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1.
The mechanism by which the ribosome catalyze peptide bond formation remains controversial. Here we describe the synthesis of dinucleotides that can be used in kinetic isotope effect experiments to assess the transition state of ribosome catalyzed peptide bond formation. These substrates are the isotopically labeled dinucleotide cytidylyl-(3′-5′)-3′-amino-3′-deoxy-3′-l-phenylalanyl- N6, N6-dimethyladenosine (Cm 6A NPhe-NH 2) and cytidylyl-(3′-5′)-3′-amino-3′-deoxy-3′-(l-2-hydroxy-3-phenylpropionyl)- N6, N6-dimethyladenosine (Cm 6A NPhe-OH). These substrates are active in peptide bond formation and can be used to measure kinetic isotope effects in ribosome catalyzed protein synthesis. 相似文献
2.
We present a preliminary work for a general method of computing the partition of σ and π electronic effects of a given atom A or substituent R on a given substrate. In this aim, the nuclear charge Z* of a fictitious hydrogen atom H * is fitted in order that the A–H * (or R–H *) bond be purely covalent, i.e. the Mulliken electron population be one electron on H *. We obtain this way entities of the same electronegativity as A or R, thus having a comparable σ effect, without any π effect. The values of Z* obtained for A–H* diatomic molecules (A=H–Br) exhibit a good linear correlation with the Allred–Rochow scale of electronegativity, as it could be expected on theoretical grounds. The method, applied to R–H* molecules, allows a determination of the electronegativity of a variety of polyatomic R substituents, and provides H*(R) having the same inductive effect as R. These results are discussed by comparison with some previous theoretical and experimental data. As an example of application, the partition of σ and π contributions of R on the 13C chemical shifts in a series of monosubstituted benzenes RC6H5 has been computed. 相似文献
3.
Third-order nonlinear optical properties of a disazo dye attached polymer (3R) were evaluated and compared with that of a monoazo (Disperse Red 1) dye attached polymer (2R). The third-order nonlinear optical coefficient (ξ (3)) of the 3R is more than three times higher than that of the 2R over the fundamental wavelengths between 1.5 and 2.0 μm. This is explained by π-conjugation elongation. For both polymers, the ξ (3) dependence on the fundamental wavelength corresponds to their absorption spectra. This is attributable to a three-photon resonance effect. At the 1.50 μm resonant wavelength, the maximum ξ (3) of 4.8 × 10 -11 esu is obtained for 3R at a dye content of 17 mol%. 相似文献
4.
The magnetic susceptibility of 1,1′,2,2′-tetramethylcobaltocene, Co[C 5H 3(CH 3) 2] 2, and 1,1′-diethylcobaltocene, Co(C 5H 4C 2H 5) 2, has been studied between 0.99 and 296 K. The data are well reproduced by a calculation of the dynamic Jahn-Teller effect for the 2E 1g(a 1g2e 2g4e 1g) ground state of D 5d symmetry. A suitable set of parameter values is given by ζ = 100 cm −1, δ = 150 cm −1, kJT = 0.40, κ = 0.70. The magnetism of cobaltocene, Co(C 5H 5) 2, may be described by parameter values of comparable magnitude. The results imply a significantly larger reduction of the spin-orbit coupling parameter ζ due to covalency than of the orbital reduction factor κ. 相似文献
5.
Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, Ru 3(CO) 9(μ 2-H)[μ 3-NS(O)MePh] (1), Ru 3(CO) 10(μ 2-H)[μ 3-NS(O)MePh] (2), Ru 3(CO) 8(μ 3-η 2-CPhCHBu)[μ 3-NS(O)MePh] (3), Ru 3(CO) 9(μ 3-η 2-PhCCCCHPh)[μ 2-NS(O)MePh] (4), and Ru 3(CO) 7(μ 2-CO)(μ 3-η 2-PhCCCCHPh)[μ 3-NS(O)MePh] (5) have been examined by EHT and DFT calculations in order to analyze the bonding present in the clusters and to establish the electron counting. They clearly show that a μ 3-sulfoximido group is not a 3e − ligand as one may be led to think at first sight, but rather acts as a three-orbital/5e − system, i.e. should be considered as isolobal to an N---R − ligand. Because of some delocalization of its π-type orbitals on the sulfur and oxygen atoms, it is expected to bind slightly less strongly to metal atoms than classical imido ligands. Once in a μ 2 coordination mode, the sulfoximido ligand retains a lone pair on its pyramidalized N atom and becomes a two-orbital/3e − ligand. It follows that clusters 1, 2, 4 and 5 are electron-precise, whereas cluster 3 is electron deficient with respect to the 18e − rule but obeys the polyhedral skeletal electron pair electron-counting rules. Consistently, all the calculated clusters exhibit large HOMO–LUMO gaps and no trace of electron deficiency can be found in their electronic structures. 相似文献
6.
The reactions of the diruthenium carbonyl complexes [Ru 2(μ-dppm) 2(CO) 4(μ,η 2-O 2CMe)]X (X=BF 4− (1a) or PF 6− (1b)) with neutral or anionic bidentate ligands (L,L) afford a series of the diruthenium bridging carbonyl complexes [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-(L,L)) 2]X n ((L,L)=acetate (O 2CMe), 2,2′-bipyridine (bpy), acetylacetonate (acac), 8-quinolinolate (quin); n=0, 1, 2). Apparently with coordination of the bidentate ligands, the bound acetate ligand of [Ru 2(μ-dppm) 2(CO) 4(μ,η 2-O 2CMe)] + either migrates within the same complex or into a different one, or is simply replaced. The reaction of [Ru 2(μ-dppm) 2(CO) 4(μ,η 2-O 2CMe)] + (1) with 2,2′-bipyridine produces [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-O 2CMe) 2] (2), [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-O 2CMe)(η 2-bpy)] + (3), and [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-bpy) 2] 2+ (4). Alternatively compound 2 can be prepared from the reaction of 1a with MeCO 2H–Et 3N, while compound 4 can be obtained from the reaction of 3 with bpy. The reaction of 1b with acetylacetone–Et 3N produces [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-O 2CMe)(η 2-acac)] (5) and [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-acac) 2] (6). Compound 2 can also react with acetylacetone–Et 3N to produce 6. Surprisingly [Ru 2(μ-dppm) 2(μ-CO) 2(η 2-quin) 2] (7) was obtained stereospecifically as the only one product from the reaction of 1b with 8-quinolinol–Et 3N. The structure of 7 has been established by X-ray crystallography and found to adopt a cis geometry. Further, the stereospecific reaction is probably caused by the second-sphere π–π face-to-face stacking interactions between the phenyl rings of dppm and the electron-deficient six-membered ring moiety of the bound quinolinate (i.e. the N-included six-membered ring) in 7. The presence of such interactions is indeed supported by an observed charge-transfer band in a UV–vis spectrum. 相似文献
7.
The resistance of a novel silica-based N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) polymeric adsorption material (TODGA/SiO 2-P) against nitric acid, temperature and γ-irradiation had been investigated. The adsorption property of the treated TODGA/SiO 2-P was evaluated by a 3 M HNO 3 solution containing 0.01 M Nd(III). It was found that both 3 and 0.01 M HNO 3 concentrations did not decrease the stability of TODGA/SiO 2-P at 25°C. The quantity of TODGA leaked from TODGA/SiO 2-P was equivalent to its solubility in the corresponding HNO 3 aqueous solution. The effect of 3 M HNO 3 on the leakage of TODGA at 80°C was significantly higher than that in 0.01 M HNO 3 as well as in all cases at 25°C. The amount of Nd(III) adsorbed towards the treated TODGA/SiO 2-P was determined in the range of 0.143–0.148 mmol/g for the HNO 3 concentration effect and 0.142–0.0506 mmol/g for the temperature effect. γ-Irradiation showed a more noticeable destruction effect on TODGA/SiO 2-P. The content of TODGA leaked increased with an increase in the γ-irradiation dose (ID) from 1.06 to 3.72 MGy in terms of the linear equation [TODGA]=794.5ID+84.0. The amount of Nd(III) adsorbed onto the irradiated TODGA/SiO 2-P decreased rapidly from 0.134 to 0.0438 mmol/g, which was lower than 0.153 mmol/g, the adsorption of fresh TODGA/SiO 2-P for Nd(III), according to the equation QNd(III)=−0.0301ID+0.160, showing that a large quantity of TODGA leaked from TODGA/SiO 2-P. The adsorbed amount of Nd(III) decreased obviously in this order: the HNO 3 concentration effect, temperature effect and γ-irradiation. 相似文献
8.
The bis(μ 3-ethylidyne) tricobalt cluster [(CpCo) 3(μ 3-CCH 3) 2] (1b) is protonated by trifluoroacetic acid to give the dicobalt edge-protonated cation [H(CpCo) 3(μ 3-CCH 3) 2] + [lb + H] +. Protonation of the μ 3-ethylidyne tetracobalt cluster hydride [H(CpCo) 4(μ 3-CCH 3)] (3) takes place in two consecutive steps. At low temperature [H 2(CpCo) 4(μ 3-CCH 3)] + [3 + H] + is formed first, and is then slowly converted into [H 3(CpCo) 4(μ 3-CCH 3)] 2+ [3 + 2H] 2+ by an excess of acid. As judged by the 1H NMR data and the crystal structure of [3 + X] +[(CF 3COO) 2X] − (X = H or D) the endo hydrogens in [3 + H] + and [3 + 2H] 2+ occupy μ 3-(Co 3) face capping hydridic positions. The cations [1b + H] + and [3 + H] + show hydride fluxionality in solution, which in the case of [3 + H] + can be frozen out on the NMR timescale at low temperature (ΔG ≠ (203 K) = 40.8 kJ/mol). The structure of [3 + X] + [(CF 3COO) 2X] − (X = H or D) was determined by X-ray crystallography. One of the hydrides/deuterides is located on the crystallographic mirror plane, capping a tricobalt face of the cluster cation. The other endo hydrogen atom is believed to be disordered between the other two μ 3-(Co 3) sites, which are related by space group symmetry. Deuteronation of 3 shows a strong normal kinetic deuterium isotope effect. From the temperature independence of the 1H NMR spectrum of [3 + 2D] 2+ a non-fluxional solution structure can be inferred. In all the systems studied, hydridic (μ 2- or μ 3-) sites are thermodynamically preferred to possible isomeric agostic CoHC or Co 2HC sites for the endo hydrogens. Agostic interactions cannot, however, be ruled out in transient intermediates during the course of the protonations. 相似文献
9.
An investigation of the frontier molecular orbitais of o- and p-RC 6H 4NC (R=H, CH 3, NO 2, F, Cl, CF 3, OCH 3) was carried out so that a thorough understanding of the intricacies of σ donation and π acceptance could be developed and used to modify subtly the electron density on metal centers. The results of this study-Indicate that the substituent position (ortho vs. para ) does alter the electron density in the ligand appreciably and that substitution of the phenyl ring with the groups indicated has a smaller effect on the σ-donating ability than it does on the π-accepting ability of the isonitrile ligand. The π-accepting abilities of the isonitrile ligands increase in the order o-, p-CH3OC6H4NC, o-, p-CH3C6H4NC, o-, p-C6H5NC, o-, p-FC6H4NC, o-, p-CF3C6NC, o-, p-ClC6H4NC, o-, p-NO2C6H4NC while the σ-donating ability decreases in this order. The energies of the σ-donor and π-acceptor orbitais are shown to correlate well with observed E
values of Cr(RC6H4NC)6 and Mn(RC6H4NC)6+1 complexes. This demonstrates how the theoretical results can be useful in understanding the observed physical properties of isonitrile-metal complexes. 相似文献
10.
The absorption spectrum of neodymium (III) (Nd 3+ doped in poly(methyl methacrylate) was measured. The Nephlauxetic effect was found in the spectrum compared to other spectra of Nd 3+ doped in various matrices. The experimental data used, Slater-Condon parameters ( F2, F4, F6) and Lander parameter (ζ 4f) were calculated by the Taylor series expansion based on the assumption that the energy separation between J-levels of the 4f n-configuration is a function of F2, F4, F6 and ζ 4f. The Judd-Ofled intensity parameters (Ω 2,Ω 4,Ω 6) were also calculated. Analysis of the Nephlauxetic effect and the parameters variation on the host matrices was carried out. 相似文献
11.
Gas-phase reaction of C(1)F 3S(2)O 2O(3)C(4)H 2C(5)F 3 and F −(16) is investigated using DFT method. The geometries of various stationary points and their relative energies are obtained from 6-31+G*, 6-311G**, and 6-311++G** levels. In the S N2(C) reaction leading to the cleavage of the C(4)–O(3) bond, the reaction complex results from attacking of F − at a hydrogen atom H11 attached to carbon atom C(4). Afterward, F − is attacking the atom C(4) from the backside of the atom O(3) with the help of the neighboring effect, and meanwhile a multi-membered ring, F(16)–H(11)–C(4)–C(5)–F(16), is being formed. The S N2(C) reaction is irreversible. On the contrary, the S N2(S) reaction leading to the cleavage of the S(2)–O(3) bond is reversible, and it is initiated by attacking of F − at the atom S(2) from the backside of the atom O(3). The products of the reaction CF 3SO 3CH 2CF 3 +F − should be, thermodynamically, controlled due to the reversibility of the S N2(S) reaction, and those result, chemospecifically, from the cleavage of the C–O bond. At last, the SCRF calculations confirm that the solvent effect is preferable to the S N2(C) reaction. 相似文献
12.
It has been shown that bis(cyclopentadienyl)(μ-cyclopentadiene)dinickel, (NiCP) 2(η-C 5H 6), and (η 5-cyclopentadienyl) (η 3-cyclopentenyl)nickel, CpNi(η 3-C 5H 7), are formed in the reaction of nickelocene with methyl-lithium and with 1-phenyl-2-methyl-propenyl-lithium. The compound (NiCp) 2(μ-C 5H 6) can be only formed as a result of the reduction of the cyclopentadienyl ring bonded to the nickel atom whereas the formation of CpNi(η 3-C 5H 7) can be explained by the further hydrogeneration of cyclopentadiene formed in the earlier reaction steps. (NiCp) 2(μ-C 5H 6) has been fully characterised spectrometrically and its X-ray structure determined. It crystallises in the orthorhombic system, space group Pnma, with four molecules per unit cell. 相似文献
13.
The spectrum of CD 2HF was measured by high-resolution interferometric Fourier-transform IR (FTIR) spectroscopy (apodised instrumental band with:0.004 cm −1 fwhm) between 800 and 1200 cm −1 covering the four lowest fundamentals. A complete rotational analysis using a semi-automatic assignment procedure yields accurate band centres (ν 9: 912.2028 cm −1, ν 6:964.4994 cm −1, ν 5: 1050.5104 cm −1, ν 4: 1093.8632 cm −1) and a complete set of first-order Coriolis coupling constants. The most important couplings occur between ν 9 and ν 6 (ξ a= 1.069 cm −1, ξ c= −0.3535 cm −1) and between ν 5 and ν 4 (ξ b= −0.80606 cm −1). The analysis was guided by and compared with results from our ab initio calculations for Coriolis constants and transition moments using CADPAC at TZP/MP2 level. 相似文献
14.
Large-scale MRD CI calculations assign to AlP the ground state X 3Σ − (9σ 23π 2) and a close-lying state 1 3Π (9σ3π 3) ( Te = 0.08 eV). Up to transition energies of 2.0 eV, other states are described by the configurations 9σ3π 3 (1 1Π), 8σ 23π 4 (1 1Σ +), 9σ 23π 2 (1 1Δ and 2 1Σ +) and 9σ3π 24π (1 5Π). The 2 3Π state, located at ≈ 2.30 eV, shows a shallow double minimum. Numerous perturbations are expected to induce predissociation upon 2 3Π. Multiplets arising from the occupation 8σ 23π 34π are clustered in the 3.25–3.50 eV region. Quintet states with the configuration 8σ9σ3π 34π are bound, with Te values (in eV) of 3.80 (1 5Σ +), 4.44 (1 5Δ) and 4.88 (3 5Σ −), respectively. The 9σ → 4s Rydberg members 5Σ − and 3Σ − lie in the 4.58–4.72 eV energy region. The first ionization potential (ionization to X 4Σ − of AlP +, 9σ → ∞) is estimated to be 7.65 eV. Ionization to the 1 2Σ − and 1 2Π states of AlP + is suggested to occur between 8.0 and 8.8 eV. The dipole moments of X 3Σ −, 1 1Δ and 2 1Σ + are close to 1.0 D, whereas the 1 1Σ + state has μ = 3.49 D; 1 3Π and 1 1Π have dipole moments from 2.45 to 2.91 D. All low-lying states show a polarity Al +P −. Finally, the electronic structure and transition energies of AlP are compared with those of the isoelectronic species BN, AIN, and SiP +. 相似文献
15.
The chemistry of the di-μ-methylene-bis(pentamethylcyclopentadienyl-rhodium) complexes is reviewed. The complex [{(η 5-C 5Me 5)RhCl 2} 2] (1a) reacted with MeLi to give, after oxidative work-up, blood-red cis-[{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(Me) 2], 2. This has the two rhodiums in the +4 oxidation state ( d5), and linked by a metal-metal bond (2.620 Å). Trans-2 was formed on isomerisation of cis-2 in the presence of Lewis acids, or by direct reaction of 1a with Al 2Me 6, followed by dehydrogenation with acetone. The Rh-methyls in [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(Me) 2] were readily replaced under acidic conditions (HX) to give [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(X) 2] (X = Cl, Br or I); these latter complexes reacted with a variety of RMgX to give [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(R) 2] (R = alkyl, Ph, vinyl, etc.). Trans-2 also reacted with HBF 4 in the presence of L to give first [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(Me)(L)] + and then [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(L) 2] 2+ (L = MeCN, CO, etc.). The {(η 5-C 5Me 5)Rh(μ-CH 2)} 2 core is rather kinetically inert and also forms a variety of complexes with oxy-ligands, both cis-, e.g. [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(μ-OAc)] + and trans-, such as [(η 5-C 5Me 5)Rh(μ-CH 2)} 2(H 2O) 2] 2+. The complexes [{(η 5-C 5Me 5)Rh(μ-CH 2)} 2(R)L] + (R = Me or aryl) in the presence of CO, or [{(η 5-C 4Me 5)Rh(μ-CH 2)} 2(R) 2] (R = Me, Ph or CO 2Me) in the presence of mild oxidants, readily yield the C---C---C coupled products RCH=CH 2. The mechanisms of these couplings have been elucidated by detailed labelling studies: they are more complex than expected, but allow direct analogies to be drawn to C---C couplints that occur during Fischer-Tropsch reactions on rhodium surfaces. 相似文献
16.
The first example of an alkyne-substituted tricarbonyl(η 5-cyclohexadienyl)iron(1+) complex has been prepared and the ω directing effect of the phenylethynyl substituent has been demonstrated in nucleophile addition reactions. Addition of NC − also occurs at the a position to form an unusual η 1, η 3-structure. 相似文献
17.
The acid–base chemistry of some ruthenium ethyne-1,2-diyl complexes, [{Ru(CO) 2(η-C 5H 4R)} 2(μ 2-CC)] (R=H, Me) has been investigated. Initial protonation of [{Ru(CO) 2{η-C 5H 4R}} 2(μ 2-CC)] gave the unexpected complex cation, crystallised as the BF 4 salt, [{Ru(CO) 2(η-C 5H 4R}} 3(μ 3-CC)][BF 4] (R=Me structurally characterised). This synthesis proved to be unreliable but subsequent, careful protonation experiments gave excellent yields of the protonated ethyne-1,2-diyl complexes, [{Ru(CO) 2{η-C 5H 4R)} 2(μ 2-η 1:η 2-CCH)](BF 4) (R=Me structurally characterised) which could be deprotonated in high yield to return the starting ethyne-1,2-diyl complexes. 相似文献
18.
The state-selected reaction of CH(X 2Πν″ = 0, 1) with H 2 has been studied, in which CH was generated by IRMPD of a precursor gas, CH 3OH. The subsequent evolution of CH (ν″ = 0, 1) was monitored by the sensitive LIF technique. For the ground state and vibrationally excited state CH, the reaction with H 2 is found to depend on the total pressure in the sample cell at room temperature, which suggests that the reaction proceeds through an intermediate adduct, CH 3. The backward dissociation process is found to depend on the buffer pressure, which can be rationalized via a collision-induced backward dissociation. The decay rates of CH (ν″ = 0, 1) due to collisions with H 2 and Ar at a buffer pressure of 10 Torr are kH2 (ν″ = 1) = (2.3±0.1) × 10 −1 cm 3 molecule −1 s −1 and kAr (ν″ = 1) = (4.4±0.1) × 10 −13 cm 3 molecule −1 s −1. Possible effects of the vibrational excitation on the reaction rate of CH (ν″ = 1) are discussed. 相似文献
19.
The emission intensity at 517 nm from Pt 2(pop) 44− (pop = P 2O 5H 22−) is quenched by the addition of sulphur dioxide. The sulphur dioxide coordinates at the axial platinum(II) sites by a η 1-SO 2 bond. This coordination is supported by 31p NMR and Raman spectroscopy of aqueous solutions. The electronic spectrum of a sulphur dioxide saturated solution of Pt 2(pop) 44− shows an absorption at 428.5nm ( = 4.1 × 10 4). From the decrease in the chromophore for uncomplexed Pt 2(pop) 44− the equilibrium constant for SO 2 binding is estimated to be 1.74 M 2l −2. The effect of adding different quenchers to aqueous solutions of Pt 2(pop)44− is discussed. The compound Pt 2(pop) 44− will undergo 2-electron reduction with chromous ion. 相似文献
20.
ESR spectra of the radical species derived from 60Co γ-ray irradiation of 1,3,5-cycloheptatriene (1,3,5-CYT) and 1,3-cycloheptadiene (1,3-CYD) in halocarbon matrices have been studied in the temperature range 70–103 K. Ring inversion across the molecular plane occurs in the radical cation 1,3,5-CYT +√ in CCl 3CF 3, the activation energy being 1.7 kcal/mol. Above 90 K, 1,3,5-CYT +√ is deprotonated thermally in CCl 2FCClF 2. No dynamical effect has been observed for 1,3-CYD +√. 相似文献
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