An FT-IR study of water hydrating dipalmitoylphosphatidylcholine multibilayers and reversed micelles

Long-Range Lipid-Water Interaction as Observed by ATR-FTIR Spectroscopy

It is commonly assumed that the structure of water at a lipid-water interface is influenced mostly in the first hydration layer. However, recent results from different experimental methods show that perturbation extends through several hydration layers. Due to its low light penetration depth, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is specifically suited to study interlamellar water structure in multibilayers. Results obtained by this technique confirm the long-range water structure disturbance. Consequently, in confined membrane environments nearly all water molecules can be perturbed. It is important to note that the behavior of confined water molecules differs significantly in samples prepared in excess water and in partially hydrated samples. We show in what manner the interlamellar water perturbation is influenced by the hydration level and how it is sequentially modified with a step-by-step dehydration of samples either by water evaporation or by osmotic pressure. Our results also indicate that besides different levels of hydration the lipid-water interaction is modulated by different lipid headgroups and different lipid phases as well. Therefore, modification of interlamellar water properties may clarify the role of water-mediated effects in biological processes.

Weakened hydrogen bonds in water confined between lipid bilayers: the existence of a long-range attractive hydration force

Stronger or weaker H-bonds? H-bonds in interlamellar water in partially hydrated lipid multibilayers are stronger with respect to bulk water. In contrast, the authors show by ATR-FTIR spectroscopy that the H-bonds are weaker in multibilayers in excess water (see spectra). This finding is of biological relevance for water-mediated phenomena in membranes.

Water near lipid membranes as seen by infrared spectroscopy

The ordering and H-bonding characteristics of the hydration water of the lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) were studied using polarized infrared spectroscopy by varying either the temperature or the relative humidity of the ambient atmosphere of multibilayer samples. The OH-stretching band of lipid-bound water was interpreted by a simplified two-state model of well-structured, low density "network" water and of less-structured dense "multimer" water. The IR-spectroscopic data reflect a rather continuous change of the water properties with increasing distance from the membrane and with changing temperature. Network and multimer water distribute across the whole polar interphase with changing composition and orientation. Upon dehydration the fraction of network water increases from about 30 to 60%, a value which is similar to that in supercooled water at -25 degrees C. The highly ordered gel phase gives rise to an increased fraction of structured network water compared with the liquid crystalline phase. The IR order parameter shows that the water dipoles rearrange from a more parallel towards a more perpendicular orientation with respect to the membrane normal with progressive hydration.

Hydration and Lyotropic Melting of Amphiphilic Molecules: A Thermodynamic Study Using Humidity Titration Calorimetry

The hydration of the lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and of the cationic detergent dodecyltrimethylammonium bromide (DTAB) has been studied by means of isothermal titration calorimetry (ITC), gravimetry, and infrared (IR) spectroscopy. During the experiments films of the amphiphiles are perfused by an inert gas of variable relative humidity. The measurement of adsorption heats using ITC represents a new adaptation of adsorption calorimetry which has been called the humidity titration technique. This method yields the partial molar enthalpy of water upon adsorption. It is found to be endothermic with respect to the molar enthalpy of water on condensation for the water molecules which interact directly with the headgroups of POPC and DTAB. Consequently, the spontaneous hydration of the amphiphiles is entropy driven in an aqueous environment. IR spectroscopy shows that hydration is accompanied by the increase in the conformational and/or motional freedom of the amphiphilic molecules upon water binding. In particular, a lyotropic chain melting transition is induced at a certain characteristic relative humidity. This event is paralleled by the adsorption of water. The corresponding exothermic adsorption heat is consumed completely (POPC) or partially (DTAB) by the hydrocarbon chains upon melting. Differential scanning calorimetry was used as an independent method to determine transition enthalpies of the amphiphiles at a definite hydration degree. Water binding onto the headgroups is discussed in terms of hydrogen bonding and polar interactions. The adsorption isotherms yield a number of approximately 2.6 tightly bound water molecules per POPC and DTAB molecule. Copyright 1999 Academic Press.

FT infrared and Raman investigation of saccharide-phosphatidylcholine interactions using novel structure probes

Conformational consequences of adduct formation between saccharides (trehalose, glucose, raffinose) and sorbitol with dipalmitoylphosphatidylcholine (DPPC) in multibilayers are revealed by relative intensity changes of the band components corresponding to the nu asN(CH3)3 and nu sC-N(CH3)3 stretching modes of the choline chain terminal and those of the nu C = O band. The conformational sensitivity of those modes was demonstrated previously (J. Grdadolnik et al., Chem. Phys. Lipids 65 (1993) 121) and used to demonstrate the effects of stepwise hydration of phosphatidylcholines. The latter are compared with the effects of saccharide binding and found to be qualitatively similar, but not identical. The same is true of the low frequency shifts of the nu asPO2- vibration: the shifts due to saccharide binding correspond to the binding of six to seven water molecules per phosphate which is about 20 cm-1 less than the shift caused by full hydration. A particularly interesting finding concerns the appearance of two bands in the nu asPO2- region of the DPPC-saccharide adducts. The relative intensities of the two bands (1243 and 1223 cm-1) change on additional hydration; it is the one at 1223 cm-1 that prevails at high hydration levels. Major changes in saccharide conformation are not detectable but minor differences between the DPPC bound and crystal spectra are observed.

Fourier transform infrared spectroscopy as a probe for the study of the hydration of lipid self-assemblies. I. Methodology and general phenomena

An algorithm for the study of the gradual hydration of phospholipid assemblies by means of Fourier transform infrared (FTIR) spectroscopy is presented. A complete series of diacyl phosphatidylcholines (PCs) including all possible analogues with palmitoyl and oleoyl residues, namely DPPC, DOPC, POPC, and OPPC, was investigated at room temperature. The lipid samples were prepared as cast films probably consisting of aligned multilamellar bilayers. The range of water activities studied in these films was regulated by adsorption via the gas phase corresponding to relative humidities of between 0 and 100%. Analyses of the IR-spectroscopic data have concentrated mainly on determining the amounts of water incorporated by each lipid as well as the hydration-induced response observed for some absorption bands of the different lipids. The water uptake at high relative humidity (RH) increases with the portion of unsaturated acyl chains in the molecular structure of the PCs. Isothermal phase transitions triggered lyotropically have been detected in demonstrating the occurrence of the main transition in POPC and OPPC films at room temperature. Moreover, it appears that both lamellar phases, the gel as well as the liquid-crystalline phase, are not uniform. They seem to comprise an amazingly large span of order/disorder states of the lipid chains generally depending on the degree of hydration. As exemplified by the significant variation in the onset of wavenumber shifts for the PO2- and C=O stretching-vibration modes, obtained as a function of hydration, a sequence of attachment to polar lipid binding sites by water molecules was established for DPPC.