These findings suggest an essential role of loops in the physical and biochemical properties of bundled actin networks. Loops constitute only a small fraction of the residues, suggesting that actin is highly structured. The depletion interaction of PEG 20 kDa induces attraction among filaments, rearranging the backbones of F-actin, and causing part of the β -sheet to transition into loops in a strongly bundled network. In filaments and weakly bundled networks, the loop structure is exposed to solvent molecules, resulting in relatively fast hydrogen bonding dynamics in the local environment. High solvent exposure may be correlated with a high propensity to bind other proteins in solution. In fact, many actin-binding proteins interact with actin loops in monomers and filaments, such as the DNase-I loop and actin-binding cleft . In strongly bundled networks, part of the β -sheet regions likely contains short loops, which gives rise to a static environment of loops in this morphology compared to filament and weakly bundled networks (Figure ). As discussed above, increasingly immobilized water can also contribute to more rigid environments of loops in strongly bundled networks . It is likely that these extended loops are distributed across buried regions, which are more constrained and less solvent-exposed than the loops at the solvent interface. Therefore, we hypothesize that loops in strongly bundled networks contribute more to network rigidity and less to binding capability, such as protein binding and solvent exchange, in contrast to the loops in monomers and filaments. To this end, the spatial distribution of different secondary structures in crowded environments is deduced, where the hydration of the actin backbone is altered by polymer depletion interactions. It is worth noting that solely based on our findings, the effects of loops on the mechanical and biochemical properties of actin networks remain elusive. Nevertheless, all results in this work point towards the non-negligible role of the loop structure, which has a substantial impact on the functions of actin bundles despite its minor fraction in the backbone structures.