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1.
Alexopoulos T Allen C Anderson EW Areti H Banerjee S Beery PD Biswas NN Bujak A Carmony DD Carter T Cole P Choi Y De Bonte RJ Erwin AR Findeisen C Goshaw AT Gutay LJ Hirsch AS Hojvat C Kenney VP Lindsey CS LoSecco JM McMahon T McManus AP Morgan N Nelson KS Oh SH Piekarz J Porile NT Reeves D Scharenberg RP Stampke SR Stringfellow BC Thompson MA Turkot F Walker WD Wang CH Wesson DK 《Physical review letters》1990,64(9):991-994
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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
3.
Lazarus EA Navratil GA Greenfield CM Strait EJ Austin ME Burrell KH Casper TA Baker DR DeBoo JC Doyle EJ Durst R Ferron JR Forest CB Gohil P Groebner RJ Heidbrink WW Hong R Houlberg WA Howald AW Hsieh C Hyatt AW Jackson GL Kim J Lao LL Lasnier CJ Leonard AW Lohr J La Haye RJ Maingi R Miller RL Murakami M Osborne TH Perkins LJ Petty CC Rettig CL Rhodes TL Rice BW Sabbagh SA Schissel DP Scoville JT Snider RT Staebler GM Stallard BW Stambaugh RD St John HE Stockdale RE Taylor PL Thomas DM 《Physical review letters》1996,77(13):2714-2717
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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
8.
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
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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