## 3 Results
### Dppe  
In Figure 4, the far-IR spectra of DPPE/H\({}_{2}\)O samples at four distinct hydration degrees (dry, RH = 11%, 53%, 75%) with the corresponding fitting curve and the Gaussian components for the dry and mostly hydrated samples are reported. The acquired spectra at the different hydrations (Figure 4a) showed complex profiles, with the absorbance increasing as the hydration increased. Analogously to the procedure followed for the DMPC samples and described above, we performed the curve fitting of the spectra as a linear combination of individual Gaussian component bands. While in the DMPC samples eight components were needed to properly analyze the data, in the case of the DPPE samples we found that ten components must be considered for an appropriate description of the absorbance profiles. However, similarly to the DMPC  
Figure 4: Far-infrared spectra of the DPPE/H\({}_{2}\)O samples. (**a**) Spectra of pure water (dashed line) and of the samples at four distinct hydration degrees: dry (black full line), RH = 11% (red), RH = 53% (green), and RH = 75% (blue); (**b**) The spectra of the dry sample and of the wet samples are shown separately with the corresponding fitting curve (solid black line) obtained as the sum of the individual Gaussian peaks (dashed black lines). samples, the same number of band components (ten) were needed for the data analysis of the samples, independent of the relative humidity. Most interestingly, eight out of ten frequencies were almost coincident with those of the DMPC samples. We thus labelled the corresponding eight bands with the same letters used for the DMPC samples, and we used the Greek letters, \(\alpha\) and \(\beta\), for the bands that are characteristic only of the DPPE samples. Furthermore, no drift in frequency was observed, independent of the relative humidity of the samples. Thus, the frequencies of all the DPPE samples are: 46, 61, 89, 118, 144, 162, 189, 218, 236, and 263 cm\({}^{-1}\) (named in the following as A, B, C, D, E, \(\alpha\), F, G, \(\beta\), and H (see Table 2)). The \(\alpha\) and \(\beta\) bands have a frequency of 162 and 236 cm\({}^{-1}\), respectively. Although the frequency of the vibrational modes was not influenced by the hydration degrees, notable differences among the DPPE samples emerged when comparing the areas and widths of the excitations. In other words, the same vibrational modes characterized the DPPE samples at different hydration degrees, including the dry state, by their vibrational areas, and widths are affected by the level of hydration. The almost total coincidence of the frequencies of the DMPC bands with those of the DPPE, together with the very similar molecular structures of the two phospholipids (apart from the polar head structure), leads us to assign the vibrations observed in the DPPE as previously conducted for the DMPC. On the other hand, the \(\alpha\) and \(\beta\) bands, observed exclusively in the spectra of the DPPE, require additional considerations for their assignment. The DPPE membrane exhibits an enhanced lipid-lipid hydrogen-bonding capability of the PE group with respect to the PC group, which can differently form hydrogen bonds exclusively with water. When PE phospholipids aggregate, the headgroups align so that the ammonium group interacts directly with the phosphate group of the adjacent lipid via an intermolecular hydrogen bond, though other combinations are possible--e.g., interactions involving the carbonyl oxygen of ester groups [8; 9]. Additionally, though DPPE lipids mainly interact via direct intermolecular \(-\)NH\({}_{3}^{+}\)\(\ldots-\)PO\({}_{4}^{-}\) or \(-\)NH\({}_{3}^{+}\)\(\ldots-\) CO\({}_{2}\) contacts, water molecules may act as bridges in mediating these interactions. It is well known that in DPPE, the ensemble of the hydrogen bond linkages between adjacent phospholipids produces a very compact and rigid structure at the bilayer surface that is largely unaffected by hydration, contrary to the considerably flexible network of DMPC [8].