley Sudhirkumar A. Shah 《The Biochemical journal》1973,136(2):387-393
Cultures of a purine-requiring mutant of Chinese hamster ovary cells (CHO-104b), randomly bred hamster embryo cells, or Escherichia coli Bs−1 were treated with non-toxic doses of 3H-labelled O6-methylguanine. DNA and RNA were isolated and subjected to enzymic digestion to nucleosides at pH8. The products of digestion were analysed by ion-exchange chromatography on columns of Dowex 50 (NH4+ form) at pH8.9. No 3H-labelled O6-methylguanosine was detected in nucleic acid digests. 3H-labelled O6-methylguanine was O-demethylated yielding [3H]guanine in CHO-104b cells. Radioactivity in nucleic acid digests was associated with thymidine, guanosine, deoxyguanosine and an unidentified early-eluting product. Reports of similar unidentified products from nucleic acids labelled with various agents are discussed. 相似文献
7.
Kabuto H Nishizawa M Tada M Higashio C Shishibori T Kohno M 《Neurochemical research》2005,30(3):325-332
As superoxide (·O2–) and hydroxyl radical (·OH) have been implicated in pathogenesis of Parkinsons disease, free radical scavenging, antioxidant, and neuroprotective agents have attracted attention as ways to prevent progression. We examined effects of zingerone, an alkaloid extracted from ginger root, on 6-hydroxydopamine (6-OHDA)-induced dopamine (DA) reduction in mouse striatum. Zingerone administration 1 h before and for 6 more days following one intracerebroventricular 6-OHDA injection prevented reductions of striatal DA and its metabolites, and increased serum ·O2– scavenging activity. Zingerone did not change activities of catalase or glutathione peroxidase in striatum or serum, or ·O2– scavenging activity in striatum. Treatment with diethyldithiocarbamate, SOD inhibitor, abolished the protective effect of zingerone against 6-OHDA-induced DA reduction. In vitro, zingerone scavenged ·O2– and ·OH and suppressed lipid peroxidation only weakly. Thus, direct antioxidant effects may be a minor component of its putative neuroprotective effect; instead, zingerone acted mainly by increasing systemic superoxide dismutase activity. Effects of zingerone treatment in this model suggest possible value in treatment of Parkinsons disease. 相似文献
8.
O6-methylguanine (O6-MeG) is a miscoding DNA lesion arising from the alkylation of guanine. This report uses the bacteriophage T4 DNA polymerase as a model to probe the roles of hydrogen-bonding interactions, shape/size, and nucleobase desolvation during the replication of this miscoding lesion. This was accomplished by using transient kinetic techniques to monitor the kinetic parameters for incorporating and extending natural and nonnatural nucleotides. In general, the efficiency of nucleotide incorporation does not depend on the hydrogen-bonding potential of the incoming nucleotide. Instead, nucleobase hydrophobicity and shape complementarity appear to be the preeminent factors controlling nucleotide incorporation. In addition, shape complementarity plays a large role in controlling the extension of various mispairs containing O6-MeG. This is evident as the rate constants for extension correlate with proper interglycosyl distances and symmetry between the base angles of the formed mispair. Base pairs not conforming to an acceptable geometry within the polymerase's active site are refractory to elongation and are processed via exonuclease proofreading. The collective data set encompassing nucleotide incorporation, extension, and excision is used to generate a model accounting for the mutagenic potential of O6-MeG observed in vivo. In addition, kinetic studies monitoring the incorporation and extension of nonnatural nucleotides identified an analog that displays high selectivity for incorporation opposite O6-MeG compared to unmodified purines. The unusual selectivity of this analog for replicating damaged DNA provides a novel biochemical tool to study translesion DNA synthesis. 相似文献
9.
《Inorganica chimica acta》2006,359(9):3020-3028
Optimization of the design of half-sandwich organometallic RuII arene complexes as anticancer agents depends on control of ligand exchange reactions. We have studied the aqueous chemistry of complexes containing O,O-chelate rings. The presence of the four-membered O,O-chelate ring from acetate (AcO) in [η6-p-cymene)Ru(AcO)Cl] was confirmed by X-ray crystallography, but in solution the acetate ligand was labile and the hydroxo-bridged dimer [((η6-p-cymene)Ru)2(μ-OH)3]+ readily formed. The dimer was relatively unreactive towards 9-ethyl guanine. The tropolonato (trop) complex [(η6-p-cymene)Ru(trop)Cl] was stable in aqueous media and the X-ray crystal structure of the aqua adduct [(η6-p-cymene)Ru(trop)(H2O)]CF3SO3, containing a five-membered O,O-chelate ring from trop, was determined. [(η6-p-cymene)Ru(trop)Cl] reacted with guanosine to form N7 adducts and with adenosine to form both N7 and N1 adducts. Competitive reactions with guanosine and adenosine gave rise to guanosine:adenosine adducts in a ca. 1.3:1 mol ratio. 相似文献
10.
Production of Reactive Oxygen Intermediates (O2˙−, H2O2, and ˙OH) by Maize Roots and Their Role in Wall Loosening and Elongation Growth
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Cell extension in the growing zone of plant roots typically takes place with a maximum local growth rate of 50% length increase per hour. The biochemical mechanism of this dramatic growth process is still poorly understood. Here we test the hypothesis that the wall-loosening reaction controlling root elongation is effected by the production of reactive oxygen intermediates, initiated by a NAD(P)H oxidase-catalyzed formation of superoxide radicals (O2˙−) at the plasma membrane and culminating in the generation of polysaccharide-cleaving hydroxyl radicals (˙OH) by cell wall peroxidase. The following results were obtained using primary roots of maize (Zea mays) seedlings as experimental material. (1) Production of O2˙−, H2O2, and ˙OH can be demonstrated in the growing zone using specific histochemical assays and electron paramagnetic resonance spectroscopy. (2) Auxin-induced inhibition of growth is accompanied by a reduction of O2˙− production. (3) Experimental generation of ˙OH in the cell walls with the Fenton reaction causes wall loosening (cell wall creep), specifically in the growing zone. Alternatively, wall loosening can be induced by ˙OH produced by endogenous cell wall peroxidase in the presence of NADH and H2O2. (4) Inhibition of endogenous ˙OH formation by O2˙− or ˙OH scavengers, or inhibitors of NAD(P)H oxidase or peroxidase activity, suppress elongation growth. These results show that juvenile root cells transiently express the ability to generate ˙OH, and to respond to ˙OH by wall loosening, in passing through the growing zone. Moreover, inhibitor studies indicate that ˙OH formation is essential for normal root growth. 相似文献
11.
Pentaaqua complexes of Cu(I) with guanine were optimized at the DFT B3PW91/6-31G(d) level. For the most stable structures, vibration frequencies and NBO charges were computed followed by energy analyses. The order of individual conformers was very sensitive to the method and basis sets used for the calculation. Several conformers are practically degenerated in energy. The inclusion of an entropy term changes the order of the conformers stability. Water molecules associated at the N9 position of guanine are favored by the inclusion of the entropy correction. Bonding energies of Cu–O(aqua) interactions were estimated to be about 60 kcal mol–1 and for Cu–N7 bonding in the range of 75–83 kcal mol–1. The broad range in Cu–N interaction energies demonstrates the role of induction effects caused by water molecules associated at the various sites of guanine. The charge distribution of the guanine molecule is changed remarkably by the coordination of a Cu(I) cation, which can also change the base-pairing pattern of the guanine. 相似文献
12.
《Life sciences》1996,58(19):PL303-PL308
O6-Alkylguanine derivatives sensitize tumor cells to chloroethylnitrosourea (CENU) chemotherapy by inactivation of O6-methylguanine-DNA methyltransferase (MGMT), which repairs CENU-induced O6-alkylguanines in DNA by accepting the alkyl group at a cysteine moiety. To test the biological significance of synthesized O6-fluorobenzylguanine derivatives, we measured their ability of inactivation of MGMT activity and their effects on the cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) in comparison with the effects of O6-benzylguanine and O6-phenylguanine. The O6-(4-and 3-fluorobenzyl)guanines considerably reduced the MGMT activity of HeLa S3 cell-free extract as did O6-benzylguanine. In contrast, O6-(2-fluorobenzyl)guanine and O6-phenylguanine had less of an effect on the activity. Two-hour pretreatment of O6-(4-and 3-fluorobenzyl)guanines potentiated ACNU cytotoxicity in HeLa S3 cells to a greater extent than did O6-(2-fluorobenzyl)guanine and O6-phenylguanine. The enhancement effects were consistent with the depletion of MGMT activity after the pretreatment of O6-fluorobenzylguanine derivatives. O6-Fluorobenzylguanines with a fluoro-substitution at the 4- or 3-position of the benzyl group were comparable to O6-benzylguanine and were powerful MGMT inactivators. The chemical features of the O6-benzyl group are a biologically important determinant in the reaction evolution with MGMT. 相似文献
13.
Verdemato PE Brannigan JA Damblon C Zuccotto F Moody PC Lian LY 《Nucleic acids research》2000,28(19):3710-3718
The C-terminal domain of the Escherichia coli Ada protein (Ada-C) aids in the maintenance of genomic integrity by efficiently repairing pre-mutagenic O6-alkylguanine lesions in DNA. Structural and thermodynamic studies were carried out to obtain a model of the DNA-binding process. Nuclear magnetic resonance (NMR) studies map the DNA-binding site to helix 5, and a loop region (residues 151–160) which form the recognition helix and the ‘wing’ of a helix–turn–wing motif, respectively. The NMR data also suggest the absence of a large conformational change in the protein upon binding to DNA. Hence, an O6-methylguanine (O6meG) lesion would be inaccessible to active site nucleophile Cys146 if the modified base remained stacked within the DNA duplex. The experimentally determined DNA-binding face of Ada-C was used in combination with homology modelling, based on the catabolite activator protein, and the accepted base-flipping mechanism, to construct a model of how Ada-C binds to DNA in a productive manner. To complement the structural studies, thermodynamic data were obtained which demonstrate that binding to unmethylated DNA was entropically driven, whilst the demethylation reaction provoked an exothermic heat change. Methylation of Cys146 leads to a loss of structural integrity of the DNA-binding subdomain. 相似文献
14.
O6-methylguanine (O6meG) is one of the most premutagenic, precarcinogenic, and precytotoxic DNA lesions formed by alkylating agents. Repair of this DNA damage is achieved by the protein MGMT, which transfers the alkyl groups from the O6 position of guanine to a cysteine residue in its active center. Because O6meG repair by MGMT is a stoichiometric reaction that irreversibly inactivates MGMT, which is subsequently degraded, the repair capacity of O6meG lesions is dependent on existing active MGMT molecules. In the absence of active MGMT, O6meG is not repaired, and during replication, O6meG:T mispairs are formed. The MMR system recognizes these mispairs and introduces a gap into the strand. If O6meG remains in one of the template strands the futile MMR repair process will be repeated, generating more strand breaks (SBs). The toxicity of O6meG is, therefore, dependent on MMR and DNA SB induction of cell death. MGMT, on the other hand, protects against O6meG toxicity by removing the methyl residue from the guanine. Although removal of O6meG makes MGMT an important anticarcinogenic mechanism of DNA repair, its activity significantly decreases the efficacy of cancer chemotherapeutic drugs that aim at achieving cell death through the action of the MMR system on unrepaired O6meG lesions. Here, we report on a modification of the comet assay (CoMeth) that allows the qualitative assessment of O6meG lesions after their conversion to strand breaks in proliferating MMR-proficient cells after MGMT inhibition. This functional assay allows the testing of compounds with effects on O6meG levels, as well as on MGMT or MMR activity, in a proliferating cell system. The expression of MGMT and MMR genes is often altered by promoter methylation, and new epigenetically active compounds are being designed to increase chemotherapeutic efficacy. The CoMeth assay allows the testing of compounds with effects on O6meG, MGMT, or MMR activity. This proliferating cell system complements other methodologies that look at effects on these parameters individually through analytical chemistry or in vitro assays with recombinant proteins. 相似文献
15.
Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis
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An O2 electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O2 concentrations on photosynthetic O2 evolution. At pH = 5.5 and 20% O2, cells grown in HCO3− medium (low CO2, pH ≥ 9.0) exhibited a higher affinity for external CO2 (K½(CO2) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO2 medium (pH = 6.5), (K½(CO2) = 94 ± 16 micromolar). With O2 ≤ 2% in contrast, both types of cells showed a high apparent affinity (K½(CO2) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO2 medium (pH = 6.5). The high-pH, HCO3−-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO2 which exceeded the CO2 externally supplied, and the reverse situation has been observed in high-CO2-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO3− (K½[HCO3−]) was 0.6 ± 0.2 millimolar, at 20% O2. But under low O2 concentrations (≤2%), surprisingly, an inhibition of net O2 evolution was elicited, which was maximal at low HCO3− concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO2 medium, (b) Chara cells are able to accumulate CO2 internally by means of a process apparently independent of the plasmalemma HCO3− transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO3−: pseudocyclic electron flow, sustained by O2 photoreduction, may produce the additional ATP needed for the HCO3− transport. 相似文献
16.
The mutagenic and carcinogenic effects of simple alkylating agents are mainly due to O6-alkylation of guanine in DNA. This lesion results in transition mutations. In both prokaryotic and eukaryotic cells, repair is effected by direct reversal of the damage by a suicide protein, O6-alkylguanine-DNA alkyltransferase. The alkyltransferase removes the alkyl group to one of its own cysteine residues. However, this mechanism for preserving genomic integrity limits the effectiveness of certain alkylating anticancer agents. A high level of the alkyltransferase in many tumour cells renders them resistant to such drugs. Here we report the X-ray structure of the human alkyltransferase solved using the technique of multiple wavelength anomalous dispersion. This structure explains the markedly different specificities towards various O6-alkyl lesions and inhibitors when compared with the Escherichia coli protein (for which the structure has already been determined). It is also used to interpret the behaviour of certain mutant alkyltransferases to enhance biochemical understanding of the protein. Further examination of the various models proposed for DNA binding is also permitted. This structure may be useful for the design and refinement of drugs as chemoenhancers of alkylating agent chemotherapy. 相似文献
17.
Abstract O6-(4-Nitrophenyl)inosine (la), O6 -(4-nitrophenyl)guanosine (1c) and O6 -(4-methylumbelliferonyl)inosine (2) were obtained by reaction of 6-chloro-9-(β-D-ribofuranosyl)purine (3a) or 2-amino-6-chloro-9-(β-D-ribofuranosyl)purine (3c) with sodium salts of 4-nitrophenol or 4-methylumbelliferone in N,N-dimethylformamide. Similarly, 6-chloro-9-(β-D-2,3-isopropylideneribofuranosyl)purine (3b) was transformed to 2′,3′-O-isopropylidene-O6-(4-nitrophenyl)inosine (1b). Deprotection of 1b with CF3COOH gave compound la and O6 -(4-nitrophenyl)hypoxanthine (4). Compounds 1a and 1c are substrates for adenosine deaminase releasing 4-nitrophenol which is readily detected visually or spectrophotomemcally. Rate and extent of hydrolysis of la are significantly increased in the presence of purine nucleoside phosphorylase but xanthine oxidase has no influence. A potential fluorogenic analogue 2 is not a substrate for adenosine deaminase. 相似文献
18.
Phosphonium zwitterions of a known type were obtained in high yield via a 1:1 reaction of p-benzoquinone or methoxy-p-benzoquinone with the tertiary phosphines R3P [R = (CH2)3OH, Ph, Et, Me] and Ph2MeP, in acetone or benzene at room temperature. In all cases, attack of the P-atom occurs at a C-atom rather than at an O-atom. The products were characterized to various degrees by elemental analysis, 31P{1H}, 1H and 13C NMR spectroscopies, and mass spectrometry, and two of the zwitterions, the new [HO(CH2)3]3P+C6H2(O−)(OH)(MeO) and the known Ph3P+C6H3(O−)(OH), were structurally characterized by X-ray analysis. The PEt3 reaction also produces small amounts of the ‘dimeric’, μ-oxo co-product Et3P+C6H2(O−)(OH)-O-C6H3(O−)P+Et3 that is tentatively characterized by 1D- and 2D-NMR data. 2,5-Di-tert-butyl- and 2,3,5,6-tetramethyl-p-benzoquinone do not react with [HO(CH2)3]3P under the conditions noted above. Heating D2O solutions of the water-soluble zwitterions R3P+C6H3(O−)(OH) [R = (CH2)3OH, Et] at 90 °C for 72 h leads to complete H/D exchange of the H-atom in the position ortho to the phosphonium center. 相似文献
19.
Ricardo González Natalia Barboza Raúl Chiozzone Carlos Kremer Donatella Armentano Giovanni De Munno Juan Faus 《Inorganica chimica acta》2008,361(9-10):2715-2720
The linkage isomers [Re(NCS)6]2? and [Re(NCS)5(SCN)]2? are obtained by the reaction of [ReBr6]2? with NCS? in dimethylformamide. Some differences in the chemical behavior allowed their separation and structural characterization in the form of (NBu4)2[Re(NCS)6] (1) and [Zn(NO3)(Me2phen)2]2[Re(NCS)5(SCN)] (2), respectively (Bu = n-C4H9 and Me2phen = 2,9-dimethyl-1,10-phenanthroline). 相似文献
20.
《Inorganica chimica acta》1986,113(1):55-60
trans-[Ru(NH3)4P(OR)3(H2O)]2+ (R = Me, Pr, iPr, and Bu) reacts with isonicotinamide at second-order- specific rates k1 of 1.2, 2.3, 7.4 and 8.1 M−1 s−(25 °C, μ = 0.10 NaCF3COO/CH3COOH), respectively, for R = Me, Pr, iPr and Bu. The products trans- [Ru(NH3)4P(OR)3isn](PF6)2 have been isolated and characterized by micro analysis, cyclic voltammetry, and electronic spectral data. The aquation rates k−1 for the isonicotinamide (isn) derivatives are 5.2 × 10−2, 5.9 × 10−2, 2.0 × 10−1 and 3.4 × 10−1 s−1 for R= Me, Pf, Bu and iPr, respectively. The activation parameters for the forward and backward reactions indicate the same mechanism for all of them. The substitution proceeds by a dissociative mechanism with a significant outer-sphere association of trans-[Ru(NH3)4P(OR)3(H2O)]2+ complexes with isn. Assuming k1 as indicative of the lability of the coordinated water molecule on the monophosphite complexes, the following sequence of increasing trans-effect mav be proposed: P(OMe)3 <P(OEt)3 <P(OPr)3 <P(OiPr)3 <P(OBu)3. The affinity of the monophosphite complexes for isn increases according to P(OMe)3 ⋍ P(OiPr)3 < P(OEt)3 < P(OPr)3 ⋍ P(OBu)3. 相似文献