Solid-state 13C-c.p.—m.a.s. n.m.r. of starches

Structural investigation of amylose complexes with small ligands: helical conformation, crystalline structure and thermostability

Crystalline amylose complexes were prepared with decanal, 1-butanol, menthone and alpha-naphtol. Their crystalline structure and the related helical conformation, determined by wide angle X-ray diffraction (WAXD) and 13C CPMAS solid state NMR, were assigned to V6I, V6II, V6III and V8 types, respectively. It was possible to propose some hypotheses on the possible nature of interactions and especially intra-/inter-helical inclusion. Some shifts in the NMR C1 carbon signals were attributed to the presence of ligand in specific sites inside the structure for a same type of V6 helical conformation. Moreover, the crystallinity and polymorphic changes induced by desorption/rehydration were studied. A general increase of the carbon resonances sharpness upon rehydration has been observed, but also a V6II-V6I transition when decreasing the water content. Differential scanning calorimetry (DSC) experiments were also performed to approach the thermostability of the four types of complex and also the way they form again after melting/cooling sequences.

Structural investigation of amylose complexes with small ligands: inter- or intra-helical associations?

Highly crystalline amylose complexes with menthone (1) and linalool (2) were analysed by wide-angle X-ray diffraction and solid-state nuclear magnetic resonance (NMR). The complexes, after partial water desorption in a controlled atmosphere (aw=0.75), displayed a typical V-isopropanol structure, showing the presence of ligand inside or between the helices in the crystalline domains. Sequential washing of the powdered complexes with ethanol before and after desorption permitted probing the intra- and inter-helical inclusions. High resolution magic angle spinning (HRMAS) recordings were used to compare the chemical shifts of free and bound aroma which allowed a proposal that some hydrogen bonding is involved in the amylose complexing. Moreover, it showed that free aroma was completely removed by ethanol washing. Using cross polarization magic angle spinning (CPMAS) and X-ray scattering experiments, it was demonstrated that the V-isopropanol type was retained for linalool whatever the treatment used. On the contrary, the measurement shifts toward the V-6I amylose hydrate (V-h) type for menthone after ethanol washing before the desorption step, reflecting the disappearance of inter-helical associations between menthone and amylose. The stability of the complex prepared with linalool shows that this ligand is more strongly associated to amylose helices. The discrepancies observed in the chemical shifts attributed to carbons C1 and C4 in CPMAS spectra of V-isopropanol and V-h forms could be attributed either to a deformation of the single helix (with possible inclusion of the ligand inside) or to the presence of the ligand between helices (only water molecules are present in the V-h form).

Effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase

Following a previous publication, the present paper reports additional results on the effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak AD) chiral stationary phase (CSP). Solid-state NMR (1H/13C CPMAS) was utilized to identify and compare structural differences in Chiralpak AD caused by the various alcohol mobile-phase modifiers, many of which were not studied in the previous publication. The influences of the various alcohol modifiers (in hexane-based mobile phase) on the structure and chiral selectivity of the CSP were studied and compared. CPMAS spectra of Chiralpak AD flushed with the mobile phases displayed clear evidence of solvent incorporation into the CSP. When alcohol modifiers with varying size and bulkiness were used in the mobile phase, differences in structure and chiral selectivity were observed on Chiralpak AD based on solid-state NMR and chromatographic data. The change of t-butanol concentration in the t-butanol/hexane mobile phase caused changes of structure and chiral selectivity of the Chiralpak AD. These data further support our belief that the different chiral selectivities of the CSP associated with the use of different alcohol modifiers are due to different alterations of the steric environment of the chiral cavities in the CSP by the different mobile-phase modifiers.

Solid-state NMR characterization of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary-phase structure as a function of mobile-phase composition

Solid-state NMR (1H/13C CPMAS) was utilized to identify structural differences in amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (Chiralpak AD), as a function of mobile-phase composition. Dry Chiralpak AD stationary phase displayed an amorphous CPMAS NMR spectrum. However, CPMAS spectra of Chiralpak AD flushed with organic mobile phases clearly displayed evidence of solvent complexes. Chiralpak AD flushed with nonpolar hexane exhibited solvent complexes with minimal structural perturbation. For Chiralpak AD flushed with hexane containing alcohol modifiers, however, solvent incorporation caused significant difference in conformation distribution as evidenced by increased resolution of 13C peaks in the CPMAS spectrum of the stationary phase. 2-Propanol modifier displayed more efficient displacement of incorporated hexane while forming relatively more distinct/ordered solvent complexes with Chiralpak AD in comparison to ethanol modifier. Reversed elution order and unusual retention behavior on Chiralpak AD as a function of mobile-phase modifier was reported earlier. These chromatographic behaviors are believed to be due to different alterations of the steric environment of the chiral cavities in the CSP by the different mobile-phase modifiers. In addition, on the basis of the chemical shift of C-1 carbon on the amylose backbone, it is possible that Chiralpak AD's structure is a helix with a number of fold less than six.

The relationship between internal chain length of amylopectin and crystallinity in starch

Molecular models of amylopectin were created and investigated by computer simulation. First, single and double helices of various lengths were constructed. The 1 --> 6 branching in double and single helices of amylopectin was studied. Subunits of single helices, double helices, and branch points were used as building blocks of larger systems. The possible makeup of amylopectin unit clusters was investigated via a series of models, including single-single, double-single, and double-double helix systems. The lengths of the single helix section that linked two branch points (internal chains) was systematically varied between values of 0-10 glucose residues. It was found that certain internal chain lengths lead to parallel double helices. Thus, it was postulated that the length of internal chains may determine the degree of local crystallinity. Furthermore, it was noted that some of the low-energy arrangement of double helices could be superimposed on either the two adjacent and nonadjacent double helices of crystalline A and B starch polymorphs. In other cases, the distance between the double helices is so large that it may in fact be a model for branching between two amylopectin crystals or unit clusters. Results obtained through this work were corroborated, where possible, with information available from crystallographic, branching, and enzymatic studies.

Solid-state 13C NMR investigations of the glycosidic linkage in alpha-alpha' trehalose

The sugar trehalose is known to play a central role in the desiccation tolerance of many organisms. Essential to trehalose's role are its glass forming abilities and ability to directly interact with lipid molecules. Detailed information on the structure and dynamics of glassy trehalose and its interactions with lipids, however, have been elusive. We have used solid-state NMR and ab initio quantum mechanical methods (Gaussian 94) in order to characterize the possible molecular conformations of trehalose. Using a simplified structure (2-(tetrahydropyran-2-yloxy) tetrahydropyran) as a model we have calculated the energy and 13C magnetic shielding parameters as a function of the two glycosidic torsion angles. Combining ab initio derived maps and using the 13C lineshape as constraints we were able to construct the torsion angle distribution map for alpha-alpha' trehalose. We believe measurements of 13C isotropic chemical shift and other solid-state NMR tensor parameter distributions in combination with ab initio methods can prove useful in identifying sources of structural disorder in glassy trehalose. By monitoring these structural distributions new information about the membrane surface associative properties of trehalose and other sugars should be accessible.

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.

Molecular behavior of water in a flour-water baked model system

The molecular behavior of water in complex food systems, via bonding and solvation reactions with low molecular weight solutes or high molecular weight macromolecules, is intimately linked with both the palatability and storage stability of those systems. Due to the difficulty in interpreting data derived from multicomponent finished food products, water behavior in a baked flour-water "matzo" model system was studied. Water behavior was assessed from high resolution [1H] nuclear magnetic resonance (NMR) spin-spin relaxation studies and differential scanning calorimetric (DSC) measurements of unfreezable water content. The unfreezable water capacity of "matzo" model crackers, as measured by DSC, ranged from ca. 24-30% (w/w). These results were corroborated by NMR data. Only one exponent (less than 0.46 msec greater than) is required to fit spin-echo evolution curves below a total moisture content of ca. 20% (w/w), whereas two exponents (less than 0.46 greater than and 1.6 msec) are observed when the moisture content exceeds 20% (w/w). Expert sensory texture assessments parallel unfreezable (= total) water contents between 2.9% (w/w) and 20.1% (w/w). This relationship may be explained by the known tendency for water to plasticize biological polymers, e.g. wheat starch and proteins, and to render these macromolecules incrementally more mobile with increasing water concentration. The similarities (if any) between water of plasticization, immobile water, and "bound" water are discussed, in terms of theoretical physicochemical "states" of water and the various techniques utilized to assess (define) those "states."

Conformational analysis and molecular modelling of the branching point of amylopectin

The conformational properties of 6(2) alpha-D-glucosylmaltotriose have been studied using energy calculations that include van der Waals interactions, hydrogen bond stabilization, exo-anometric effect and torsional potential contributions. The calculations focused mainly on the conformational properties displayed at the alpha(1----6) linkage within the tetrasaccharide for which the conformational space is reported. The tetrasaccharide molecule was then considered as a model compound of the branching point in amylopectin. From molecular modelling, some basic structural features associated with branching were clearly established. It was found that, among the low energy arrangements, the side chain would fold back onto the main backbone, thereby producing dense three-dimensional structures in which a 'parallel' arrangement is achieved. The branching between two strands of the double helix, as found in the crystalline moiety of A and B starches, was further investigated. It was found that one particular set of conformations about the glycosidic linkages in the two different strands, could result in an arrangement such that strands could be connected through an alpha(1----6) glycosidic linkage, with a minimum of distortion. The three-dimensional features derived from the molecular modelling agree with the physical properties and mode of biogenesis within the starch granule; they are in accord with a 'cluster' type of structure.