The total area of each PSA spectrum is normalized to 1. The relative percentage of each subpeak is calculated as the subpeak area over the total area, so that we can compare the differences among the three samples in Figure . The area fraction of α-helix was approximately 72% with a 1% difference for all three samples, indicating the dominance of α-helical structure. The difference in the area percentage of β -sheet is statistically indistinguishable for filaments (27%) and weakly bundled networks (28%, Supplementary Figure ) but decreases slightly to 24% by 4% when strongly bundled networks are formed. Interestingly, the area percentage of the loop and turn structures is negligible and remains at 0.8% for filaments and weakly bundled networks, but it becomes more pronounced and almost triples (2%) when bundles become strong. The opposite trend of changes in the peak area between β -sheet and loop seems to indicate a correlation between the two backbone structures upon morphology change, where a fraction of β -sheets transitions into loops when the bundles are strong. However, the lineshape difference is relatively small for a concrete conclusion on the structural transition, considering that both β -sheet and the loop are overpowered by the α-helix. Furthermore, the fitting of each structure with one Gaussian profile is a simplified model because there could be more than one hydrogen-bonded species under each fitted peak that would not be well fitted by a single Gaussian profile. Thus, the area percentage calculated from Gaussian fitting cannot be treated as a quantification of the change in the backbone population. Rather, it is a good indicator for the structural comparison of filamentous and bundled actin networks, providing evidence for the structural transition in the actin backbones. Overall, PSA analysis offers insights into the actin backbone structure undergoing morphological changes from filaments to strongly bundled networks, which implies a structural transition from β -sheet to loop structures.