Radial variations in mechanical behaviour and fibrillar structure in annulus fibrosus has foundations at molecular length-scale: Insights from molecular dynamics simulations of type I and type II collagen molecules

In annulus fibrosus (AF), type I collagen (Col-I) dominates outer AF (OAF) while type II collagen (Col-II) dominates inner AF (IAF). Using molecular dynamics simulations, this work reports how molecular-level structural and mechanical differences between Col-I and Col-II contribute to AF exhibiting radially varying structure and mechanical properties. Results show that differences in interpolypeptide forces and residue solvent accessibility contributes to Col-I showing intact triple-helix, lower backbone kinks and interpolypeptide separation (IPS), and thus contributing to expression of lower intermolecular spacing in collagen fibrils, lower lamellae thickness and lower water content in OAF. Furthermore, Col-I primarily exhibits backbone stretch while Col-II exhibit backbone straightening under tension, despite having comparable elastic moduli of ~ 3.5 GPa and ~ 3.2 GPa during backbone stretch. Such differences contribute to OAF showing stiffer stress–strain characteristics under tension. Furthermore, higher slenderness ratio of Col-I leads to buckling under compression—contributing to larger radial bulge being exhibited by OAF lamellae.