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The question of the nature of the proton bridge involved in general acid-base catalysis in both enzymic and non-enzymic systems is considered in the light of long-known but insufficiently appreciated work of Jencks and his coworkers and of more recent results from neutron-diffraction crystallography and NMR spectroscopic studies, as well as results from(More)
Covalent conjugates of polyvinylpyrrolidone (PVP) with para-nitroaniline (PNA) were synthesized as a model PVP-drug conjugate, and PNA release was evaluated in vitro. Pyrrolidone ring opening with subsequent t-BOC protection of the pyrrolidone nitrogen and reaction with PNA in methylene chloride (CH2Cl2) produced a PVP-PNA conjugate with 3% of the(More)
The deamidation kinetics of four model peptides (AcGQNGG, AcGQNDG, AcGQNEG, and AcGQNQG) were studied in solution (70 degrees C, pH 5-10) and in lyophilized solids [70 degrees C, 50% relative humidity, "effective pH" ('pH') 5-10] containing polyvinyl pyrrolidone. AcGQNGG, AcGQNEG, and AcGQNQG degraded exclusively through Asn deamidation, whereas AcGQNDG(More)
The deamidation reactions of asparagine residues in alpha-helical and beta-turn secondary structural environments of peptides and proteins are reviewed. Both kinds of secondary structure tend to stabilize asparagine residues against deamidation, although the effects are not large. The effect of beta-sheet structures on asparagine stability is unclear,(More)
The rate of Asn deamidation of a model hexapeptide (L-Val-L-Tyr-L-Pro-L-Asn-Gly-L-Ala) was measured as a function of effective pH ('pH') in glassy and rubbery polymeric solids containing poly(vinyl pyrrolidone) (PVP) and in solution controls at 70 degrees C. The reaction exhibited pseudo-first-order kinetics in all samples over a wide 'pH' range (0.5 < 'pH'(More)
Comparison of crystal structures of S-adenosylhomocysteine (AdoHcy) hydrolase in the substrate-free, NAD(+) form [Hu, Y., Komoto, J., Huang, Y., Gomi, T., Ogawa, H., Takata, Y., Fujioka, M., and Takusagawa, F. (1999) Biochemistry 38, 8323-8333] and a substrate-bound, NADH form [Turner, M. A., Yuan, C.-S., Borchardt, R. T., Hershfield, M. S., Smith, G. D.,(More)
The purpose of this work was to probe the rate and mechanism of rapid decarboxylation of pyruvic acid in the presence of hydrogen peroxide (H2O2) to acetic acid and carbon dioxide over the pH range 2-9 at 25 °C, utilizing UV spectrophotometry, high performance liquid chromatography (HPLC), and proton and carbon nuclear magnetic resonance spectrometry ((1)H,(More)
PURPOSE To develop a facile functional assay for quantitative determination of the apparent affinities of compounds that interact with the taxol binding site of P-glcoprotein (P-gp) in Caco-2 cell monolayers. METHODS A transport inhibition approach was taken to determine the inhibitory effects of compounds on the active transport of [3H]-taxol, a known(More)