Our modeling also provides effective angles of the ensemble of parallel and antiparallel β-sheet fractions (Figure and). Interestingly, the tilt angles differ more considerably between the membrane systems (i.e., red vs. blue in Figures and) than between parallel and antiparallel β-sheets (i.e., Figure vs. Figure ). In the case of tMLM (red data), parallel and antiparallel β-sheets adsorb at an angle of about 20° and do not change their orientation considerably over time, consistent with the maintained fraction of parallel β-sheets (Figure , red data). This suggests that the Aβ adsorbs onto the membrane interface either already as a mixture of parallel or antiparallel β-sheet species or that the associated parallel/antiparallel β-sheet transitions occur within the first minute, i.e., beyond our current time resolution. On tBLMs (blue data), we observe quite different orientations. The orientation of parallel β-sheet species is maintained at around 50° (when considering the intervals), showing that we indeed probe membrane-incorporated Aβ species with a different alignment than for interfacial species. Instead, antiparallel βsheet species shows an orientation around 35°, which may increase slightly coinciding with the sigmoidal transition at 1 h in Figure . However, it should be noted that the determination of the orientation after 1h is quite error-prone due to < 5% content of antiparallel β-sheets in the SEIRA spectra. As the initial orientation is somewhat larger than the 20° at the tMLMs, it is likely associated with membrane-incorporated structures. We want to note that the absolute values of fractions and orientations rely on the intensity ratios obtained from DFT. As such, deviations of approx. 10% are possible when the DFTbased reference system is changed, for instance, by introducing amino acid side chains or by changing lengths and number of strands. Testing such factors in our previous work led only to small deviation and unchanged conclusions. Importantly, this quantitative analysis is consistent with the qualitative assessment we made above based on the unprocessed data and the temporal evolution of the β-sheet specific SEIRA signals in Figure .