## 3 Results
### Dppe
We thus tentatively assigned the \(\alpha\) and \(\beta\) bands to vibrations of the network of DPPE phospholipids linked by intermolecular hydrogen bonds. In particular, the frequency of the \(\alpha\) band (162 cm\({}^{-1}\)) was found to be very close to the frequency (166 cm\({}^{-1}\)) measured in a proteinic liquid for the hydrogen bond stretching (N\(-\)H\(\cdots\)O) occurring between the N\(-\)H group from the N-methylpyrrolidinium cation and the oxygen on the acetate anion [16]. Based on that, we tentatively ascribed the \(\alpha\) mode to the hydrogen bond stretching N\(-\)H\(\cdots\)O where the N\(-\)H group from the ammonium group acted as a hydrogen bond donor, while the oxygen on the carbonyl esterified group acted as a hydrogen bond acceptor. Furthermore, we tentatively assigned the higher frequency \(\beta\) band to the N\(-\)H\(\cdots\)O\(-\)P hydrogen bond stretching between the ammonium group and the oxygen of the phosphate moiety. This assignment was based on the following considerations: the N\(-\)H\(\cdots\)O\(-\)P hydrogen bond is usually stronger than the N\(-\)H\(\cdots\)O\(-\)C bond [17], so a higher frequency is expected for the corresponding stretching band; it is more favorable for DPPE to form intermolecular hydrogen bonds between amine groups and phosphate groups than between amine groups and carbonyl groups [8; 9]; this finding corresponds with the higher intensity of the \(\beta\) band with respect to the \(\alpha\) band (Figure 4). It is worth noting that such a rigid network of lipid-lipid intermolecular contacts dramatically reduces the capability of hydration in DPPE. This is evident when analyzing the number of water molecules per lipid that only achieves the value n\({}_{W}\) = 1.03 at the highest relative humidity (RH = 75%). On the other hand, the DMPC lipids have a higher number of water molecules per lipid, n\({}_{W}\) = 4.0, that may act as bridges between lipids, contributing to establish an extended and more flexible network in the bilayer [10]. Thus, in DPPE, differently from DMPC, we observed a reduced capability of hydration together with a lower sensitivity to the water content of the spectral fingerprints at THz frequencies. This situation was reflected in the minor changes observed for the fwhm of all 10 identified far-infrared components (data not shown) when the hydration was changed. The impenetrability to water maintained the bilayer rigidity and kept the acyl chains well-ordered. However, the influence of the hydration degree on the low-frequency vibrations can be investigated through the inspection of the area of the Gaussian bands as a function of the relative humidity, as shown in Figure 5. This trend can be more easily examined by looking at Figure 5b, which shows the relative variation in the areas in the wet state (RH = 75%) compared to the dry state. It is immediately noticeable how this trend differs considerably from that observed in the DMPC samples. Firstly, the range of the fractional variation for the different components was much smaller in the case of DPPE, and this confirmed the lower sensitivity of these samples to hydration compared to DMPC. Secondly, while for the DMPC samples all of the higher frequency bands (from D to H) were significantly affected by the water content, for DPPE, only two bands, i.e., F and H, that correspond to 189 and 263 cm\({}^{-1}\), showed an important variation with respect to the dry sample, albeit very limited when compared to DMPC. The frequencies of these two bands were very close to the two bands at higher frequencies that characterized the connectivity band of pure water (195, 251 cm\({}^{-1}\)), as detailed above in Section 3.1. Interestingly, the intensities of the \(\alpha\) and \(\beta\) bands showed a negative increase with increasing hydration. This can be interpreted as the result of competition between the water molecules and the \(-\mathrm{PO}_{4}^{-}\) or carbonyl groups in forming hydrogen bonding with \(-\mathrm{NH}_{3}^{+}\) moieties.