Overshoot-hump

Contrary to the accumulated knowledge on the static or quasi-static characteristics of thin lipid films at air/water interface, less attention has been paid to the dynamical or nonequilibrium behavior of the film. Studies on the dynamical characteristics of thin lipid films may be quite important, because the life phenomena are maintained under nonequilibrium conditions. According to the modern biochemistry [11,12], thin lipid membrane in living cells is not a rigid wall but a thermally fluctuating barrier with high fluidity. In the present section, we will show that thin lipid film exhibits the various interesting dynamical tc-A characteristics, such as the "overshoot hump", the "zero surface pressure", and the "flat plateau". 

Such a large overshoot hump, or zero surface pressure, was observed only for the subphase containing both CdCl2 and KHCO3. This result suggests that the stabilization effect of cadmium ion is enhanced in the presence of KHCO3 due to its buffering capacity of the subphase. 

As has been mentioned above, the 7t-A curves of the PhDA2-8 thin film show the existence of the zero pressure region after the formation of the overshoot hump. It is noteworthy that the remarkable overshoot hump and the subsequent zero surface pressure are observable only in a particular range of compression rates, suggesting that the inclusion of the effect of non-... 

It is evident that the above simulation results can reproduce essentially all characteristic features such as "flat plateau", "zero surface pressure , and "overshoot hump observed in the actual it-A curves. These properties are characteristic examples of nonlinearity in the nonequilibrium state of a thin... 

An overshoot-hump in a ir-A curve and the cooperative aggregation among dinitrophenyl moieties in phospholipids at an air/water interface... 

Abstract We observed the surface pressure (n) - surface area (A) curves of the DNP-DPPE monolayers at an air/water interface. An overshoot-hump and an inflection point were markedly shown in the n-A curve with a successive compression. To elucidate the mechanism of the characteristic changes in the curve, we measured the absorption spectra of the thin film which was deposited on a quartz plate using a Langmuir-Blodgett (LB) technique. It was found that the maximum peak in the spectrum shifted to longer wavelengths at 3 nm in the subsequent overshoot-hump, and that the peak intensity increased threefold after the inflection point. Therefore, it was considered that the overshoot-hump indicated a change of the... 

Key words Overshoot-hump -dinitrophenyl phospholipid -Langmuir-Blodgett technique -cooperative aggregation... 

We have studied the viscoelastic behavior of lipid thin films at an air/water interface from surface pressure (tt) vs. surface area (A) curves, since an observation of the k-A curve of a monolayer is one of the most convenient methods to elucidate the viscoelastic behavior of the monolayer. Recently, in spite of compression process, we observed an overshoot-hump, a zero surface pressure, and a flat plateau in the rt-A curve of a synthetic fatty acid [3]. These characteristic features in the curve were explained by using a kinetic model representing the formation of aggregates of the fatty-adds molecules at an air/water interface. 

As shown in Fig. 2, an interesting characteristic, an overshoot-hump, was observed in the n-A curves of the DNP-DPPE monolayer with a constant-speed compression. It is clear that the shape of the overshoot-hump became more marked with an increase in the subphase temperature, and that the surface pressure, at which the overshoot-hump was observed, increased from 12.7 to 31.4 mN/m. Furthermore, it was found that both the... 

To elucidate the origin of the characteristic features in the tz-A curves such as the overshoot-hump and the inflection point, we observed the absorption spectra of DNP-DPPE thin film by using the LB technique [5,6]. The DNP-DPPE monolayer was deposited onto a quartz plate at different surface pressures (see (a)-(c) in Fig. 3A). It was found that the maximum peak in the absorption spectrum shifted to long wavelengths at 3 nm after the overshoot-hump had been observed. However, the intensity of the peak little increased. On the other hand, we observed that the intensity became threefold after the inflection point. 

From these experimental findings, it may be considered that the overshoot-hump in the tc-A curve indicates the formation of a condensed layer such as aggregates of the DNP-DPPE molecules, and that the inflection point represents a collapse from a monolayer to some multilayers at the air/water interface. As an overshoot-hump has not been found in the tc-A curves of dipalmitoylphos-phatidylethanolamine (DPPE), the dinitrophenyl moiety in the phospholipids may play an important role in the mechanism of the aggregation between the DNP-DPPE molecules during the compression process. For example, a stacking of the moieties may be indueed at ca. 20 mN/m. 

This step corresponds with the aggregation of the DNP-DPPE molecules, and the interaction between the dinitrophenyl moieties in the molecules at the air/water interface. We consider that the aggregation process at 20 °C changes to a much faster process than the dissociation one at ca. 20 mN/m, since a temporal decrease in the surface pressure (an overshoot-hump) was observed at the pressure. In other words, the aggregation process sensitively depends on the surface pressure around 20 mN/m. The rapid increase in the value of ki indicates a cooperative behavior on the aggregation process and induces a temporal decrease of the surface pressure inspite of compression. On the basis of this kinetic model, the time dependence of [S] and [D] is given by Eqs. (1) and (2), respectively. 

The overshoot-hump in the n-A curve of DNP DPPE monolayer was foimd with constant-speed compression. From the absorption spectra of the lipid thin film, the hump reflected the formation of condensed layers at the critical surface pressure. A computer simulation was carried out on the basis of a cooperative aggregation process between the DNP-DPPE molecules at the air/water interface, the characteristic feature in the n-A curve was able to be reproduced. 

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