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.

It has been suggested that water in confined regions presents different properties than bulk water, mainly because of the changes in water population species that may be induced by the adjacent walls of different polarities in terms of hydrogen bond formation. In this context, it would be expected that lipids in the gel and the fluid states should offer different templates for water organization. The presence of water pockets or defects in lipid bilayers has been proposed to explain the insertion of charged and polar peptides and amino acids in membranes. In this work, we provide direct evidence by means of FTIR spectroscopy that water band profiles are changed whether lipids are in the solid state, in the gel state after heating and cooling across the phase transition, or in the fluid state. The different bands found in each case were assigned to different H-bonded water populations in agreement with the exposure of carbonyl groups.

We employ molecular dynamics simulations to study the hydration properties of Dipalmitoylphosphatidylcholine (DPPC) bilayers, both in the gel and the liquid crystalline states. We show that while the tight hydration centers (PO and CO moieties) are significantly hydrated in both phases, the gel-fluid transition involves significant changes at the second hydration shell, particularly at the buried region between the hydrocarbon tails. Thus, while almost no buried water population exists in the gel state below the carbonyls, this hydrophobic region becomes partially water accesible in the liquid crystalline state. We shall also show that such water molecules present a lower H-bond coordination as compared to the molecules at the primary hydration shell. This means that, while the latter are arranged in relatively compact nanoclusters (as already proposed), the buried water molecules tend to organize themselves in less compact structures, typically strings or branched strings, with a scarce population of isolated molecules. This behavior is similar to that observed in other hydration contexts, like water penetrating carbon nanotubes or model hydrophobic channels or pores, and reflects the reluctance of water to sacrifice HB coordination.

The hydration of solid dimyristoylphosphatidylethanolamine (DMPE) produces a negligible shift in the asymmetric stretching frequency of the phosphate groups in contrast to dimyristoylphosphatidylcholine (DMPC). This suggests that the hydration of DMPE is not a consequence of the disruption of the solid lattice of the phosphate groups as occurs in DMPC. The strong lateral interactions between NH(3) and PO(2)(-) groups present in the solid PEs remain when the lipids are fully hydrated and seem to be a limiting factor for the hydration of the phosphate group hindering the reorientation of the polar heads. The lower mobility is reflected in a higher energy to translocate the phosphoethanolamine (P-N) dipoles in an electrical field. This energy is decreased in the presence of increasing ratios of PCs of saturated chains in phosphoethanolamine monolayer. The association of PC and PE in the membrane affecting the reorientation of the P-N groups is dependent of the chain-chain interaction. The dipole potentials of PCs and PEs mixtures show different behaviors according to the saturation of the acyl chain. This was correlated with the area in monolayers and the hydration of the P-N groups. In spite of the low hydration, DMPE is still able to adsorb fully hydrated proteins, although in a lower rate than DMPC at the same surface pressure. This indicates that PE interfaces possess an excess of surface free energy to drive protein interaction. The relation of this free energy with the low water content is discussed.

Analysis of the band corresponding to the frequency of vibrational symmetric stretching mode of methylene groups in the lipid acyl chains and the bands of water below and above the phase transition of different lipids by Fourier transform infrared spectroscopy gives strong support to the formation of confined water pockets in between the lipid acyl chains. Our measures and analysis consolidate the mechanism early proposed by Traüble, in the sense that water is present in kinks formed by trans-gauche isomers along the hydrocarbon tails. The formation of these regions depends on the acyl lipid composition, which determines the presence of different populations of water species, characterized by its degree of H bond coordination in fluid saturated or unsaturated lipids. The free energy excess due to the reinforcement of the water structure along few water molecules in the adjacencies of exposed membrane residues near the phase transition is a reasonable base to explain the insertion and translocation of polar peptides and amino acid residues through the biomembrane on thermodynamic and structural grounds.

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The classical view of a cell membrane is as a hydrophobic slab in which only nonpolar solutes can dissolve and permeate. However, water-soluble non-electrolytes such as glycerol, erythritol, urea and others can permeate lipid membranes in the liquid crystalline state. Moreover, recently polar amino acid's penetration has been explained by means of molecular dynamics in which appearance of water pockets is postulated. According to Träuble (1971), water diffuses across the lipid membranes by occupying holes formed in the lipid matrix due to fluctuations of the acyl chain trans-gauche isomers. These holes, named "kinks" have the molecular dimension of CH2 vacancies. The condensation of kinks may form aqueous spaces into which molecular species of the size of low molecular weight can dissolve. This molecular view can explain permeability properties considering that water may be distributed along the hydrocarbon chains in the lipid matrix. The purpose of this review is to consolidate the mechanism anticipated by Träuble by discussing recent data in literature that directly correlates the molecular state of methylene groups of the lipids with the state of water in each of them. In addition, the structural properties of water near the lipid residues can be related with the water activity triggering kink formation by changes in the head group conformation that induces the propagation along the acyl chains and hence to the diffusion of water.

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