Oligosaccharide Analogues of Polysaccharides, Part 20 , NMR Analysis of Templated Cellodextrins Possessing Two Parallel Chains: A Mimic for Cellulose I?

Abstract

Für Albert, den Meister des Komplexen an sich Naphthalene-1-ethanol and naphthalene-1,8-diethanol carrying one or two glycosidically bonded cellodextrin chains, T-x and T-x-x, resp. (x 1 ± 4, 8) were analyzed by NMR spectroscopy. For solutions in (D 6 )DMSO and (D 5 )pyridine, analysis was based on a comparison of chemical shifts, coupling constants, temperature dependence of OH signals, and ROESY spectra of the singly and doubly substituted T-x and T-x-x. The characteristic strong intrachain inter-residue O(3)ÀH´´´O(5') H-bond of celluloses was detected in the singly and doubly substituted naphthalenes. Also detected was a weakly persistent flip-flop H-bond between HO(2') and HO(6). Weak interchain interactions were, however, observed only for the units closest to the link of T-x-x in (D 6 )DMSO and for parallel units of T-1-1 and T-3-3 in (D 5 )pyridine. Interchain interactions in T-x-x are stronger in (D 5 )pyridine than in (D 6 )DMSO and decrease with increasing distance from the link. The solidstate CP/MAS 13 C-NMR spectra of T-x-x were compared with those of T-x and of celluloses. The spectrum of T-8 and, surprisingly, also of T-8-8 strongly resembles that of cellulose II and not that of cellulose I b , evidencing that a flexible template possessing parallel cellodextrin chains does not impose sufficient constraints on the structure of supramolecular assemblies to mimic cellulose I b , but leads to a valuable mimic of cellulose II.Introduction. ± There are at least four polymorphs of celluloses, namely cellulose I ± IV [2 ± 4]. The two most common polymorphs are cellulose I, the native form, and cellulose II, the mercerised or regenerated form. Cellulose II is the most stable polymorph. There are two allomorphs of cellulose I. Cellulose I a predominates in algal celluloses [5] [6] and cellulose I b in plant celluloses. Cellulose I b is more stable than cellulose I a [7]. Common to all celluloses is the 4 C 1 conformation of all 1,4-linked b-dglucopyranosyl moieties and the intrachain inter-residue O(3)ÀH´´´O(5') H-bond 2 ). Alternating glucopyranosyl units are rotated by ca. 1808 relative to each other; thus, cellobiosyl moieties are the repeating unit of celluloses. Current three-dimensional structures of celluloses are mainly based on limited X-ray-diffraction data of polycrystalline samples (a few tens of reflections) and computer modeling. Additional models for cellulose II are derived from the crystal structure of b-cellotetraose hemihydrate and from a neutron-diffraction analysis of deuteriated cellulose II fibres (see below).

Association of symmetrical alkane diols with pyridine: DFT/GIAO calculation of 1H NMR chemical shifts

“…In the more limited context of physical organic chemistry, it is often associated with hydrogen bonding, each hydrogen bond in a chain of linked hydrogen bonds (two or more) being stronger than it would be in the absence of the others. Because of the importance of carbohydrates in molecular recognition, crystallization, self‐assembly, cryoprotection etc., largely controlled by the OH groups, cooperativity continues to be a subject of considerable interest . The proximity of several OH groups favours intramolecular hydrogen bonding and cooperativity, although it is not always clear whether these features are the cause or simply the opportunistic consequence of conformational preferences .…”

Intramolecular O―H⋯O and C―H⋯O hydrogen bond cooperativity in D‐glucopyranose and D‐galactopyranose—A DFT/GIAO, QTAIM/IQA, and NCI approach

“…Extension of this concept to carbohydrates, which have several OH groups in 1,2‐diol and 1,3‐diol motifs, has led to the idea that molecular recognition and binding with acceptor species, as well as many other features, are the result of intramolecular O―H ⋯ O hydrogen bond cooperativity. Structures with chains of OH groups are found, both experimentally and theoretically, to be the most stable, but the question remains as to whether connectivity implies cooperativity. Despite the importance of the oxygen atoms in O―H ⋯ O hydrogen bonding, there has been remarkably little 17 O NMR work on carbohydrates …”

Radially oriented cellulose triacetate chains on gold nanoparticles

“…However, it has been difficult to control supramolecular structure, such as orientation, of cellulose chains. Cellulose crystals with parallel orientation have not prepared from regenerated cellulose or via chemical synthetic pathway yet, although there have been some attempts to build up parallel orientation of the cellulose chains using cellooligosaccharide analogues (Bernet et al 2000;Murty et al 2006).…”