The negative free energy previously reported is explained by the stabilization of a PC-Phe (phosphocholine-phenylalanine) complex in the presence of water shown by the decrease in the symmetric stretching frequency of the phosphate group of the lipid (PO2(-)). An entropic contribution due to the disruption of the water network around the phenyl and in the membrane defect may be invoked. The dipole potential decrease is explained by the orientation of the carboxylate opposing to the CO of the lipids with oxygen moiety toward the low hydrated hydrocarbon core. The symmetric bending frequency of NH3(+) group of Phe, decreases in 5.2 cm(-1) in relation to water congruent with zeta potential shift to positive values. The Phe to DPPC dissociation constant is Kd = 2.23 ± 0.09 mM, from which the free energy change is about -4.54 kcal/mol at 25 °C. This may be due to hydrophobic contributions and two hydrogen bonds.

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Lipid membranes are one of the most important biological matrixes in which biochemical processes take place. This particular lipid arrangement is driven by different water disposition interacting with it, which is related to different water states with different energy levels at the interphase. In our work, we report changes in water content and distinctive water states by Fourier transform infrared (FTIR) spectroscopy of this self-assembled matrix at different water contents and temperatures. To determine whether water properties at lipid interphases depend on the group of the lipid molecule at which it is bound the phase-transition temperature of 1,2-dimiristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (14:0 diether PC) was followed by the changes in frequency of the different groups of the lipids by attenuated total reflection (ATR)-FTIR spectroscopy at different humidities. A comparison of these two lipids enables us to put into relevance the contribution of the CO groups as a hydration site. These changes were compared with those occurring at the water band, and a value of the enthalpic change was evaluated from them. The −OH stretching in the liquid water IR spectrum is the principal region used to understand its molecular organization (4000–3000 cm–1). The strength of hydrogen bonding depends on the cooperative/anticooperative nature of the surrounding hydrogen bonds, with the strongest hydrogen bonds giving the lowest vibrational frequencies. Thus, we can use water as a mirror of the membrane state in this kind of biological systems. Different phospholipids associate water at particular modes according to their structures. This may produce modulation of packing and hydration suitable for the incorporation of amino acids, peptides, and enzymes.

The lack of carbonyl groups and the presence of ether bonds give the lipid interphase a different water organization around the phosphate groups that affects the compressibility and electrical properties of lipid membranes. Generalized polarization of 14:0 Diether PC in correlation with FTIR analysis indicates a higher level of polarizability of water molecules in the membrane phase around the phosphate groups both below and above T m . This reorganization of water promotes a different response in compressibility and dipole moment of the interphase which is related to different H-bonding of water molecules with PO and CO groups.

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This review is an attempt to incorporate water as a structural and thermodynamic component of biomembranes. With this purpose, the consideration of the membrane interphase as a bidimensional hydrated polar head group solution, coupled to the hydrocarbon region allows for the reconciliation of two theories on cells in dispute today: one considering the membrane as an essential part in terms of compartmentalization, and another in which lipid membranes are not necessary and cells can be treated as a colloidal system. The criterium followed is to describe the membrane state as an open, non-autonomous and responsive system using the approach of Thermodynamic of Irreversible Processes. The concept of an open/non-autonomous membrane system allows for the visualization of the interrelationship between metabolic events and membrane polymorphic changes. Therefore, the Association Induction Hypothesis (AIH) and lipid properties interplay should consider hydration in terms of free energy modulated by water activity and surface (lateral) pressure. Water in restricted regions at the lipid interphase has thermodynamic properties that explain the role of H-bonding networks in the propagation of events between membrane and cytoplasm that appears to be relevant in the context of crowded systems.

Water plays a key role in the functioning of natural and synthetic molecular systems. Despite several hydration studies, different techniques are employed individually for monitoring different physical features such as kinetics, dynamics, and absorption. This study describes a compact hydration cell that enables simultaneous dielectric relaxation spectroscopy (DRS) and mass loss/uptake measurements in thin organic layers under controlled humidity conditions and in a wide temperature range. This approach enabled us to correlate the physical quantities obtained during the same experiment by complementary techniques. To demonstrate the performance of this device, a 200 nm thick poly(methyl methacrylate) (PMMA) layer was measured at various relative humidity levels (0%–75%), temperatures (25–75 °C), and frequencies (DRS: 0.1 Hz–1 MHz) to study how hydration and dehydration processes affect its molecular dynamics. The results show the capability of this setup to study the changes in the PMMA film regarding the kinetics and molecular dynamics upon variation of the water content.

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