A solid-state NMR application of the anomeric effect in carbohydrates: galactosamine, glucosamine, and N-acetyl-glucosamine

Determination of glucosamine and monitoring of its mutarotation by hydrophilic interaction liquid chromatography with evaporative light scattering detector

Saccharides and their derivatives are typical polar analytes without a suitable UV-chromophore that are nowadays analyzed by HPLC (high-performance liquid chromatography) under HILIC (hydrophilic interaction liquid chromatography) mode. Usually an evaporative light scattering detector (ELSD) is utilized which, however, gives a nonlinear response. A procedure to overcome the problem of mutarotating (time-varying) analytes recorded with such a nonlinear response detector is described. The procedure was applied for determination of glucosamine in two commercially available pharmaceutical formulations containing the common inorganic ions that the detector gives a response to. Under optimized conditions, both the anomers of glucosamine were separated and could be determined separately. Owing to the short retention time of the analyte (a run time <4 min) and relatively slow kinetics of the anomeric conversion (equilibration time 2.5 h), mutarotation could be monitored and corresponding rate constants calculated.

Bandwidth in double cross-polarization MAS NMR spectroscopy

The signal intensity in double cross-polarization (DCP) NMR experiments is critically dependent on the experimental parameters, which include the rf field strength, carrier frequency, and magic-angle spinning (MAS) frequency. In this systematic study, we have monitored {(1)H}/(31)P/(13)C DCP signals from monosaccharide α-D-[UL-(13)C(6)] galactopyranosyl 1-phosphate (GalP) at a MAS frequency of 13 kHz, at which only double quantum cross-polarization (CP) coherence transfer is allowed. To lessen the stringent requirements for these experimental parameters, we have implemented linear ramp pulse, adiabatic ramp-shaped pulse, and block pulse during the period of (31)P/(13)C CP. We unravel the CP matching profiles with respect to these parameters by monitoring the (31)P/(13)C signal while varying the rf field strength and carrier frequency. For comparison, we extracted the selectivity bandwidth from the full width at half maximum (FWHM) of the matching profiles, in units of frequency (kHz), and found bandwidths of 1.1, 14, and 22 kHz for the matching profiles of the (13)C rf field strength and the (13)C and (31)P carrier frequencies, respectively, for a linear ramp pulse CP. These bandwidths are broader than the measured values in an adiabatic-shaped pulse CP (0.8, 10, and 12 kHz), as well as in block CP (0.3, 7, and 10 kHz) experiments. We demonstrate that the linear ramp pulse CP is superior to both block CP and adiabatic-shaped CP in lessening the stringent requirements of the aforementioned experimental settings for DCP experiments.

Getting to know the nitrogen next door: HNMBC measurements of amino sugars

Long-range ¹H-¹⁵N correlations detected by the heteronuclear multiple-bond correlation (HMBC) experiment are explored for the characterization of amino sugars. The gradient-enhanced HMBC, IMPACT-HMBC, and a modified pulse sequence with the ¹J-filters removed, IMPACT-HNMBC, are compared for sensitivity and resolution. ¹⁵N chemical shifts and long-range proton correlations are reported using the IMPACT-HNMBC experiment for N-acetyl-glucosamine, N-acetyl-galactosamine, and for a series of glucosamine analogs with an N-sulfo substitution, unmodified amino group, and 6-O-sulfonation. As is common with sugars, for all the compounds examined both anomeric forms are present in solution. For each compound studied, the ¹⁵N chemical shifts of the α anomer are downfield of the β form. For the N-acetylated sugars, the β anomer has a unique long-range ¹⁵N correlation to the anomeric proton not observed for the α anomer. Though N-sulfonation results in a significant change in the ¹⁵N chemical shift of the glucosamine analogs, 6-O sulfo substitution has no significant effect on the local environment of the amino nitrogen. For N-acetylated sugars in D₂O solution, peaks in the ¹⁵N projection of the HMBC spectrum appear as triplets as a result of J-modulation due to ²H-¹⁵N coupling.

Coherence selection in double CP MAS NMR spectroscopy

Applications of double cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy, via (1)H/(15)N and then (15)N/(13)C coherence transfers, for (13)C coherence selection are demonstrated on a (15)N/(13)C-labeled N-acetyl-glucosamine (GlcNAc) compound. The (15)N/(13)C coherence transfer is very sensitive to the settings of the experimental parameters. To resolve explicitly these parameter dependences, we have systematically monitored the (13)C{(15)N/(1)H} signal as a function of the rf field strength and the MAS frequency. The data reveal that the zero-quantum coherence transfer, with which the (13)C effective rf field is larger than that of the (15)N by the spinning frequency, would give better signal sensitivity. We demonstrate in one- and two-dimensional double CP experiments that spectral editing can be achieved by tailoring the experimental parameters, such as the rf field strengths and/or the MAS frequency.

Conformations of dimethoxydimethylsilane: matrix isolation infrared and ab initio studies

Conformations of dimethoxydimethylsilane (DMDMS) were studied using matrix isolation infrared spectroscopy, by trapping the silane in argon and nitrogen matrixes. The matrix was deposited using both an effusive and a supersonic jet source. The effusive source was maintained at two different temperatures, viz. 298 and 433 K, during deposition to alter the conformational population of the silane. The experimental results were supported by computations performed at both the HF and B3LYP levels, using 6-31++G** basis set. Vibrational frequency calculations were carried out to assign the experimental features and also to ensure that the computed structures did indeed correspond to minima. A conformer with a G+/-G-/+ structure was found to be the ground state, while G+/-T and G+/-G+/- structures were the next higher energy conformers with energies of 1.32 and 1.48 kcal/mol, respectively. Natural bond orbital analysis was carried out at both HF/6-31++G** and B3LYP/6-31++G** level which indicated that the charge-transfer hyperconjugative interactions largely determine the conformational preferences in this molecule. This interaction appears to be smaller in DMDMS than in the corresponding carbon analogue, dimethoxypropane (DMP).