A high-resolution c.p.-m.a.s. 13C-n.m.r. study of solid-state cyclomaltohexaose inclusion-complexes: Chemical shifts and structure of the host cyclomaltohexaose

NMR studies of inclusion complexes: naphthalene and natural cyclodextrins

Inclusion complexes of naphthalene (NP) with cyclodextrins (CD) have been investigated so far using non-NMR techniques resulting in inconsistent data. Here, the first application of high-field NMR spectroscopy in combination with a precise analysis of the results has allowed us to determine accurately the stoichiometry of complexes and their association constants. Titration measurements have been performed by ¹H NMR spectroscopy in D₂O at a magnetic field B₀ of 18.8 T. NP and αCD form a 1 : 2 complex in which a single NP molecule is closed in a capsule made up of two αCD macrocycles. NP and βCD build coexisting 2 : 1 and 2 : 2 complexes with large binding constants. Larger γCD host molecules form essentially similar complexes with NP as the βCD but corresponding binding constants are smaller.

Shape adaptation of quinine in cyclodextrin cavities: NMR studies

Complex formation between quinine and natural cyclodextrins (CD) was studied using NMR spectroscopy.

Use of 13C chemical shift surfaces in the study of carbohydrate conformation. Application to cyclomaltooligosaccharides (cyclodextrins) in the solid state and in solution

The anomeric carbon chemical shifts of free cyclomaltohexaose, -heptaose, -octaose, -decaose, and -tetradecaose (alpha-, beta-, gamma-, epsilon-, and eta-cyclodextrin, respectively), and of alpha-cyclodextrin inclusion complexes, both in the solid state and in solution, were computed using ab initio 13C chemical shift surfaces for the D-Glcp-alpha-(1-->4)-D-Glcp linkage as a function of the glycosidic bond <Phi,Psi> dihedral angles. Chemical shift calculations in the solid state used <Phi,Psi> angle pairs measured from cyclodextrin X-ray structures as input. For estimations in the liquid state two different approaches were employed to account for dynamic averaging. In one, the computed solid-state anomeric carbon chemical shifts for each cyclodextrin D-Glcp monomer were simply averaged to obtain an estimate of the 13C shifts in solution. In the other, chemical shifts for the anomeric carbons were determined by averaging back-calculated 13C shift trajectories derived from a series of 5 ns molecular dynamic simulations for the oligosaccharides with explicit representation of water. Good agreement between calculated and experimental 13C shifts was found in all cases. Furthermore, our results show that the ab initio 13C chemical shift surfaces are sufficiently sensitive to reproduce the small variations observed for the anomeric 13C shifts of the different cyclodextrin D-Glcp units in the solid state with excellent accuracy. The use of chemical shift surfaces as tools in conformational studies of oligosaccharides is discussed.

Solid state NMR spectroscopy study of molecular motion in cyclomaltoheptaose (beta-cyclodextrin) crosslinked with epichlorohydrin

Dry and hydrated insoluble cyclomaltoheptaose (beta-cyclodextrin, beta-CD) polymers have been investigated by solid state 13C NMR spectroscopy techniques such as cross polarization/magic angle spinning with dipolar decoupling (CP/MAS), magic angle spinning both with (DD-MAS) and without (MAS) dipolar decoupling and CP/MAS dipolar dephasing (dd-CP/MAS) to allow the assignment of the main 13C signals. In the solid state, the presence of water in the samples resulted in a better resolution reflecting increased mobility. Two distinct components (crosslinked beta-CD and polymerized epichlorohydrin) have been found. The molecular mobility of these two components has been analyzed in terms of relaxation parameters such as 13C spin lattice relaxation (T1) and 1H spin lattice relaxation in the rotating frame (T1 rho). The T1 values of the polymers show that the beta-CD trapped inside the polymers does not seem to undergo changes in its mobility whatever the amount of epichlorohydrin. The addition of water to beta-CD significantly increases the T1 values reflecting strong interaction between beta-CD and the solvent. The T1P values obtained reflect the homogeneous nature of the materials.

13C-CP/MAS NMR studies of the cyclomalto-oligosaccharide (cyclodextrin) hydrates

The 13C-CP/MAS NMR spectra of cyclomaltohexaose (alpha-cyclodextrin) hexahydrate, cyclomaltoheptaose (beta-cyclodextrin) "undecahydrate", cyclomalto-octaose (gamma-cyclodextrin) "octadecahydrate", and of the same materials at lower levels of hydration are compared with solution NMR data, structures obtained from single crystal diffraction studies, and with previous reports of the 13C-CP/MAS NMR spectra. The chemical shifts of the C-1 and C-4 resonances can be correlated with the conformation about the (1----4) linkage. The chemical shifts of the C-6 resonances are also sensitive to hydrogen-bonding interactions, as shown by the spectral changes on loss of water from the structures. The results suggest that, for resonances of carbon atoms close to a centre of significant conformational change, chemical shifts may be predicted on the basis of conformation alone, but for the resonances of more distant atoms, changes in chemical shift due to conformational change may be masked by the effects of alterations in the local environment.

The double-helical nature of the crystalline part of A-starch

A new three-dimensional structure of the crystalline part of A-starch is described in which the unit cell contains 12 glucose residues located in two left-handed, parallel-stranded double helices packed in a parallel fashion; four water molecules are located between these helices. Chains are crystallized in a monoclinic lattice with a = 2.124 nm, b = 1.172 nm, c = 1.069 nm and gamma = 123.5 degrees, the c axis being parallel to the helix axis. Systematic absences are consistent with the space group B2. The structure was derived from joint use of electron diffraction of single crystals, X-ray powder patterns decomposed into individual peaks and previously reported X-ray fibre diffraction data after adequate re-indexing. The repeating unit consists of a maltotriose moiety where the glucose residues have the 4C1 pyranose conformation and are alpha(1----4) linked. The conformation of the glycosidic linkage is characterized by torsion angles (phi, psi) which take the values (91.8, -153.2), (85.7, -145.3) and 91.8, -151.3); all the primary hydroxyl groups exist in a gauche-gauche conformation. There are no intramolecular hydrogen bonds. Within the double helix, interstrand stabilization is achieved without any steric conflict and through the occurrence of O(2)...O(6) type hydrogen bonds. The present structure is consistent with both physicochemical and biochemical aspects of the crystalline component of the cereal starch granules.