The sensitivity of the amide I modes helps gain access to the degree of hydration within the different backbone structures. Among the three secondary structures, α-helix is the dominant species, followed by the β -sheet and loops. As discussed above, the relative populations of different structures remain invariant across filaments and weakly bundled networks. However, for strongly bundled networks, there is a transition from β -sheet to loops corresponding to more disordered backbones, which can be associated with an increase in structural heterogeneity within the actin network. Compared to β -sheet and loop structures, α-helix features are the most preserved across morphologies (Figures and). As shown in the dynamics analysis, the α-helices of actin biopolymers are the most solvent-exposed, with fast water fluctuations in the local environment. In contrast, the β -sheet structure was shown to be another abundant backbone structure in the actin network, whose measured dynamics indicate a very static local environment (Figure ). Upon strongly bundled network formation, β -sheets in the actin backbones become increasingly rigid, as there are essentially no dynamics within the picosecond scale of the measurement. Because the dynamics measured by 2D IR report on the hydrogen bonding dynamics of local water molecules, static dynamics are explained as low exposure to solvents around β -sheets. This finding allows us to hypothesize that β -sheets are generally less solvent-exposed across all morphologies; in strongly bundled networks, they are shielded from water solvation by being buried in the bundle core. Based on this evidence, we also hypothesize that β -sheets of actin bundles play a minor role in protein binding or solvent exchange. The more static dynamics around β -sheets in the strongly bundled network can also result from more immobilized water molecules in the local environment. It is possible that the water molecules are more ordered and less mobile between β -sheets, leading to a more static dynamics in 2D IR analysis .