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 similar to 3.5 GPa and similar to 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.