Monolayer collapse

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. 

The monolayer stability limit is defined as the maximum pressure attainable in a film spread from solution before the monolayer collapses (Gaines, 1966). This limit may in some cases correspond directly to the ESP, suggesting that the mechanism of film collapse is a return to the bulk crystalline state, or may be at surface pressures higher than the ESP if the film is metastable with respect to the bulk phase. In either case, the monolayer stability limit must be known before such properties as work of compression, isothermal compressibility, or monolayer viscosity can be determined. 

Non-Newtonian flow may result if the monolayer array consists of molecules that interact by specific Coulombic or dipole interactions to form floating islands , which in turn may interact by van der Waals forces around their peripheries (Joly, 1956). Non-Newtonian flow may also be a property of collapsed films. The resulting differences in viscosity over a range of flow rates may then reflect film-component segregation or partial monolayer collapse. 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

As described here, the monolayer of a lipid can be formed by different spreading methods. The thermodynamics of the Ilcol analysis is given in the literature (Birdi, 1989). The monolayer collapse has been shown to provide much information also in the case of protein monolayers. 

At very high values of n the monolayer collapses (buckles). Both the cross-sectional area per molecule in the monolayer and the collapse pressure can be determined. For typical fatty acids, regardless of chain length, the area covered is only 0.2 nm2 per molecule indicating that the fatty acid chains are stacked vertically to the surface in the monolayer. The collapse... 

A simple theory has been proposed to account for this behavior (15,16). It is supposed that at the plateau, the monolayer collapses under the action of the interfacial forces and the surface pressure (W), so that... 

Studies of the Fully Collapsed Monolayer. Collapsed monolayers of all polymers were examined by IR spectroscopy and in most cases by electron diffraction using a Phillips EM300 electron microscope set for selected area diffraction. Viewing of the specimens in the microscope... 

For these and other (practical and academic) reasons the behaviour ol cholesterol in monolayers and bilayers, both being effective model systems. Is oltcn studied. Here, we discuss some results regarding the fluidity of cholesterol mono-layers and the formation of cholesterol exudates upon monolayer collapse. 

In contrast with saturated-LMWE, imsaturated-LMWE monolayers present only the liquid expanded structure and the collapse. BAM images corroborate that only the homogeneous LE phase is present during the compression of unsaturated-LMWE monolayers. From the observation with BAM along the film balance no fractures were visualized after the monolayer collapse. These monolayers collapse with the formation of lenses. 

At TT > TTg the relaxation phenomena for insoluble monolayers are caused by the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase system. However, some differences exist between saturated-LMWE and unsaturated-LMWE monolayers (Eigure 14.6b). Relaxation phenomena in saturated-LMWE monolayer are controlled predominantly by the collapse mechanism because the surface pressure relaxes to TTg. Eor these systems the monolayer collapses by nucleation and growth of critical nuclei. Unsaturated-LMWE monolayers behave differently to saturated-LMWE monolayers. As the surface pressure relaxes from the collapse value, which is close to TTg, towards values lower than TTg at longer times, the collapse competes with a desorption mechanism (Patino and Nino, 1999). 

AFM was used to investigate whether a series of related molecules could be analyzed when present as SAM. The system chosen was the cholesterol molecule. The molecule cholesterol is a very important biological lipid. A cholesterol molecule with one hydroxyl group is known to oxidize into a variety of structures. These oxidized cholesterol products play an important role in many biological diseases, such as blood clots. The AFM data of the collapsed film of cholesterol (when spread on the surface of water) shows that two-dimensional crystallization takes place with very characteristic butterfly shapes. This shows, for the first time in the literature, that not all lipid monolayers collapse to give a transition from monolayer to trilayer. This shows that the collapsed state may be a two-dimensional crystal phase. 

Fig. 9 The polymer-tethered lipid monolayer (a) can swell by the up-take of significant amounts of water (b) surface-plasmon optical evidence of the swelling behavior of a polymer-cushioned monolayer by the exposure to air of different humidity (% relative humidity as indicated). Upon exposure to dry air the tethered monolayer collapses to its water-free thickness (however, can swell again if exposed to humid air (not shown))...

Future studies in this area will require the radiolabelling of intrinsic membrane so that the amount of protein entering the monolayer can be accurately measured. More information should also be obtained on the dependence of the initial monolayer surface pressure on protein mediated fusion of vesicles with monolayers. The surface pressure at which the properties of a monolayer most closely mimic the properties of a bilayer is known to be relatively high. It is clear from our studies that protein extracts penetrate monolayers upto equilibrium surface pressures approaching the monolayer collapse pressure which suggests that data can be obtained from monolayer studies at surface pressures which are directly applicable to bilayers. 

When two film-forming components are immiscible, then according to the phase rule, their mixed monolayers collapse at the same surface pressure regardless of their composition e.g. the component of the mixed film that has a lower equilibrium spreading pressure relative to the other film-forming component is squeezed out from the monolayer as the surface pressure reaches a value corresponding to its own collapse pressure [16-18]. 

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