Monolayers phase transition region

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

The results for the calculation of the orientation distribution for the hydrocarbon chains in the transferred monolayer films are presented in Figure 8. As is the case with the orientation distribution of the in-situ monolayers, the transferred films have a similar tilt angle in the expanded and phase transition regions. For the transferred monolayer, however, the tilt angle is in the range 35-40° from the surface normal, a much more oriented monolayer than the calculations indicate for the in-situ film (Figure 5). Figure 8 also shows that the condensed phase transferred monolayers are more oriented than those films transferred in the LE and LE-LC... 

The lipid monolayer is compressed into the phase transition region. This leads to the fonnation of solid-analogous lipid domains in a liquid-analogous matrix (Fig. 4 A). Injection of the labeled phospholipase A2 into the subphase (Fig. 4 B) is followed by specific recognition between the... 

Hydrolysis of a Mixed Monolayer ofL-a-DPPCIPyrenedecanoic Acid. Visualization of the hydrolysis of a mbced film of L-a-DPPC with pyrenedecanoic acid (ratio 5 1) by phospholipase A2 [20] offers tire possibility to directly see what happens with the acid during hydrolysis reaction. It could be shown that hydrolysis of such a mixed monolayer in the phase transition region leads to accumitiation of the acid above the enzyme domains. Whether the monolayer above the protein domains consists only of the acid, or of a mixture of acid and DPPC, can still not be distinguished with these measurements alone. 

Grainger, D. W., Reichert, A., Ringsdorf, H. and Salesse, C. (1990) Hydrolytic action of phospholipase A2 in monolayers in the phase transition region direct observation of enzyme domain formation using fluorescence microscopy, Biochim. Biophys. Acta 1023,365-379. 

The free energy of a monolayer domain in the coexistence region of a phase transition can be described as a balance between the dipolar electrostatic energy and the line tension between the two phases. Following the development of McConnell [168], a monolayer having n circular noninteracting domains of radius R has a free energy... 

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

Ex situ measurements in the presence of dissolved oxygen have proved that the mixed monolayer was stable in the solution free of 6TG and guanine. Madueno etal. [Ill] have also studied adsorption and phase formation of 6TG on mercury electrode. At high potentials, the molecules were chemisorbed and were able to form a self-assembled monolayer. When the potential was scanned to more negative values, reductive desorption of the monolayer was observed. Cathodic voltam-metric peaks, which are typical of a 2D condensed phase transition, divided the potential window into two regions one, in which self-assembled monolayer was stable, and the second, in which a physisorbed state existed. 

Upon further compression of the monolayer, a pronounced break, or discontinuity in the Isotherm marks a region of co-existence of the LE and the LC states. In most research papers it is stated that x(A) still increases upon compression beyond the beginning of the LE-LC transition region. Breaks may be indicative of higher-order phase transitions. The order of such a transition depends on the extent of co-operatlvity between the aliphatic chains. However, the presence of such transitions is not always well established. Pallas and Pethica found that... 

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