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1.
A reagent panel comprised of seven aryl beta-D-N-acetylneuraminides was synthesized and then used to probe the mechanisms of nonenzymatic hydrolysis. These reactions proceeded via four independent pathways: (1) acid-catalyzed hydrolysis of the neutral molecule; (2) acid-catalyzed hydrolysis of the anionic form, or its kinetic equivalent spontaneous hydrolysis of the neutral form; (3) spontaneous hydrolysis of the anionic form; and (4) a base-promoted pathway. The pH-independent spontaneous hydrolysis of 4-nitrophenyl alpha-D-N-acetylneuraminide (5) occurs at a rate that is over 100 times faster than that of the corresponding reaction of 4-nitrophenyl beta-D-N-acetylneuraminide (4a). Spontaneous hydrolyses of four aryl beta-D-N-acetylneuraminides displayed a beta(lg) value of -1.24 +/- 0.16 (pH = 8.1, T = 100 degrees C), and at a pH value of 1.0 (50 degrees C), all seven panel members gave a beta(lg) value of 0.14 +/- 0.08. The aqueous ethanolyses of 4a and 5 gave similar products and displayed sensitivity parameters (m) in a standard Winstein-Grunwald analysis of -0.04 +/- 0.01 and +0.23 +/- 0.02, respectively. These results, plus the activation parameters calculated for the spontaneous hydrolyses of the anionic forms of 5 (DeltaH() = 116 +/- 2 kJ mol(-1) and DeltaS = 27 +/- 4 J mol(-1) K(-1)) and 4a (DeltaH = 138 +/- 3 kJ mol(-1) and DeltaS = 59 +/- 8 J mol(-1) K(-1)), are inconsistent with anomeric carboxylate assistance occurring during the hydrolysis reactions, and the likely cause for the enhanced reactivity of 5 in comparison to that of 4a is an increase in ground-state steric strain. 相似文献
2.
A panel of seven isotopically substituted sialoside natural substrate analogues based on the core structure 7-(5-acetamido-3,5-dideoxy-d-glycero-α-d-galacto-non-2-ulopyranosylonic acid)-(2→6)-β-D-galactopyranosyloxy)-8-fluoro-4-methylcoumarin (1, Neu5Acα2,6GalβFMU) have been synthesized and used to probe the rate-limiting step for turnover by the M. viridifaciens sialidase. The derived kinetic isotope effects (KIEs) on k(cat) for the ring oxygen ((18)V), leaving group oxygen ((18)V), anomeric carbon ((13)V), C3-carbon ((13)V), C3-R deuterium ((D)V(R)), C3-S deuterium ((D)V(S)), and C3-dideuterium ((D)(2)V) are 0.986 ± 0.003, 1.003 ± 0.005, 1.021 ± 0.006, 1.001 ± 0.008, 1.029 ± 0.007, 0.891 ± 0.008, and 0.890 ± 0.006, respectively. The solvent deuterium KIE ((D(2)O)V) for the sialidase-catalyzed hydrolysis of 1 is 1.585 ± 0.004. In addition, a linear proton inventory was measured for the rate of hydrolysis, under saturating condition, as a function of n, the fraction of deuterium in the solvent. These KIEs are compatible with rate-determining cleavage of the enzymatic tyrosinyl β-sialoside intermediate. Moreover, the secondary deuterium KIEs are consistent with the accumulating Michaelis complex in which the sialosyl ring of the carbohydrate substrate is in a (6)S(2) skew boat conformation. These KIE measurements are also consistent with the rate-determining deglycosylation reaction occurring via an exploded transition state in which synchronous charge delocalization is occurring onto the ring oxygen atom. Finally, the proton inventory and the magnitude of the solvent KIE are consistent with deglycosylation involving general acid-catalyzed protonation of the departing tyrosine residue rather than general base-assisted attack of the nucleophilic water. 相似文献
3.
Alex Gunn E. F. Harrison P. Lemaire C. T. Bennet Paul Stoepel Gowie und Dickson 《Fresenius' Journal of Analytical Chemistry》1911,50(11):728-730
Ohne Zusammenfassung 相似文献
4.
Bennet Manvel 《Journal of Combinatorial Theory, Series B》1974,17(1):41-47
A method is given by which the connectedness category of an arbitrary digraph D with more than four points can be determined from its subdigraphs Di = D ? vi. 相似文献
5.
Two isomeric 4-methylumbelliferyl-alpha-D-N-acetylneuraminylgalactopyranosides (1 and 2) were synthesised. These compounds contain either the natural alpha-2,3 or alpha-2,6 sialyl-galactosyl linkages, as well as an attached 4-methylumbelliferone for convenient detection of their hydrolyses. These compounds were designed as natural sialoside analogues to be used in a continuous assay of sialidase activity, where the sialidase-catalysed reaction is coupled with an exo-beta-galactosidase-catalysed hydrolysis of the released galactoside to give free 4-methylumbelliferone. The kinetic parameters for 1 and 2 were measured using the wild-type and nucleophilic mutant Y370G recombinant sialidase from Micromonospora viridifaciens. Kinetic parameters for these analogues measured using the new continuous assay were in good agreement with the parameters for the natural substrate, 3'-sialyl lactose. Given the selection of commercially available exo-beta-galactosidases that possess a variety of pH optima, this new method was used to characterise the full pH profile of the wild-type sialidase with the natural sialoside analogue 1. Thus, use of these new substrates 1 and 2 in a continuous assay mode, which can be detected by UV/Vis or fluorescence spectroscopy, makes characterisation of sialidase activity with natural sialoside linkages much more facile. 相似文献
6.
7.
Chan J Lewis AR Indurugalla D Schur M Wakarchuk W Bennet AJ 《Journal of the American Chemical Society》2012,134(8):3748-3757
A series of isotopically labeled natural substrate analogues (phenyl 5-N-acetyl-α-d-neuraminyl-(2→3)-β-d-galactopyranosyl-(1→4)-1-thio-β-d-glucopyranoside; Neu5Acα2,3LacβSPh, and the corresponding 2→6 isomer) were prepared chemoenzymatically in order to characterize, by use of multiple kinetic isotope effect (KIE) measurements, the glycosylation transition states for Vibrio cholerae sialidase-catalyzed hydrolysis reactions. The derived KIEs for Neu5Acα2,3LacβSPh for the ring oxygen ((18)V/K), leaving group oxygen ((18)V/K), C3-S deuterium ((D)V/K(S)) and C3-R deuterium ((D)V/K(R)) are 1.029 ± 0.002, 0.983 ± 0.001, 1.034 ± 0.002, and 1.043 ± 0.002, respectively. In addition, the KIEs for Neu5Acα2,6βSPh for C3-S deuterium ((D)V/K(S)) and C3-R deuterium ((D)V/K(R)) are 1.021 ± 0.001 and 1.049 ± 0.001, respectively. The glycosylation transition state structures for both Neu5Acα2,3LacβSPh and Neu5Acα2,6LacβSPh were modeled computationally using the experimental KIE values as goodness of fit criteria. Both transition states are late with largely cleaved glycosidic bonds coupled to pyranosyl ring flattening ((4)H(5) half-chair conformation) with little or no nucleophilic involvement of the enzymatic tyrosine residue. Notably, the transition state for the catalyzed hydrolysis of Neu5Acα2,6βSPh appears to incorporate a lesser degree of general-acid catalysis, relative to the 2,3-isomer. 相似文献
8.
Jang DP Kim I Chang SY Min HK Arora K Marsh MP Hwang SC Kimble CJ Bennet KE Lee KH 《The Analyst》2012,137(6):1428-1435
Although fast-scan cyclic voltammetry (FSCV) has contributed to important advances in neuroscience research, the technique is encumbered by significant analytical challenges. Confounding factors such as pH change and transient effects at the microelectrode surface make it difficult to discern the analytes represented by complex voltammograms. Here we introduce paired-pulse voltammetry (PPV), that mitigates the confounding factors and simplifies the analytical task. PPV consists of a selected binary waveform with a specific time gap between each of its two comprising pulses, such that each binary wave is repeated, while holding the electrode at a negative potential between the waves. This allows two simultaneous yet very different voltammograms (primary and secondary) to be obtained, each corresponding to the two pulses in the binary waveform. PPV was evaluated in the flow cell to characterize three different analytes, (dopamine, adenosine, and pH changes). The peak oxidation current decreased by approximately 50%, 80%, and 4% for dopamine, adenosine, and pH, in the secondary voltammogram compared with the primary voltammogram, respectively. Thus, the influence of pH changes could be virtually eliminated using the difference between the primary and secondary voltammograms in the PPV technique, which discriminates analytes on the basis of their adsorption characteristics to the carbon fiber electrode. These results demonstrate that PPV can be effectively used for differentiating complex analytes. 相似文献
9.
Francesca Bennet Tara M. Lovestead Philip J. Barker Thomas P. Davis Martina H. Stenzel Christopher Barner‐Kowollik 《Macromolecular rapid communications》2007,28(16):1593-1600
The products of the thermal degradation at 95 °C over 10 months of ω‐saturated and non‐saturated poly(methyl methacrylate) (pMMA) model compounds were identified with high accuracy via quadrupole ion trap and quadrupole ion trap‐time of flight (Q‐ToF) mass spectrometry. Analysis of the samples taken via these techniques indicated that degradation of vinyl terminated pMMA proceeds via the incorporation of oxygen via the formation of ethylene oxide type end groups, which subsequently rearrange under the expulsion of formaldehyde and 2‐oxo‐propionic acid methyl ester. The corresponding saturated model compounds were demonstrated to be stable over the same time period. The present findings highlight for the first time that poly(methyl methacrylate) degradation does not necessarily and exclusively proceed via radical intermediates.
10.
Weiwu Ren Marco Farren-Dai Natalia Sannikova Katarzyna
widerek Yang Wang Oluwafemi Akintola Robert Britton Vicent Moliner Andrew J. Bennet 《Chemical science》2020,11(38):10488
Carbasugars are structural mimics of naturally occurring carbohydrates that can interact with and inhibit enzymes involved in carbohydrate processing. In particular, carbasugars have attracted attention as inhibitors of glycoside hydrolases (GHs) and as therapeutic leads in several disease areas. However, it is unclear how the carbasugars are recognized and processed by GHs. Here, we report the synthesis of three carbasugar isotopologues and provide a detailed transition state (TS) analysis for the formation of the initial GH-carbasugar covalent intermediate, as well as for hydrolysis of this intermediate, using a combination of experimentally measured kinetic isotope effects and hybrid QM/MM calculations. We find that the α-galactosidase from Thermotoga maritima effectively stabilizes TS charge development on a remote C5-allylic center acting in concert with the reacting carbasugar, and catalysis proceeds via an exploded, or loose, SN2 transition state with no discrete enzyme-bound cationic intermediate. We conclude that, in complement to what we know about the TS structures of enzyme-natural substrate complexes, knowledge of the TS structures of enzymes reacting with non-natural carbasugar substrates shows that GHs can stabilize a wider range of positively charged TS structures than previously thought. Furthermore, this enhanced understanding will enable the design of new carbasugar GH transition state analogues to be used as, for example, chemical biology tools and pharmaceutical lead compounds.Positive charge stabilized on remote C5-allylic center with catalysis occurring via a loose SN2 transition state. 相似文献