Theoretical investigations of 13C chemical shifts in glucose, cellobiose, and native cellulose by quantum chemistry calculations
Introduction
Solid-state high-resolution NMR spectroscopy is a very powerful tool for the characterization of the structure and dynamics of polymeric and related materials [1]. Sophisticated multi-dimensional or multiple quantum solid-state NMR techniques are frequently employed in more detailed and precise analyses, but isotropic chemical shifts, which are readily obtained by the conventional cross polarization/magic angle spinning (CP/MAS) method, are still very important NMR parameters to evaluate conformations, hydrogen bonding, molecular packing, or local motions in solid materials. In the field of native cellulose, CP/MAS 13C NMR characterization has greatly contributed to the proposal and confirmation of the so-called composite crystal model [2], [3], [4], which revealed that native cellulose crystals are composites of the two allomorphs, cellulose Iα and Iβ, and to the elucidation of their crystallization process and the formation of the disordered structure [5], [6], [7]. Nevertheless, possible effects of the conformation and hydrogen bonding on 13C chemical shifts of the carbons associated with the (1 → 4)-β-glycosidic linkage and the CH2OH side group are not understood very well for cellulose and related saccharides, although a clear conformational dependence of the chemical shift of the C6 carbons in the side groups was obtained experimentally [8], [9], [10]. The main reason for such a difficulty may be the limitation of appropriate model compounds to elucidate those effects through experiments in detail.
To overcome such a situation, we have recently been investigating different possible effects on 13C chemical shifts by quantum chemistry calculations in detail. In a previous paper [11], the so-called γ-gauche effect, which reveals that marked upfield shifts of 13C resonance lines are produced by carbon or oxygen atoms having the gauche conformation located at the γ positions, was successfully evaluated for simple model compounds including cyclic molecules by the ab initio gauge included atomic orbital (GIAO)-coupled Hartree–Fock (CHF) procedure [12] based on the density functional theoretical (DFT) method. In this paper, based on these previous calculations, we examine the effects of the conformation and hydrogen bonding on the 13C chemical shifts for β-d-glucose, d-cellobiose, and cellobiose units of native cellulose by similar quantum chemistry calculations. The origins of the 13C chemical shifts of the carbons associated with the (1 → 4)-β-glycosidic linkages and the side groups are also clarified theoretically for native cellulose.
Section snippets
Calculations
All calculations were performed, similar to the previous calculations [11], [13], using Gaussian03 Revision E.01 on an SGI Altix4700 super computer in the Bioinformatics Center, Institute for Chemical Research, Kyoto University. An initial model structure of each compound examined in this paper was created by isolating a single molecule or a pair of molecules from its crystal structure using GaussView 3.0. For cellobiose units of native cellulose, the H atoms were attached at both molecular
13C chemical shifts of β-d-glucose
Fig. 1 shows the calculated 13C NMR spectrum of a single β-d-glucose molecule that was isolated from the crystal structure previously determined by X-ray diffractrometry [14] without any further energy minimization. For comparison, the CP/MAS 13C NMR spectrum of β-d-glucose powder-like crystals is also shown in this figure, with the assignment of the resonance lines made according to the reported data [8], [15]. The relative resonance positions of the respective carbons are in good agreement
Conclusion
The effects of the conformation and hydrogen bonding on 13C chemical shifts were evaluated for β-d-glucose, d-cellobiose, and the cellobiose units of native cellulose by quantum chemistry calculations based on the DFT method and the following conclusions were drawn:
(1) The quantum chemistry calculations for β-d-glucose successfully revealed nonlinear relationships between the C4, C5, C6 chemical shifts and the torsion angle χ around the C6O6 bond in the CH2OH side group. However, when only the
Acknowledgment
Computation time for the quantum chemistry calculations in this work was provided by the Bioinformatics Center, Institute for Chemical Research, Kyoto University.
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