Stretch, phospholipid monolayers

Monolayer Films at the A/W Interface. Previous studies of phospholipid monolayers at gas-liquid interfaces have shown that it is possible to follow the first order thermodynamic phase transition of these monolayer films using the infrared reflectance techniques described in this manuscript (see e.g. ref. 6 and references cited therein). For long chain hydrocarbon molecules, it has been demonstrated that the frequencies of the antisymmetric and symmetric CH2 stretching vibrations are conformation-sensitive, and may be empirically correlated with the order (i.e. the trans-gauche character) of the hydrocarbon chains (9-11). 

FIGURE 2.10. Representative spectra of dipalmitoyl phospholipid monolayers at the CCU/D2O interface under the ssp polarization scheme showing the methyl and methylene symmetric stretch (a) DPPC, (b) DPPE, (c) DPPG, (d) DPPS. Spectra of the monolayers are shown in solid squares. Spectra of the monolayers with halothane are shown with solid triangles. The lines ate fits to the data. From Ref. [62]. 

Vibrational sum frequency spectroscopy in conjunction with interfacial pressure measurements provide the first direct information about the structure of phospholipid monolayers composed of DLPC, DMPC, DPPC, and DSPC, adsorbed to the interface between two immiscible liquids. Temperature controlled experiments carried out with aqueous solutions of DSPC show the lipid bilayer gel to liquid crystalline phase transition temperature to play a pivotal role in determining interfacial monolayer concentration and alkyl chain structure. Even at equivalent interfacial concentrations longer chain phosphocho-line species form more disordered monolayers with a greater number of gauche defects than shorter chain phosphocholine species, as determined from relative intensities of vibrational bands in the CH stretching region. 

The applications of PM-IRRAS also include fatty acids, phospholipids, and protein conformations. Desbat and co-workers reported on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air-water interface as a function of the subphase pH and for several cations (Cd2+, Ca2 +, Mg2 +, and Na+) with the help of the PM-IRRAS method [92]. Fig. 14 shows the PM-IRRAS spectra of Langmuir monolayer of deuterated arachidic acid in the presence of CdCb as a function of the subphase pH. At low subphase pH (pH = 3.5), the spectrum only presents absorption bands related to the acid form, i.e., the C = O stretching vibration (v(C = O)) and the OH bending (<5(0-H)) located at 1720 and 1270 cm respectively. The frequency position of the v(C = O) is characteristic of a hydrogen-bonded carbonyl group. As the subphase pH is increased, the arachidic acid is progressively deprotonated to... 

Figure 2.10 shows the methylene and methyl symmetric stretch region of the spectrum for each headgroup before (solid squares) and after (solid triangles) the exposure of the monolayer to halothane. For each spectrum shown, there is a small increase in the overall intensity when halothane is present. This increase occurs across the entire spectrum for each of the phospholipids smdied. We believe that the small change in intensity... 

Information on the conformational state of the hydrocarbon chains and their orientation has been obtained from external infrared reflection absorption spectroscopy (IRRAS). The first systematic IRRAS studies on phospholipid Langmuir monolayers were reported by Dluhy et al ) (see, for instance fig. 3.62). For DPPC monolayers in the LE phase the positions of the conformation-sensitive symmetric and anti-symmetric C-H stretching bands in the IRRAS spectra were found to be at the same positions as for bilayer systems of DPPC above the Kralft temperature. In the LC phase the frequencies of these bands indicate that the hydrocarbon chains of the lipid molecules are in the all-trans ) conformation (i.e. zig-zag) and analysis of polarized IRRAS spectra show that their average tilt is ca 35° relative to the monolayer normal. This is in reasonable agreement with the tilt angle of 30° obtciined from X-ray diffraction on DPPC monolayers (30°). 

Vibrational modes of the phospholipid polar head groups (in particular the symmetric and anti-symmetric PO stretching vibration) reflect their ionization and hydration state. The hydration state of the head group of DPPC was found to change under monolayer compression or by addition of cations such as Ca + There are indications that the transition at Ttg (to the solid state S, see fig. 3.6) involves ordering and dehydration of the head groups. 

Also using IRRAS, Mendelsohn and co-workers have studied monolayers of phospholipids with deuterated acryl chains. In such systems C-H and C-D stretching vibrations can be monitored simultaneously. This permits, for example, observation of individual components in a mixed lipid monolayer or conformational analysis of different parts of the acryl chains. Measurements on mono-layers consisting of tail-end deuterated DPPC molecules showed that the chains posses more conformational order adjacent to the head group than at their tails. ... 

The diameter, d, is calculated from the area per lipopolymer using the familiar relation Area = nr2. Substituting 650 A2 for Area gives d = 28.8 A, and from (5), L = 40.9 A. Free polymers in solution would have a value of L — R . According to (6), this solution of 30% lipopolymers where A = 65 A2 yields L/R = 1.36/1, or slightly stretched. Thus, it appears that around the density when the polymers start to interact with each other and become stretched, they cause the diffusion of lipids on the monolayer to slow down proportionally [51]. In other words, the lipopolymers start to act like obstacles instead of fellow-phospholipids. The second transition, from the second to the third diffusion regime, appears at around... 

Vesicles form spontaneously from extended sheets when phospholipids or surfactants are dissolved in water, and heated above the gel-to-liquid crystalline transition temperature. Bergstrom and Eriksson (5) suggested that the free energy required to form a spherical bilayer from sheets is made up of two contributions, i.e. an energy associated with bending the monolayers and the work required to stretch the bilayer, with the latter being determined by the planar bilayer tension. A major... 

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