Carbon-13 CP/MAS NMR studies of amylose inclusion complexes, cyclodextrins, and the amorphous phase of starch granules: relationships between glycosidic linkage conformation and solid-state carbon-13 chemical shifts

Abstract

In order to characterize molecular conformations within starch granules and to examine the relationships between polysaccharide conformation and solid state 13C chemical shifts, a range of polymeric and oligomeric a-(1r4) glucans has been examined by cross polarization and magic angle spinning (CP/MAS) 13C NMR spectroscopy. Single helical amylose (polymeric a-(1r4) glucan) polymorphs with various molecular inclusions as well as a- and a-cyclodextrin hydrates have been studied and their 13C CP/MAS spectral features compared with those of both double helical and amorphous a-(1r4) glucans. Spectra of single helical amyloses show similar features irrespective of the nature of the included molecule and have only one resolved signal for each carbon site consistent with the nearly hexagonal packing of sixfold helices as characterized by X-ray diffraction. Cyclodextrin hydrates show resolved C-1 and C-4 resonances from each of the six (a-cyclodextrin) or seven (b-cyclodextrin) a-(1r4)-linked glucose residues present in the macrocycle. Chemical shift ranges in cyclodextrins are closely similar to those of single helical amyloses with the exception of one C-1 and one C-4 resonance in a-cyclodextrin which are at unusually high field and assigned to sites adjacent to a conformationally strained glycosidic bond. A comparison of solution chemical shifts with weighted averages of solid-state shifts suggests that b-cyclodextrin adopts glycosidic solution conformations similar to those found in the crystalline state but that a-cyclodextrin may be slightly more expanded in solution than in the crystalline state. Line widths in the a-(1r4) glucans studied can be rationalized in terms of crystalline perfection, and signal multiplicity arises through either intramolecular conformational effects (a- and b-cyclodextrin) or considerations of packing symmetry (double helical a-(1r4) glucans). The wide range of chemical shifts observed for C-1 and C-4 sites together with the essentially constant chemical shifts for other sites suggests that C-1 and C-4 chemical shifts are primarily determined by glycosidic linkage conformation. Correlations are found between C-1 chemical shifts and the sum of the moduli of the two torsion angles (p and p) describing rotation about the glycosidic bonds as well as with the modulus of p. Both correlations accurately predict the range and qualitatively predict the distribution of chemical shifts found for amorphous a-(1r4) glucans assuming the equiprobable occurrence of all allowed glycosidic conformations. Similarities in C-1 and C-4 chemical shifts for single helical amyloses and amorphous materials show that starch granule amorphous phases contain a significant fraction of single-helix-like local conformations. This observation is consistent with the presence of a-(1r4) glucan/lipid inclusion complexes within starch granules.