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21.
We propose and test a pair potential that is accurate at all relevant distances and simple enough for use in large-scale computer simulations. A combination of the Rydberg potential from spectroscopy and the London inverse-sixth-power energy, the proposed form fits spectroscopically determined potentials better than the Morse, Varnshi, and Hulburt-Hirschfelder potentials and much better than the Lennard-Jones and harmonic potentials. At long distances, it goes smoothly to the London force appropriate for gases and preserves van der Waals's "continuity of the gas and liquid states," which is routinely violated by coefficients assigned to the Lennard-Jones 6-12 form. 相似文献
22.
When the Hammett-Taft equation log (k/ko)=ρq · σ is applied to the solvolysis of the 3-substituted propyl bromides 6a-6i in ethanol/water 4:1 (v/v) log k correlates linearly with σ except in cases where R exerts an anchimeric effect. The reaction constant ρq for 6 is ? 0.12 and is typical for a nucleophilic solvent-assisted ks process at a primary C-atom. The tertiary halides 1 and 3 , however, which react with little or no nucleophilic solvent assistance, i.e. by kc processes, lead to larger ρq values of ?0.71 and ?1.14, respectively. The reaction constant pq is therefore a sensitive gauge for charge development in the transition state for solvolysis of saturated compounds. 相似文献
23.
Eshelby J Goessman M Parsons PJ Pennicott L Highton A 《Organic & biomolecular chemistry》2005,3(16):2994-2997
The construction of the fourteen membered ring present in galbonolide B 1 is reported. The 10,11-diene system present in the southern portion of has been constructed using an ester enolate rearrangement/silicon mediated fragmentation cascade, whilst the macrocycle has been synthesised following a Johnson rearrangement/mercury assisted ring closure protocol. 相似文献
24.
An improvement of the signal-to-noise ratio in atomic absorption spectroscopy may be obtained by each of the following three methods of signal averaging: increase of the time constant digital signal averaging true integration of the signal The most adequate of these methods appears to be true integration. Two examples are presented to demonstrate the improved signal-to-noise ratio due to the application of this method: trace analysis of mercury in water by means of flameless AAS. The detection limit appears to be 0.2 ng which corresponds to a concentration of 2 · 10?13 g/ml. trace analysis of copper in human urine by means of flame AAS: 1,3 · 10?8 g/ml copper could be detected without any solvent extraction process. 相似文献
25.
The solvolysis rates and products of the tertiary 2-methyl-2-exo- and -2-endo-norbornyl 2,4-dinitrophenyl ethers 1 and 2 , (X = 2,4-(NO2)22C6H3O) have been determined. The different sensitivities of the rates of these ethers to the inductive effect of substituents at C(6) indicate that graded bridging of C(2) by C(6) occurs in the ionization of the exo-ethers 1 , not, however, in the ionization of the endo-ethers 2. In both cases hydrolysis leads to 2-methyl-2-exo-norbornanols only. Consequently, substitution takes place with retention at C(2) in the exo-series 1 and with inversion at C(2) in the endo-series 2. It is concluded that stereoelectronic and polar effects, rather than steric bulk effects, determine the high exo/endo rate ratios of the parent norbornyl derivatives 1a and 2a . 相似文献
26.
Szefczyk B Mulholland AJ Ranaghan KE Sokalski WA 《Journal of the American Chemical Society》2004,126(49):16148-16159
Chorismate mutase is a key model system in the development of theories of enzyme catalysis. To analyze the physical nature of catalytic interactions within the enzyme active site and to estimate the stabilization of the transition state (TS) relative to the substrate (differential transition state stabilization, DTSS), we have carried out nonempirical variation-perturbation analysis of the electrostatic, exchange, delocalization, and correlation interactions of the enzyme-bound substrate and transition-state structures derived from ab initio QM/MM modeling of Bacillus subtilis chorismate mutase. Significant TS stabilization by approximately -23 kcal/mol [MP2/6-31G(d)] relative to the bound substrate is in agreement with that of previous QM/MM modeling and contrasts with suggestions that catalysis by this enzyme arises purely from conformational selection effects. The most important contributions to DTSS come from the residues, Arg90, Arg7, Glu78, a crystallographic water molecule, Arg116, and Arg63, and are dominated by electrostatic effects. Analysis of the differential electrostatic potential of the TS and substrate allows calculation of the catalytic field, predicting the optimal location of charged groups to achieve maximal DTSS. Comparison with the active site of the enzyme from those of several species shows that the positions of charged active site residues correspond closely to the optimal catalytic field, showing that the enzyme has evolved specifically to stabilize the TS relative to the substrate. 相似文献
27.
The first kinetic study of a substrate (CN(-)) binding to the isolated active site (extracted FeMo-cofactor) of nitrogenase is described. The kinetics of the reactions between CN(-) and various derivatives of extracted FeMo-cofactor [FeMoco-L; where L is bound to Mo, and is NMF, Bu(t)NC, or imidazole (ImH)] have been followed using a stopped-flow, sequential-mix method in which the course of the reaction is followed indirectly, by monitoring the change in the rate of the reaction of the cofactor with PhS(-). The kinetic results, together with DFT calculations, indicate that the initial site of CN(-) binding to FeMoco-L is controlled by a combination of the electron-richness of the cluster core and lability of the Mo-L bond. Ultimately, the reactions between FeMoco-L and CN(-) involve displacement of L and binding of CN(-) to Mo. These reactions occur with a variety of rates and rate laws dependent on the nature of L. For FeMoco-NMF, the reaction with CN(-) is complete within the dead-time of the apparatus (ca. 4 ms), while with FeMoco-CNBu(t) the reaction is much slower and exhibits first order dependences on the concentrations of both FeMoco-CNBu(t) and CN(-) (k = 2.5 +/- 0.5 x 10(4) dm(3) mol(-1) s(-1)). The reaction of FeMoco-ImH with CN(-) occurs at a rate which exhibits a first order dependence on FeMoco-ImH but is independent of the concentration of CN(-) (k = 50 +/- 10 s(-1)). The results are interpreted in terms of CN(-) binding directly to the Mo site for FeMoco-NMF and FeMoco-ImH, but with FeMoco-CNBu(t) initial binding at an Fe site is followed by movement of CN(-) to Mo. Complementary DFT calculations are consistent with this interpretation, indicating that, in FeMoco-L, the Mo-L bond is stronger for L = ImH than for L = CNBu(t) and the binding of CN(-) to Mo is stronger than to any Fe atom in the cofactor. 相似文献
28.
Palmer FN Lach F Poriel C Pepper AG Bagley MC Slawin AM Moody CJ 《Organic & biomolecular chemistry》2005,3(20):3805-3811
Various approaches to the tyrosine-derived fragment of the marine secondary metabolite diazonamide A are described. Initial efforts were focused on the originally proposed structure of the natural product, and a feasibility study established that a model 4-aryltryptamine could be readily prepared. Protected 4-bromotryptamine underwent Pd0-catalyzed coupling with the boronic acid derived from 2-bromophenyl allyl ether by Claisen rearrangement, O-methylation and lithiation-boration. The resulting biaryl was elaborated into an alpha-diazo-beta-ketoester, dirhodium(II)-catalyzed reaction of which with N-Z-valinamide gave the desired tryptamine-oxazole following cyclodehydration of the intermediate ketoamide. A potential precursor to the benzofuran ring of the original structure of diazonamide A was prepared in eight steps from N-Z-tyrosine tert-butyl ester. Iodination, O-protection and Stille coupling gave the cinnamyl alcohol 25, converted via the bromide into the allyl aryl ether 27. Subsequent Claisen rearrangement and oxidative cleavage of the alkene gave the lactol 29, converted into the desired benzofuranone 31. The revision in the structure of diazonamide A to 2 resulted in the targeting of an alternative tyrosine-derived model benzofuranone 41 synthesized in four steps from N-Z-tyrosine methyl ester 36 by a route involving Claisen rearrangement of cinnamyl ether 37. Poor yields in this sequence prompted an investigation into the intramolecular Heck reaction as a route to benzofuranone 50. Coupling of 3-iodotyrosine 44 with 2-phenylbutenoic acid 48 gave ester 49 that readily underwent intramolecular Heck reaction to give benzofuranone 50, albeit with poor stereocontrol. 相似文献
29.
Katarzyna Kaszyńska Ewa Banachowicz Genowefa Ślósarek Adrian Morawiec Iwona Gawrońska Jan Barciszewski 《Journal of solution chemistry》2002,31(12):987-993
We analyze the structure of napin (BngNAP1), a storage protein (m.w. 14.5 kDa) from Brassica napus. On the basis of the results of 1H NMR spectroscopy and dynamic light scattering (DLS) studies, the overall shape and secondary structure of the molecule are estimated. 相似文献
30.
B. M. K. Nefkens S. D. Adrian D. B. Barlow P. DiBernardo A. D. Eichon R. S. Kessler J. N. Labrenz C. Pillai J. W. Price J. A. Wightman W. J. Briscoe L. H. Kramer M. F. Taragin D. F. Ottewell 《Few-Body Systems》1992,12(2-4):133-140
First results on –
d are reported. The measurements were made using 8 specially designed neutron counters, which were carefully calibratedin situ. The differential cross sections atT
=142, 180, 217, and 254 MeV were obtained at four angles between 0° and 90°, they are compared to +
d pp data measured at the same energies and angles with the same setup. At every beam energy, the shape of the angular distributions of –
d nn and +
d pp is the same to ±2%. The absolute cross sections differ by 1 to 10%. The error in this comparison is ±4% implying a small violation of charge symmetry.Dedicated to Prof. I. laus on the occasion of his 60th birthdayDeceased 相似文献