Comparative 1H NMR and molecular modeling study of hydroxy protons of β-d-Galp-(1→4)-β-d-GlcpNAc-(1→2)-α-d-Manp-(1→O)(CH2)7CH3 analogues in aqueous solution

Inter-residual Hydrogen Bonding in Carbohydrates Unraveled by NMR Spectroscopy and Molecular Dynamics Simulations

Carbohydrates, also known as glycans in biological systems, are omnipresent in nature where they as glycoconjugates occur as oligo- and polysaccharides linked to lipids and proteins. Their three-dimensional structure is defined by two or three torsion angles at each glycosidic linkage. In addition, transglycosidic hydrogen bonding between sugar residues may be important. Herein we investigate the presence of these inter-residue interactions by NMR spectroscopy in D₂ O/[D₆ ]DMSO (70:30) or D₂ O and by molecular dynamics (MD) simulations with explicit water as solvent for disaccharides with structural elements α-d-Manp-(1→2)-d-Manp, β-d-GlcpNAc-(1→2)-d-Manp, and α-d-Glcp-(1→4)-β-d-Glcp, all of which have been suggested to exhibit inter-residue hydrogen bonding. For the disaccharide β-d-GlcpNAc-(1→2)-β-d-Manp-OMe, the large extent of O5'⋅⋅⋅HO3 hydrogen bonding as seen from the MD simulation is implicitly supported by the ¹ H NMR chemical shift and ³ J_HO3,H3 value of the hydroxy proton. In the case of α-d-Glcp-(1→4)-β-d-Glcp-OMe, the existence of a transglycosidic hydrogen bond O2'⋅⋅⋅HO3 was proven by the presence of a cross-peak in ¹ H,¹³ C HSQC-TOCSY experiments as a result of direct TOCSY transfer between HO3 of the reducing end residue and H2' (detected at C2') of the terminal residue. The occurrence of inter-residue hydrogen bonding, albeit transient, is judged important for the stabilization of three-dimensional structures, which may be essential in maintaining a conformational state for carbohydrate-protein interactions of glycans to take place in biologically important environments.

1H NMR characterization of chitin tetrasaccharide in binary H2O:DMSO solution: Evidence for anomeric end-effect propagation

Chitin oligosaccharides, composed of homogeneous β(1 → 4)-linked N-acetyl-D-glucosamine (GlcNAc) sequences, is a well-known elicitor of plant immune defense, and also occur as structural elements of chitosan and nodulation (Nod) factor. Detailed microstructure characterization is required for understanding the function mode of these bioactive molecules. Herein, experimental conditions for detection and elucidation of the ¹H NMR resonances of amide groups in chitin oligosaccharides are presented. The binary mixture of 70% H₂O: 30% DMSO‑d₆ was found to be the optimal solvent for amide proton measurements in homogeneous GlcNAc sequences, facilitating differentiation of the local chemical microenvironments of all four amide groups of the chitin tetrasaccharide. Experimental evidence that anomeric end-effect triggers amide proton resonance differentiation at the adjacent residue has potential to provide important insights into the solution structure of chitin and other amino sugars containing GlcNAc sequences.

A Solution NMR Approach To Determine the Chemical Structures of Carbohydrates Using the Hydroxyl Groups as Starting Points

An efficient NMR approach is described for determining the chemical structures of the monosaccharide glucose and four disaccharides, namely, nigerose, gentiobiose, leucrose and isomaltulose. This approach uses the 1H resonances of the -OH groups, which are observable in the NMR spectrum of a supercooled aqueous solution, as the starting point for further analysis. The 2D-NMR technique, HSQC-TOCSY, is then applied to fully define the covalent structure (i.e., the topological relationship between C-C, C-H, and O-H bonds) that must be established for a novel carbohydrate before proceeding to further conformational studies. This process also leads to complete assignment of all 1H and 13C resonances. The approach is exemplified by analyzing the monosaccharide glucose, which is treated as if it were an "unknown", and also by fully assigning all the NMR resonances for the four disaccharides that contain glucose. It is proposed that this technique should be equally applicable to the determination of chemical structures for larger carbohydrates of unknown composition, including those that are only available in limited quantities from biological studies. The advantages of commencing the structure elucidation of a carbohydrate at the -OH groups are discussed with reference to the now well-established 2D-/3D-NMR strategy for investigation of peptides/proteins, which employs the -NH resonances as the starting point.

Lactose permease H+-lactose symporter: mechanical switch or Brownian ratchet?

Lactose permease structure is deemed consistent with a mechanical switch device for H(+)-coupled symport. Because the crystallography-assigned docking position of thiodigalactoside (TDG) does not make close contact with several amino acids essential for symport; the switch model requires allosteric interactions between the proton and sugar binding sites. The docking program, Autodock 3 reveals other lactose-docking sites. An alternative cotransport mechanism is proposed where His-322 imidazolium, positioned in the central pore equidistant (5-7 A) between six charged amino acids, Arg-302 and Lys-319 opposing Glu-269, Glu-325, Asp-237, and Asp-240, transfers a proton transiently to an H-bonded lactose hydroxyl group. Protonated lactose and its dissociation product H(3)O+ are repelled by reprotonated His-322 and drift in the electrostatic field toward the cytosol. This Brownian ratchet model, unlike the conventional carrier model, accounts for diminished symport by H322N mutant; how H322 mutants become uniporters; why exchanging Lys-319 with Asp-240 paradoxically inactivates symport; how some multiple mutants become revertant transporters; the raised export rate and affinity toward lactose of uncoupled mutants; the altered specificity toward lactose, melibiose, and galactose of some mutants, and the proton dissociation rate of H322 being 100-fold faster than the symport turnover rate.