## 3 Results
### Dmpc
Concerning the contribution of each band to the spectrum (Figure 3), the area of all the bands increased with the hydration level, thus reflecting the high absorbance of the hydration water, which could be due to both water-water and water-lipid coupled vibrations. By calculating the fractional variation of the area of each band in the wet state (75%) with respect to the dry state (Figure 3b), we found that the highest frequency bands (D, E, F, G, H) showed very high variation (3 \(\div\) 8%), and the H band (259 cm\({}^{-1}\)), attributed to the \begin{table}
\begin{tabular}{c c c c c} \hline
**Bands** & **Dry DMPC** & **Bands** & **DPPE** & **DYPE** \\
**DMPC** & **Frequency (cm\({}^{-1}\))** & **DPPE** & **Frequency (cm\({}^{-1}\))** & **Assignment** \\ \hline A & 50 & A & 46 & [PERSON] \\ \hline B & 70 & B & 61 & [PERSON] \\ \hline C & 90 & C & 89 & [PERSON] \\ \hline D & 110 & D & 118 & C–CH\({}_{3}\) motion \\ \hline E & 145 & E & 144 & van [PERSON] \\ \hline  & & \(\alpha\) & 162 & \\ \hline F & 186 & F & 189 & \(\tau\)(C–CH\({}_{2}\)) \\ \hline G & 219 & G & 218 & \\ \hline  & & \(\beta\) & 236 & \\ \hline H & 259 & H & 263 & \(\tau\)(C–CH\({}_{3}\)) \\ \hline \end{tabular}
\end{table} 
Table 2: Frequencies and tentative assignments of the bands identified in this work for both DPPE and DMPC samples. Figure 3: DMPC/H\({}_{2}\)O samples. (**a**) Dependence of the areas of the eight Gaussian band components on the relative humidity. Each panel corresponds to a different band, as named in Table 2. (**b**) Fractional variation of the area of each band evaluated as (I75% \(-\) Idry)/Idry. torsional modes of the CH\({}_{3}\) groups, was the one that shows the most significant percentage variation. Notably, the frequency range of the bands between 110 and 260 cm\({}^{-1}\), i.e., from D to H, almost matches that of the so-called \"connectivity band\" of bulk liquid water [15]. The connectivity band, a pronounced and broad peak centered at 200 cm\({}^{-1}\) as seen in the absorption spectrum of pure water, is described in terms of the collective intermolecular stretching of the hydrogen bond network [5; 15]. This shows a composite structure based on four components (88, 130, 195, 251 cm\({}^{-1}\)) corresponding to different states of connectivity [15], with the component at the highest frequency reflecting water molecules having established a maximum number of hydrogen bonds [15]. Concerning the minimal variation in the lowest energy bands (A, B, C) as a function of increasing relative humidity and thus of the number of water molecules per lipid, it is worth mentioning that in pure water, the hydrogen bond bending vibrations and the umbrella-like motion of two hydrogen bonded tetrahedra along the connecting hydrogen bond axis that both contribute to the 50-70 cm\({}^{-1}\) region are clearly visible in the Raman and optical Kerr effect spectra but are not very pronounced in the far-IR absorption spectra [5]. The main features of the vibrational excitations of the DMPC/H\({}_{2}\)O samples at THz frequencies can be summarized in the following statements: (i) eight vibrational modes have been detected independently from the hydration level; (ii) all models show a sensitivity to water, as seen in the area and in the fwhm; (iii) in particular, the highest frequency bands (from D to H, and to a greater extent the H band) that match the connectivity band of water are the ones that show the largest percentage variation upon hydration; (iv) two vibrations, namely D and E, undergo a shift to higher frequencies upon dehydration. This can be attributed to a strengthening in the corresponding intermolecular lipid-lipid interactions.