Bruce R. Adams

A detailed 1H and 13C NMR study of a repeating disaccharide of hyaluronan: the effects of temperature and counterion type

For the first time, a detailed NMR study of the conformation of methyl 2-acetamido-2-deoxy-3-O-(beta-D-glucopyranosyluronic acid)-beta-D-glucopyranoside (disaccharide 1) in aqueous solution is reported. This disaccharide is a repeating unit of hyaluronan, a polysaccharide with widespread biological and pharmaceutical applications. Relatively small changes in temperature, over typical experimental conditions (0-37 degrees C), completely change the appearance of its one-dimensional 1H NMR spectrum at 500 MHz. To determine the underlying cause for this temperature sensitivity, we analyzed 1H and 13C chemical shifts, temperature coefficients (delta gamma/delta T), 1H-1H coupling constants, and interglycosidic 1H-13C coupling constants for 1 as a function of temperature. For comparison, we measured the temperature dependence of 1H chemical shifts and coupling constants for related monosaccharides: glucuronate (GlcUA or U) and N-acetylglucosamine (GlcNAc or N), and glucose (Glc). The temperature sensitivity of the 1H spectrum of 1 is caused by relatively larger values of delta delta/delta T for some ring protons, rather than a conformational change. The effect is mediated by strong coupling. To detect the presence of long-lived intramolecular hydrogen bonds in the disaccharide, we measured chemical shifts, delta delta/delta T, and coupling constants for hydroxyl protons of 1, GlcUA, and GlcNAc in 1:1 H2O-acetone-d6 at low temperature. We compared 1H NMR parameters for 1, GlcUA, and GlcNAc in water with published values measured in Me2SO-d6 and concluded that interactions with water predominated. We found no evidence for long-lived intramolecular hydrogen bonds occurring in 1 in aqueous solution.

Intramolecular hydrogen bonding in simple diamides

Abstract Intramolecular hydrogen bonding is detected in a homologous series of diamides by NMR and IR spectroscopy. Variable temperature NMR experiments reveal a particularly favorable interaction when the hydrogen bond involves a nine-membered ring.

Determination of the Conformation and Isomeric Composition of Lignin Model Quinone Methides By NMR

Abstract Proton and 13C NMR of representative guaiacyl and syringyl β-aryl ether quinone methides have shown that guaiacyl quinone methides, generated from the corresponding benzyl bromides, exist as isomeric pairs in approximately a 70:30 ratio, the major isomer having the 3-methoxyl group syn with respect to the side chain. The ring protons at the 2- and 6-positions in the syn-and the anti-isomera respectively are markedly deshielded by 3teric compression effects; concomitant shielding of these ring carbons is observed in the C-13 NMR. Nuclear Overhauser enhancement experiments define the major solution conformation of these species, the conformation being consistent with the observed predominance of threo-products resulting from nucleophilic addition reactions.

Molybdenum-95 nuclear magnetic resonance studies on molybdenum carbonyl complexes of isonitriles and amines

Abstract The 95 Mo NMR spectra of 28 molybdenum carbonyl complexes of isonitriles and amines are reported. Within similar classes of complexes excellent correlation is found between 95 Mo chemical shifts and ligand π-acceptor ability and, more importantly, complex structures. Application to the analysis of diastereomeric complexes is reported. From analysis of linewidths and relaxation behavior, the surprising result is found that 95 Mo linewidths in isonitrile complexes are not due to coupling to the electric field gradient, but to scalar coupling between the 95 Mo and 14 N nuclei which, in turn, is inversely proportional to the substitution number n in complexes MO(CO) 6- n (CNR) n . A dependence of chemical shift on temperature of 0.3 ppm/°C is found for MO(CO) 6 .

An Interesting Rearrangement in the Cycloaddition of 1-(Methoxycarbony)Cyclopentadiene1

Abstract An attempted Diels-Alder reaction of 1-methoxycarbonylcyclopentadiene with cis-1,2-bis(phenylsulfonyl)ethylene gives a mixture of two sulfones, neither of which is the desired adduct. Instead a [1.5] sigmatropic rearrangement of the 1-methoxycarbonylcyclo-pentadiene precedes the 4 + 2 cycloaddition. Subsequent desulfonation also occurs in an unusual manner.