Molecular dynamics simulation of conformational change of poly(Ala-Gly) from silk I to silk II in relation to fiber formation mechanism of Bombyx mori silk fibroin

The fiber formation mechanism of Bombyx mori silk fibroin by silkworm is essentially the structural change from silk I (the silk fibroin structure before spinning in the solid state) to silk II (the silk fibroin structure after spinning) under external forces in both silk gland and spinneret of B. mori silkworm. Recently, we proposed structural models for silk I and silk II forms of the model peptide (Ala-Gly)(15) of B. mori silk fibroin using mainly solid-state NMR methods. In this paper, molecular dynamics (MD) calculation was performed to simulate the structural change of poly(Ala-Gly) from silk I to silk II and to clarify the detailed mechanism of the silk fiber formation. The silk I structure (repeated beta-turn type II) changes to silk II structure (heterogeneous structure, but mainly antiparallel beta-sheet) by stretching of the chain with MD simulation, but the change occurs only under very high temperature such as 1000 K and large tensile stress (1.0 GPa). However, the structural change during the MD simulation occurs more easily by taking into account several external forces (the presence of water molecules around the silk chains, and application of both shear and tensile stresses to the silk fibroin) applied to the silk fibroin simultaneously. The heterogeneous structure of the silk fiber determined previously with solid-state NMR could be reproduced well with the MD calculation and then molecular mechanics calculation after removal of water molecules.