###### Abstract  
The vibrational signature in the far-infrared region of two different phospholipids, phosphatidcholine (PC) and phosphatidylethanolamine (PE), was investigated as a function of relative humidity from 0 to 75% in order to evaluate the effect of headgroup composition on the formation of intermolecular interactions. The substructures of the frequency region between 50 and 300 cm\({}^{-1}\) were identified, and changes in the frequency and intensity of the related vibrations with hydration were analyzed. Interestingly, in PE, two additional vibrational bands with respect to PC were found at 162 and 236 cm\({}^{-1}\) and assigned to intermolecular hydrogen bonds between the hydrogen-bond-donating groups, \(-\)NH\({}_{3}^{+}\), and hydrogen-bond-accepting groups, \(-\)P\(-\)O\(-\) and \(-\)COO, of adjacent molecules. The presence of these interactions also affected the penetration of water, severely reducing the hydration capability of PE lipids. f =-IR spectroscopy; terahertz gap; phospholipid membranes; phosphatidylcholine; phosphatidylethanolamine; hydrogen-bonding continuum; hydration water +
Footnote †: journal: _Sci._**2021**, _11_, 10038.  
glycero-3-phosphocholine, C\({}_{36}\)H\({}_{72}\)NO\({}_{8}\)P) stack bilayers--and showed that this system sustains several low-frequency optical modes, most of which attributable to van der Waals intermolecular lipid-lipid vibrations [6], thus confirming the complex scenario proposed by molecular dynamics simulations [1]. With the aim of identifying the intrinsic vibrations of the membrane and evaluating the effect of the hydration water on the vibrations, we carried out measurements as a function of hydration, including the dry state. The measurements were carried out at different dehydration times, corresponding to different hydration degrees. Our findings showed that the intrinsic THz dynamics of the membrane were only slightly affected by hydration, with the same vibrational modes characterizing both the dry and the wet samples and with the hydration mainly influencing the vibrational width of the modes. Additionally, the results revealed a strong similarity between the THz dynamics of the lipids and those of water, thus, pointing to an effective biosystem-solvent dynamical coupling due to resonance mechanisms. Here, we aim to investigate the THz dynamics of a phospholipid membrane with a phosphatidylcholinetanolamine (PE) headgroup and compare them with those of a membrane with a phosphatidylcholine (PC) headgroup. As detailed below, these two headgroups mainly differ with respect to their intermolecular interaction principles [8]. We thus point to understand if and how the differences in the intermolecular interactions of these two model membranes may reflect their THz spectral signatures. PC and PE headgroups are two of four primary headgroups for the phospholipids found in eukaryotic cell membranes. PE lipids largely contribute to the total quantity of lipids in the inner membrane of E. coli (75-80%), in eukaryotic membranes (15-25%), and in mammalian brains (up to 45%) [9]. While the PC headgroup consists of a fully methylated ammonium group \((\text{CH}_{3})_{3}\)N\({}^{+}\), the PE headgroup is characterized by the presence of an ammonium group \((-\text{NH}_{3}^{+})\) (Figure 1). One substantial difference between PC and PE headgroups concerns the capability of forming intermolecular (lipid-lipid) hydrogen bonds. In PE, intermolecular hydrogen bonds between adjacent lipids are formed between ammonium nitrogen and phosphate oxygen. This linkage results in a very compact, rigid headgroup lattice at the bilayer surface that shows a reduced capability of hydration and is largely unaffected by both hydration and temperature [8]. Though in aggregated lipids with aligned headgroups the \(-\text{NH}_{3}^{+}\) group interacts with the \(-\text{PO}_{4}^{-}\) in particular, other combinations are possible--e.g., combinations involving the ester carbonyl groups [9].