Synthesis of Selectively Protected Chitobiose and Chitotriose Derivatives from one Precursor. Versatile Building Blocks for Oligosaccharide Synthesis

Abstract

The inner core region of cell surface N-glycoproteins consists of a chitobiose substructure², containing β-(1,4)-linked disaccharides of glucosamine. Such carbohydrate structures are also found as repeating units of the bacterial cell wall peptidoglycan³ and in novel tetra- and pentasaccharide plant hormones, which are nodulation factors on legume roots.⁴ Since the first synthesis of a chitobiose derivative in 1966 by Paulsen,⁵ approaches to these compounds have relied mainly on the oxazoline method.⁶ The coupling reactions of aminosugar chlorides,⁷ bromides,⁸ acetates⁹ and trichloroacetimidates¹⁰ to suitable glycosyl acceptors have also been described. Most of these syntheses¹¹ require two completely different coupling partners; only in very few examples could the glycosyl donor and acceptor be obtained from the same starting material.¹² During our investigations into the stereocontrolled synthesis of glucosamine oligosaccharides, we required an economical synthetic route to protected derivatives of chitotriose. For the purpose of easy oligomerization, the anomeric protecting group of every building block had to be exchangeable selectively with the activating group for the next glycosylation. In this paper, we report an efficient approach to chitobiose and chitotriose from a single precursor. Furthermore, the hydroxyl groups at C-1, C-3, C-4, C-6 of these oligosaccharides are differentially protected. This protecting group scenario allows a specific access to any of these functionalities by regioselective deblocking. The N-phthalimide group was chosen out of several possible amino protecting groups to ensure β-selectivity and simultaneous activation in the coupling.¹³     

Decarboxylative benzylation and arylation of nitriles

Decarboxylative benzylation of nitriles is achieved via coupling of metallated nitriles with Pd-π-benzyl complexes that are generated in situ from cyanoacetic benzyl esters. In addition, decarboxylative couplings of α,α-disubstituted 2-methylfuranyl cyanoacetates can lead to either decarboxylative arylation or benzylation depending on the reaction conditions.     

On the ergodic theory of non-integrable functions and infinite measure spaces

We prove a Chow-Robbins type result for an ergodic, non-negative SSSP, and a similar result for transformations preserving infinite measure, which implies that for these transformations, no “absolute” version of Hopf's theorem can hold.     

Constrained optimization in cartesian coordinates

Modifications are made to a previously published algorithm for constrained optimization in Cartesian coordinates (J. Comp. Chem., 13 , 240, 1992) to incorporate both fixed and dummy atoms. Standard distance and angle constraints can now be specified with respect to dummy atoms, greatly extending the range of constraints that can be handled. Fixed atoms can be eliminated from the optimization space and so there is no need to calculate their gradients resulting in potentially significant savings of CPU time in ab initio computations. Several examples illustrate the range and versatility of the modified algorithm. © John Wiley & Sons, Inc.     

Biochemical and structural insights of the early glycosylation steps in calicheamicin biosynthesis

The enediyne antibiotic calicheamicin (CLM) gamma(1)(I) is a prominent antitumor agent that is targeted to DNA by a novel aryltetrasaccharide comprised of an aromatic unit and four unusual carbohydrates. Herein we report the heterologous expression and the biochemical characterization of the two "internal" glycosyltransferases CalG3 and CalG2 and the structural elucidation of an enediyne glycosyltransferase (CalG3). In conjunction with the previous characterization of the "external" CLM GTs CalG1 and CalG4, this study completes the functional assignment of all four CLM GTs, extends the utility of enediyne GT-catalyzed reaction reversibility, and presents conclusive evidence of a sequential glycosylation pathway in CLM biosynthesis. This work also reveals the common GT-B structural fold can now be extended to include enediyne GTs.     

Bilirubin Nanoparticles as a Nanomedicine for Anti‐inflammation Therapy

Despite the high potency of bilirubin as an endogenous anti‐inflammatory compound, its clinical translation has been hampered because of its insolubility in water. Bilirubin‐based nanoparticles that may overcome this critical issue are presented. A polyethylene glycol compound (PEG) was covalently attached to bilirubin, yielding PEGylated bilirubin (PEG‐BR). The PEG‐BR self‐assembled into nanoscale particles with a size of approximately 110 nm, termed bilirubin nanoparticles (BRNPs). BRNPs are highly efficient hydrogen peroxide scavengers, thereby protecting cells from H₂O₂‐induced cytotoxicity. In a murine model of ulcerative colitis, intravenous injection of BRNPs showed preferential accumulation of nanoparticles at the sites of inflammation and significantly inhibited the progression of acute inflammation in the colon. Taken together, BRNPs show potential for use as a therapeutic nanomedicine in various inflammatory diseases.     

Versatile inlet system for on-line compound-specific deltaD and delta13C gas chromatography-oxidation/reduction-isotope ratio mass spectrometry analysis of gaseous mixtures

Compound-specific deltaD and delta13C analyses of gas mixtures are useful indicators of geochemical and environmental factors. However, the relative concentrations of individual components in gas mixtures (e.g., H2, CO2, methane, ethane, propane, i-butane, n-butane) may vary over several orders of magnitude. The determination of hydrogen and carbon compound-specific stable isotope ratios requires that the hydrogen and carbon dioxide produced from each separated component has a concentration adjusted to match the dynamic range of the stable isotope mass spectrometer. We present a custom-built gas sampling and injection system (GASIS) linked with a Delta Plus XP mass spectrometer that provides flexibility, ease of operation, and economical use of small gas samples with wide ranges of analyte concentrations. The overall on-line GC-ox/red-IRMS (Gas Chromatography - oxidation/reduction - Isotope Ratio Mass Spectrometry) system consists of (i) a customized GASIS inlet system and (ii) two alternative reactors, namely an oxidative Cu-Ni-Pt reactor at 950 degrees C for production of CO2 and a reductive graphitized Al2O3 reactor at 1420 degrees C for production of H2. In addition, the system is equipped with (iii) a liquid nitrogen spray-cooling unit for cryo-GC-focusing at -20 degrees C, and (iv) a Nafion dryer for removal of water vapor from product CO2. The three injection loops of the GASIS inlet allow flexibility in the volume of injected analyte gas (e.g., from 0.06 to 500 microL) in order to measure reproducible deltaD and delta13C values for gases at concentrations ranging from 100% down to 10 ppm. We calibrate our GC-ox/red-IRMS system with two isotopically distinct methane references gases that are combusted off-line and characterized using dual-inlet IRMS.     

Chiral separation of highly negatively charged enantiomers by capillary electrophoresis

The separation of two highly negatively charged enantiomeric organic disulfates containing two chiral centers was investigated by capillary electrophoresis using cyclodextrin based chiral selectors added to the run buffer. The optimum separation for the enantiomers was achieved in less than 3 min at 25 degrees C with a run buffer of 10 mM glycine pH 2.4 and 5 mM QA-beta-CD, which is a positively charged quaternary ammonium beta-cyclodextrin derivative. The method resulted in baseline resolution, excellent linearity, and highly reproducible migration times allowing facile evaluation of the enantiomeric purity of the individual isomers. Detection limits for the enantiomeric pair were determined to be 0.3 ng/microl (S/N = 3). The nature of the selector-enantiomer interaction and a quantitative measurement of the apparent stability constants that governed chiral discrimination of the enantiomers with QA-beta-CD were also investigated by UV-Vis spectroscopy and electrospray ionization mass spectrometry.     

In Appreciation¶A Lifetime of Connections: Otto Herbert Schmitt, 1913 - 1998

Otto H. Schmitt was born in St. Louis, Missouri, in 1913. As a youth, he displayed an affinity for electrical engineering but also pursued a wide range of other interests. He applied his multi-disciplinary talents as an undergraduate and graduate student at Washington University, where he worked in three departments: physics, zoology, and mathematics. For his doctoral research, Schmitt designed and built an electronic device to mimic the propagation of action potentials along nerve fibers. His most famous invention, now called the Schmitt trigger, arose from this early research. Schmitt spent most of his career at the University of Minnesota, where he did pioneering work in biophysics and bioengineering. He also worked at national and international levels to place biophysics and bioengineering on sound institutional footings. His years at Minnesota were interrupted by World War II. During that conflict - and the initial months of the Cold War to follow - Schmitt carried out defense-related research at the Airborne Instruments Laboratory in New York. Toward the end of his career at Minnesota, Schmitt coined the term biomimetics. He died in 1998. RID="*" ID="*"Jon M. Harkness received his Ph.D. degree in the history of science from the University of Wisconsin in 1996. During the spring of 2002, he is an adjunct assistant professor of the history of medicine at the University of Minnesota.     

A study of the thermal denaturation of ribonuclease S by electrospray ionization mass spectrometry

The thermal stability of ribonuclease S (RNase S), an enzymatically active noncovalent complex composed of a 2166-u peptide (S-peptide) and a 11,534-u protein (S-protein), was investigated by electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis ESI-MS (CE-ESI-MS). The intensities of peaks corresponding to the RNase S complex were inversely related to both the applied nozzle-skimmer (or capillary-skimmer) voltage bias in the atmosphere-vacuum interface and the temperature of the RNase S solution. By using a heated metal capillary-skimmer interface and a room temperature solution of RNase S, the intensities of RNase S molecular ion peaks were observed to decrease with increasing metal capillary temperature. Mass spectrometric studies with both the nozzle-skimmer and capillary-skimmer interface designs allowed determination of phenomenological enthalpies for dissociation of the RNase S complex in both solution and for the electrosprayed microdroplet-gas phase species. Intact RNase S complex could also be detected with CE-ESI-MS separations by using a 10-mM ammonium bicarbonate (pH 7.9) solution as the electrophoretic buffer. These studies provide new insights into the stability of multiply charged noncovalent complexes in the gas phase and the mass spectrometric conditions required for such studies, and suggest that information regarding solution properties can be obtained by ESI-MS.