Figure 5. Salt bridges in mineralized and non-mineralized collagen fibrils.

(a) Number of salt bridges per unit cell. In the non-mineralized model, the number of salt bridges is ~260, due to interactions between charged side chains of collagen: lysine (+), arginine (+), glutamic acid (−) and aspartic acid (−). In the mineralized models, there is a high number of mineral–mineral salt bridges, due to the fact that the moieties forming HAP are highly charged: Ca²⁺, PO₄³⁻ and OH⁻. Salt bridges are also found relevant between collagen and mineral phase (about 220 and 520 in 20% HAP and 40% HAP case, respectively), contributing to the increase in mechanical properties of mineralized fibrils. Panel b shows an example of salt bridge between a lysine (+) side chain and mineral PO₄³⁻ group.