Crystallographic Approach and Molecular Models

The XRD patterns of starch grains have broad peaks and a large amorphous fraction. This is because of the low crystallinity and the small size of crystals. The data obtained from crystalline powders are therefore difficult to interpret. Crystallographic studies have rather been conducted on crystals of fibrillar amylose by XRD (Sarko and Wu, 1978; Sarko and Biloski, 1980). Achieving monocrystals of amylose (Buleon et al., 1984) and electron diffraction studies have permitted to propose lattice parameters and space groups. The first modeling work of French and Murphy (1977) showed that amylose could be formed in a double helix. For type A and type B, this arrangement corresponds to an energy minimum. Wu and Sarko (1978a,b) proposed models from amylose A and B involving right double helix with an antiparallel stacking. The difference between the two polymorphs is the compactness of the double helices in the crystalline stacking. These models, however, do not take into account the cluster model of amylopectin or the steric hindrance that results. Models proposed by Imberty et al. (1988) and Imberty and Perez (1988), based on data from Wu and Sarko (1978a,b), and enriched by data obtained from electron diffraction propose a parallel stacking of left double helices with parallel strands for types A and B. The most stable conformation for the amylose is a double helix, which comprises 2X6 glucose units per turn with a pitch of about 2.1 nm. The corresponding dihedral angles are 83.8 degrees for F and -144.6 degrees for ψ.

Leave a Reply

Your email address will not be published. Required fields are marked *