Dependence on saccharide conformation of the one-bond and three-bond carbonproton coupling constants

A review of NMR analysis in polysaccharide structure and conformation: Progress, challenge and perspective

Nuclear magnetic resonance (NMR) has been widely used as an analytical chemistry technique to investigate the molecular structure and conformation of polysaccharides. Combined with 1D spectra, chemical shifts and coupling constants in both homo- and heteronuclear 2D NMR spectra are able to infer the linkage and sequence of sugar residues. Besides, NMR has also been applied in conformation, quantitative analysis, cell wall in situ, degradation, polysaccharide mixture interaction analysis, as well as carbohydrates impurities profiling. This review summarizes the principle and development of NMR in polysaccharides analysis, and provides NMR spectra data collections of some common polysaccharides. It will help to promote the application of NMR in complex polysaccharides of biochemical interest, and provide valuable information on commercial polysaccharide products.

Validation and Comparison of Force Fields for Native Cyclodextrins in Aqueous Solution

Molecular dynamics simulations of native α-, β-, and γ-cyclodextrin in aqueous solution have been conducted with the goal to investigate the performance of the CHARMM36 force field, the AMBER-compatible q4md-CD force field, and five variants of the GROMOS force field. The properties analyzed are structural parameters derived from X-ray diffraction and NMR experiments as well as hydrogen bonds and hydration patterns, including hydration free enthalpies. Recent revisions of the torsional-angle parameters for carbohydrate systems within the GROMOS family of force fields lead to a significant improvement of the agreement between simulated and experimental NMR data. Therefore, we recommend using the variant 53A6_GLYC instead of 53A6 and 56A6_{CARBO_R} or 2016H66 instead of 56A6_CARBO to simulate cyclodextrins in solution. The CHARMM36 and q4md-CD force fields show a similar performance as the three recommended GROMOS parameter sets. A significant difference is the more flexible nature of the cyclodextrins modeled with the CHARMM36 and q4md-CD force fields compared to the three recommended GROMOS parameter sets.

NMR of glycans: shedding new light on old problems

The diversity in molecular arrangements and dynamics displayed by glycans renders traditional NMR strategies, employed for proteins and nucleic acids, insufficient. Because of the unique properties of glycans, structural studies often require the adoption of a different repertoire of tailor-made experiments and protocols. We present an account of recent developments in NMR techniques that will deepen our understanding of structure-function relations in glycans. We open with a survey and comparison of methods utilized to determine the structure of proteins, nucleic acids and carbohydrates. Next, we discuss the structural information obtained from traditional NMR techniques like chemical shifts, NOEs/ROEs, and coupling-constants, along with the limitations imposed by the unique intrinsic characteristics of glycan structure on these approaches: flexibility, range of conformers, signal overlap, and non-first-order scalar (strong) coupling. Novel experiments taking advantage of isotopic labeling are presented as an option for overcoming spectral overlap and raising sensitivity. Computational tools used to explore conformational averaging in conjunction with NMR parameters are described. In addition, recent developments in hydroxyl detection and hydrogen bond detection in protonated solvents, in contrast to traditional sample preparations in D2O for carbohydrates, further increase the tools available for both structure information and chemical shift assignments. We also include previously unpublished data in this context. Accurate determination of couplings in carbohydrates has been historically challenging due to the common presence of strong-couplings. We present new strategies proposed for dealing with their influence on NMR signals. We close with a discussion of residual dipolar couplings (RDCs) and the advantages of using (13)C isotope labeling that allows gathering one-bond (13)C-(13)C couplings with a recently improved constant-time COSY technique, in addition to the commonly measured (1)H-(13)C RDCs.

HR-HSBC: Measuring heteronuclear one-bond couplings with enhanced resolution

Heteronuclear one-bond couplings have a variety of applications, and their accurate determination is the basis for obtaining specific structural information of mostly small organic compounds. In this context, it is of utmost importance to reduce signal overlap to a minimum, and a number of techniques has been introduced during the last decades. Here, we introduce a modified version of the HR-HMBC (Magn. Reson. Chem. 2010, 48, 179-183) for heteronuclear one-bond coupling measurements with improved resolution because of the J-resolved-like tilt of corresponding multiplet patterns. The pulse sequence is introduced, and its performance is compared with a standard ω(2)-coupled HSQC experiment. Example spectra on glucose and maltose demonstrate that signals can be resolved that overlap otherwise. The approach is discussed in detail.

Measurement of long-range2 13C-1H coupling constants of 95% uniformly 13C-labeled polysaccharide from Streptococcus mitis J22

The coaggregation of Streptococcus mitis strain J22 in the early stages of dental plaque formation has been shown to result from interaction of cell wall polysaccharides with lectins on the surface of other oral bacterial species. This bacterium was grown in a medium containing 13C as the sole carbon source. We have isolated the lectin receptor polysaccharide from this strain with full enrichment in 13C and have determined a number of two-bond and three-bond 13C-1H coupling constants from measurements of the offsets in two-dimensional homonuclear nmr spectra [exclusive correlated spectroscopy (E-COSY) method]. A scheme for reliable extraction of these coupling constants from homonuclear Hartmann-Hahn and nuclear Overhauser effect spectroscopy spectra is tested in model compounds. We interpret the three-bond coupling across the glycosidic linkage in terms of dihedral angles in order to provide conformational information to supplement molecular modeling and nuclear Overhauser effect data. We show that the E-COSY method works well even for coupling constants smaller than the nmr line width and that a number of the 3JCH across the glycosidic linkage are in the range of 1-2 Hz, which is much smaller than many previously reported values.

Conformational equilibria of 4-thiomaltose and nitrogen analogues of maltose in aqueous solutions

The 1H and 13C NMR data at neutral pH are presented for methyl 4-thio-beta- and alpha-maltoside (1 and 2) together with methyl 1-thio-alpha-D-glucopyranoside (3) and methyl 4-thio-alpha-D-glucopyranoside (4) as reference compounds. Furthermore, the NMR data at high and low pH are presented for the 4-amino-4-deoxy analogues of methyl alpha-maltoside (5 and 6) and the 5-amino-5-deoxy analogue (8) together with reference compounds methyl 4-amino-4-deoxy-alpha-D-glucopyranoside (7) and 1-deoxynojirimycin (9). The experimental NMR data are assigned by 1- and 2-dimensional spectroscopy at 500 and 600 MHz. The conformational preferences of the maltose analogues 1, 2, 5, 6 and 8 are evaluated by difference NOE experiments, 13C-1H long-range coupling constants, chemical-shift comparison with model compounds and hard-sphere force field calculations for 1 using Monte Carlo simulations. Additionally, the results are compared with extensive experimental NOE data for methyl alpha- and beta-maltoside and the results discussed in light of earlier studies.

Synthesis and spectroscopic characterization of hydroxycinnamoylated methyl alpha-L-arabinofuranosyl-(1-->2)- and (1-->3)-beta-D-xylopyranosides

A reaction sequence for the preparation of methyl 5-O-feruloyl-alpha-L- arabinofuranosyl-(1-->3)-beta-D-xylopyranoside, the companion 5-O-p-coumaroyl disaccharide, and their (1-->2) analogs has been developed. The (1-->3) hydroxycinnamoylated disaccharides are available in 11 steps from L-arabinose and methyl beta-D-xylopyranoside in 17% overall yield (based on methyl beta-D-xylopyranoside). The corresponding (1-->2) materials were prepared in 9 steps in > 37% overall yield. Complete spectral characterization provides unambiguous assignments for comparison with analogous materials isolated from plant cell-walls. Conformational aspects of the prepared materials are discussed in relation to coupling-constant information.

One-bond carbon-proton coupling constants: angular dependence in alpha-linked oligosaccharides

An investigation is presented of the angular dependence of 1JC,H in model compounds related to alpha-linked oligosaccharides. Values calculated by FPT formulation in the semi-empirical INDO method for models of (1----1)-, (1----2)-, (1----3)-, and (1----4)-linked disaccharides were compared, and the effect of the orientation of HO-2 was elucidated. The angular dependence of 1JC,H on the glycosidic dihedral angles phi H and psi H was characterised in the form 1JC,H = A cos 2 phi + B cos phi + C sin 2 phi + D sin phi + E. The 1JC,H values, measured by DEPT methods for C-1-H-1 and C-X'-H-X' in various rigid carbohydrates, were used to adjust the calculated angular dependences and to determine solvent effects. Based on the abundance of the conformers for maltose and isomaltose, the thermodynamically averaged 1JC,H values have been calculated. The results obtained (less than 1JC-1,H-1 greater than 169.9, less than 1JC-4',H-4' greater than 147.7 Hz for methyl beta-maltoside and less than 1JC-1,H-1 greater than 169.8 Hz for methyl beta-isomaltoside) agree with the experimental values of 172.4, 147.7, and 170.3 Hz, respectively.