Amylose chain behavior in an interacting context I. Influence of a nonchair ring on the maltose conformations

In the presence of steric constraints, flexible forms, i.e., skew (S), boat (B) or half-boat (H), were evoked from experimental data and conformational analyses by molecular mechanics calculations for glucopyranose rings of amylose fragments. This important case occurring, for example, in amylose-amylase complexes, requires careful analysis of these flexible ring forms prior to any further conformational study. The influence of a nonchair (flexible) form on the maltose conformation is systematically evaluated, with an appropriate strategy using 'Semirelaxed'' maps and comparing them with those obtained from already known chair-chair (C-4(1)-C-4(1)) maps. Therefore, new low-energy maltose conformations are described and classified from flexible-chair and chair-flexible maps. These conformations are well dispersed inside significantly larger contours in the (phi, psi) projection. In a second stage, the consequence of these flexible ring forms is discussed in terms of amylose propagation apart from these new maltose conformations. Two propagation parameters are defined (tau, Omega), related to the local curvature of the chain and the relative orientation of the two mean ring planes. Low-energy conformations of C-4(1)-C-4(1) maltose have almost the same curvature between the two rings, whereas their relative orientations have well-identified Omega values. On the contrary, the presence of one flexible conformations considerably increases the variation range of both propagation parameters. Thus, new low-energy conformations allow local curvatures yielding from almost perpendicular to linear pairs of glucopyranose rings with relative orientation coveting about three-fourths of the total domain. This description is essential to understand amylose conformations in catalytic site and subsites of the amylases. .