Collapsed monolayers, structure

While collapsed films of this polymer can be lifted off the surface on electron microscope grids, viewed under the light microscope they are seen to break under the action of surface forces within a few minutes. Electron diffraction observations are evidently not feasible, but good polarized IR spectra are obtainable (Figure 2). The parallel dichroism of the Amide A band (3300 cm ) and the Amide I band (1660 cm ), and the perpendicular dichroism of the Amide II band (1555 cm ) is strong evidence that the collapsed monolayer is in the a-helical conformation with the molecules aligned on the water surface more or less parallel to the barrier. There is not sufficient dichroism in the bands associated with the n-decyl side chain for it to be orientated predominantly either parallel or perpendicular to the backbone. Since the side chains are very flexible it is probable that during collapse of the monolayer the side chains fold to form a more compact non-dichroic structure. 

Structure of the Collapsed Monolayers. IR spectra of specimens prepared from air dried collapsed monolayers were typical of specimens in the a-helical conformation with no indication of any p conformation. Electron diffraction patterns gave a similar result. The patterns for poly-(L-leucine) and poly(L-norleucine) are similar to poly(L-norvaline) (12) with low crystallinity. A strong equatorial reflection at 10.94 0.10 A is observed in poly(L-leucine). If we assume as previously (5) that this is the 100 reflection from a hexagonal cell, the calculated area per residue in the monolayer is 17.3 A, assuming the molecular separation is the same as in the collapsed film. This figure is in agreement with the observed area of 16 A in view of the difficulties encountered in spreading the monolayer. 

The structure of collapsed lipid monolayers has not been investigated at the molecular scale. AFM studies of collapsed monolayers on HOPG clearly showed steps of heights that correspond to the trilayer of Mg-(stearate)2... 

Other interesting Langmuir monolayer systems include spread thermotropic liquid crystals where a foam structure forms on expansion from a collapsed state [23]. Spread monolayers of clay dispersions form a layer of overlapping clay platelets that can be subsequently deposited onto solid substrates [24]. 

Initially, the compression does not result in surface pressure variations. Molecnles at the air/water interface are rather far from each other and do not interact. This state is referred to as a two-dimensional gas. Farther compression results in an increase in snrface pressure. Molecules begin to interact. This state of the monolayer is referred as two-dimensional liquid. For some compounds it is also possible to distingnish liqnid-expanded and liquid-condensed phases. Continnation of the compression resnlts in the appearance of a two-dimensional solid-state phase, characterized by a sharp increase in snrface pressure, even with small decreases in area per molecule. Dense packing of molecnles in the mono-layer is reached. Further compression results in the collapse of the monolayer. Two-dimensional structure does not exist anymore, and the mnltilayers form themselves in a non-con trollable way. 

Compression of the PS II membrane monolayer shows that the monolayer collapses at a relatively low surface pressure, at around 20mN/m. This can be attributed to the formation of a multilayered structure [8] and some of PS II membrane fragments diffuse into the subphase. This observation further indicates that PS II membranes can only marginally stay at the air-water interface and one must be very careful in choosing the experimental parameters. 

We studied the surface pressure area isotherms of PS II core complex at different concentrations of NaCl in the subphase (Fig. 2). Addition of NaCl solution greatly enhanced the stability of monolayer of PS II core complex particles at the air-water interface. The n-A curves at subphases of 100 mM and 200 mM NaCl clearly demonstrated that PS II core complexes can be compressed to a relatively high surface pressure (40mN/m), before the monolayer collapses under our experimental conditions. Moreover, the average particle size calculated from tt-A curves using the total amount of protein complex is about 320 nm. This observation agrees well with the particle size directly observed using atomic force microscopy [8], and indicates that nearly all the protein complexes stay at the water surface and form a well-structured monolayer. 

In contrast to the uniform dendrimer, a polydisperse hyperbranched polymer with OH-terminal groups showed only one transition corresponding to the collapse from the monolayer of flatly spread molecules to a disordered liquid. Dendrimer 1 is similar to dendrimer 2 in Fig. 8, except the difference in branch ends (CH3 vs OH), did not spread and exhibited no structural transitions upon compression. 

The measurements of n versus A isotherms generally exhibit, when compressed, a sharp break in the isotherms that has been connected to the collapse of the mono-layer under given experimental conditions. The monolayer of some lipids, such as cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of FI increases very little as compression takes place. In fact, the collapse state or point is the most useful molecular information from such studies. It has been found that this is the only method that can provide information about the structure and orientation of amphiphile molecules at the surface of water (Birdi, 1989). 

This value of Aco corresponds to the cholesterol molecule oriented with the hydroxyl group pointing toward the water phase. Atomic force microscope (AFM) studies of cholesterol in Langmuir-Blodgett (LB) films has shown that there exist domain structures (see Chapter X). This has been found for different collapse lipid monolayers (Birdi, 2003). Different data have provided much information about the orientation of lipid on water (Table 4.1). 

Grazing incidence x-ray diffraction (GID) measurements have indicated that both precollapse and collapsed state monolayers at the air-water interface can be crystalline (Birdi, 1989). A general procedure was delineated that could provide near-atomic resolution of two-dimensional crystal structures of -triacontanoic acid (C29H59COOH). A monolayer composed of rod-like molecules would generally pack in such a way that each molecule has six nearest neighbors, that is, hexagonal cell. 

This arrangement projects hydrophobic and hydrophilic surfaces above and below the plane of the dendron in a display that mimics the two-dimensional structure of lipid clusters. Accordingly, the amphiphile can efficiently spread out along the surface like a spider web. The II-A isotherms revealed that these molecules form stable monolayers with collapse pressures in the range of 40-60 mN/m. The addition... 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

Liger-Belair, G., Robillard, B., Vignes-Adler, M., and Jeandet, P. (2001b). Flower-shaped structures around bubbles collapsing in a bubble monolayer. C. R. Phys. 2, 775-780. 

It is neither feasible nor appropriate in a book like this to give a detailed presentation of biological membranes, which compartmentalize living matter and perform numerous cell functions as well. However, because of the impetus to the study of surfactants that the membrane-mimetic properties of surfactant structures have provided, it would be a mistake to exclude some mention of membranes in this chapter. We have already noted in connection with Figure 7.7 that a monolayer may collapse into a bilayer that leaves the surfactant in a tail-to-tail configuration. This is exactly the arrangement of molecules in the lipid portion of a cell... 

For the mixture of diamine (43) with diester (44) miscibility is necessary for a successful polycondensation. Miscibility was demonstrated by showing that the mixed monolayer obeyed Crisp s rule53 (see 4.2.1). For a 1 1 molar mixture of diamine (43) and diester (44) only a slight area change during monolayer polycondensation could be observed. The compression isotherm of the polymer film exhibits a diminished collapse area and pressure (Fig. 14). The structure of the final polymer is shown in Scheme 3. 

The structure and molecular orientation of the monolayers have been studied taking in account the different conformations of the repeating unit in the collapse area. Molecular Dynamic Simulation (MDS) has been also reported. The MDS un-... 

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