Figure 1.

A simplified mechanical model of the pyranose ring. An external force (F) is applied to the ring through the oxygen atoms O₁ and O₄ as in an (1a → 4e)-linked polysaccharide chain (e.g., amylose). The direction of F coincides with the direction of the residue vector O₁O₄. The force applied to the axial oxygen O₁ has a large torque τ_P1 relative to the pivot point P₁ (P₁ lies on the imaginary axis of rotation passing through C₂ and O₅) and promotes a transition of the ring to the boat conformation. The axial bond C₁O₁ works as a lever arm. The force applied to the equatorial oxygen O₄ has a negligible torque relative to P₂ (P₂ lies on the imaginary axis of rotation that passes through C₃ and C₅) because its line of action passes in the vicinity of P₂. Therefore the equatorial bond cannot mediate a conformational transition; however, a force applied to it will strain the ring. (Inset) Definition of torque as the cross product between the distance vector l and the force vector F.