Abstract
The vibrational signature in the far-infrared region of two different phospholipids, phosphatidylcholine (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, -NH3+, 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.
Highlights
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
In agreement with our previous work, we found that eight components were needed to accurately fit the DMPC samples independently of their hydration degree [6], while ten components were necessary for all of the DPPE samples
By comparing the two investigated systems, DMPC and DPPE phospholipid membranes, we can expect that differences in their spectral signatures will mainly reflect different intermolecular interaction principles
Summary
The hydrogen bond network of water exhibits vibrational resonances in the THz frequency range, and, the dynamical coupling between biomolecules and their hydration water is effective in this range [5]. Attention has been focused both on the identification of the intrinsic low-frequency vibrations of the biosystems—e.g., by means of studies on dry systems—and on the effect of hydration on these vibrations. In this regard, studies as a function of hydration, ranging from the dry state to full hydration, have allowed us to investigate both issues.
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