Automated synthesis of a protected N-linked glycoprotein core pentasaccharide

[structure: see text] Described is the first automated solid-phase synthesis of the core N-linked pentasaccharide, common to all N-linked glycoproteins via stepwise assembly from mono- and disaccharide building blocks. The challenging beta-mannosidic linkage was incorporated by the inclusion of a disaccharide trichloroacetimidate. This automated synthesis provides rapid access to an oligosaccharide common to an entire class of glycoconjugates.     

High-resolution spectra of the 1(1,1) ← 0(0,0) rotational transition of water in argon and krypton matrices

The high-resolution spectra of H₂¹⁶O, H₂¹⁸O, HD¹⁶O, HD¹⁸O, D₂¹⁶O, and D₂¹⁸O isolated in argon and krypton matrices are reported in the 10–60 cm^?1 region. The high resolution is obtained by observing very dilute mixtures of water in the rare gases. No discrete absorbances attributable to pure argon or krypton are found in this region. In all cases the relative gas—matrix shifts of the isotopimers are consistent with the rotation—translation coupling model. The linewidths are found to correlate with rotation—translation coupling. The integrated intensity of the 1(1,1) — 0(0,0) transition of HDO exhibits a temperature dependence which does not deviate significantly from that expected from a Boltzmann distribution in rigid rotor energy levels.     

Luminescence techniques and characterization of the morphology of polymer latices 2. Fluorescence lifetime, phosphorescence and fluorescence anisotropy studies

Five poly(n-butyl methacrylate), PBMA, latex dispersions have been prepared, each incorporating a different fluorescent label, via a two-stage seeded emulsion polymerization. The resultant latices contain ca. 35% by weight total solids and are of 80 (+/-10) nm diameter as determined by photon correlation spectrometry. Luminescence spectroscopic techniques, namely fluorescence (and phosphorescence) excited state lifetime measurements in addition to time-resolved anisotropy experiments have provided useful information regarding the morphology, microviscosity and water permeability of the resultant particles. A picture of the PBMA colloid emerges of an interior which is highly viscous and water impermeable in nature. Indeed, the environment is protective enough to sustain room temperature stabilized phosphorescence from both an acenaphthylene and 9-phenanthrylmethyl methacrylate labeled dispersion through simple nitrogen purging of the solutions. However, the current spectroscopic measurements should be viewed with the knowledge that each luminescent label may fashion its own distinctive microenvironment within the latex during polymerization.     

Kinetics and mechanism of the addition of nucleophiles to alpha,beta-unsaturated thiol esters

Compounds containing the UV-absorbing chromophores p-methoxycinnamate, p-methoxycinnamide, or anthranilate and an alpha,beta- or alpha,beta,gamma,delta-unsaturated thiol ester (crotonyl or sorboyl) have been prepared. These compounds are subject to nucleophilic attack at the C=C conjugated to the thiol ester carbonyl group. The kinetics of the reactions of these thiol esters with N-acetyl-l-cysteine (NAC), N-acetylcysteamine, and N(2)-acetyl-L-lysine (NAL) have been studied, and the thiol addition products have been identified. The reaction rates increased at higher pH, and the reaction of NAC thiolate with a crotonyl thiol ester in 1:1 (v/v) acetonitrile/aqueous HEPES exhibited buffer catalysis as a result of protonation of the enolate intermediate. At the same concentration, NAC underwent approximately 300-fold more reaction than NAL with a crotonyl thiol ester at pH 9.8. Additionally, a crotonyl thiol ester was found to be 7.9 times more reactive than a sorboyl thiol ester toward NAC addition. These unsaturated thiol esters may serve as a means of covalently binding UVA and UVB sunscreens to the outer layer of skin to provide long-lasting protection.     

Proteasome-inhibitory and cytotoxic constituents of Garcinia lateriflora: absolute configuration of caged xanthones

A new biflavonoid (1), a new xanthone enantiomer (2), five new caged xanthones (3-7), and several known compounds were isolated from the stem bark of Garcinia lateriflora, collected in Indonesia. The structures of the new compounds were determined by analysis of spectroscopic data, and the absolute configuration of the caged xanthones was shown for the first time at carbons 5, 7, 8, 8a, 10a, and 27, by analysis of COSY and NOESY NMR and ECD spectra. The biflavonoids exhibited proteasome-inhibitory activity, and the known compound, morelloflavone (8) was found to have the greatest potency (IC₅₀=1.3 μM). The caged xanthones were cytotoxic toward HT-29 cells, with the known compound, morellic acid (10) being the most active (ED₅₀=0.36 μM). However, when tested in an in vivo hollow fiber assay, it was inactive at the highest dose tested (20 mg/kg).     

Intricate role of water in proton transport through cytochrome c oxidase

Cytochrome c oxidase (CytcO), the final electron acceptor in the respiratory chain, catalyzes the reduction of O(2) to H(2)O while simultaneously pumping protons across the inner mitochondrial or bacterial membrane to maintain a transmembrane electrochemical gradient that drives, for example, ATP synthesis. In this work mutations that were predicted to alter proton translocation and enzyme activity in preliminary computational studies are characterized with extensive experimental and computational analysis. The mutations were introduced in the D pathway, one of two proton-uptake pathways, in CytcO from Rhodobacter sphaeroides . Serine residues 200 and 201, which are hydrogen-bonded to crystallographically resolved water molecules halfway up the D pathway, were replaced by more bulky hydrophobic residues (Ser200Ile, Ser200Val/Ser201Val, and Ser200Val/Ser201Tyr) to query the effects of changing the local structure on enzyme activity as well as proton uptake, release, and intermediate transitions. In addition, the effects of these mutations on internal proton transfer were investigated by blocking proton uptake at the pathway entrance (Asp132Asn replacement in addition to the above-mentioned mutations). Even though the overall activities of all mutant CytcO's were lowered, both the Ser200Ile and Ser200Val/Ser201Val variants maintained the ability to pump protons. The lowered activities were shown to be due to slowed oxidation kinetics during the P(R) → F and F → O transitions (P(R) is the "peroxy" intermediate formed at the catalytic site upon reaction of the four-electron-reduced CytcO with O(2), F is the oxoferryl intermediate, and O is the fully oxidized CytcO). Furthermore, the P(R) → F transition is shown to be essentially pH independent up to pH 12 (i.e., the apparent pK(a) of Glu286 is increased from 9.4 by at least 3 pK(a) units) in the Ser200Val/Ser201Val mutant. Explicit simulations of proton transport in the mutated enzymes revealed that the solvation dynamics can cause intriguing energetic consequences and hence provide mechanistic insights that would never be detected in static structures or simulations of the system with fixed protonation states (i.e., lacking explicit proton transport). The results are discussed in terms of the proton-pumping mechanism of CytcO.     

Gold-catalyzed rearrangement of substituted allyl aryl ethers

Triphenylphosphinegold(I) complexes catalyze the Claisen-type rearrangement of aryl allyl ethers to the corresponding branched and linear products. The product distribution depends on the olefin geometry of the allylic ether. Stereochemical transfer experiments support an ionic mechanism.