The global fitting routine (Global Peak and 2 nd Derivative Fit -GP2DF.py; see detailed description in SI section 1) is based on the simultaneous modelling of spectra and their 2 nd derivatives with Gaussian band shapes and 2 nd derivatives thereof, respectively. As argued above, 2 nd derivatives considerably enhance narrow signals, which are in the present case due to Aβ. Therefore, fitting both results simultaneously provides additional restraints on peak positions and band shapes to model the narrower peptide features with increased accuracy. Furthermore, as the water signal is associated with a drastically larger linewidth (HWHM > 41.2 cm -1 ; i.e., σ > 35 cm -1 ) than the amide I peaks (HWHM < 23.6 cm -1 ; i.e., σ < 20 cm -1 ), one can conveniently extract the contribution due to Aβ. Figure and show the SEIRA spectra after removal of the overlapping water bands for the membrane systems. The main peaks are again indicated in the spectra and agree with the 2 nd derivative peaks identified in Figure (see comparison of 2 nd derivative before and after post-processing in Figure ). That is, with the tMLM system, we observe both peaks, 1635 1694 cm -1 , consistent with anti-parallel β-sheets; instead, in the tBLM system, we note a dominating peak at 1635 cm -1 with only low intensity at ~1694 cm -1 indicative of parallel β-sheets. Interestingly, recording SEIRA spectra in D2O (Figure and), we find remarkably similar shapes of the amide I and amide I' envelopes (in H2O and D2O, respectively), as the underlying peaks adopt very similar relative intensities (see labelled peaks in Figure and). Accordingly, as expected, the amide I components at roughly 1694 and 1635 cm -1 (β-sheets), and at 1670 -1655 cm -1 (β-turns, α-helices and random coils) are redshifted by up to 10 cm -1 to 1684 and 1632 cm -1 , and 1660 -1650 cm -1 via the H/D-based vibrational isotope shift. Consequently, also in D2O we arrive at the conclusion that higher fraction of parallel β-sheets when incorporation into the membrane is enabled. It should be noted that the spectrum of Aβ with tBLMs in D2O shows a more clearly identifiable peak at 1683 cm -1 (Figure ) than the corresponding 1694 cm -1 peak in H2O (Figure ). On the one hand, this can be due to the vibrational isotope shift upon H/D-exchange, which exposes this absorption in the background of unstructured Aβ fragments. On the other hand, one must take into account H/D-effects on the Aβ aggregation dynamics, as it was shown recently that D2O modulates the aggregation kinetics due increased H-bond strengths upon H/D-exchange. We observe the consequences of this effect in our experiments on neutral membranes, where we detect significant Aβ signals due to association only with the membrane in H2O, but negligible amounts in D2O (Figure ). We interpret this observation as accelerated folding dynamics in solution before the interaction with the membrane can take its effect. Negatively charged membranes shown in Figure appear to compensate for this H/D-effect to considerable extent; still the kinetics of the antiparallel to parallel β-sheet transition may be affected.