Monolayers force-area curve

Fig. 13 Force/area curves of dipalmitoylphosphatidyl choline monolayers spread on pure water at 25°C (solid line) and 45°C (dashed line). The compression rate is 7.2 A2/molecule per minute. The shape of the isotherms is identical for homochiral and heterochiral films.

Fig. 15 Force/area curves for monolayers of dipalmitoylphosphatidyl choline (solid line), dimyristoylphosphatidyl choline (dashed line), and dilauroylphosphatidyl choline (dotted line) at 25°C.

The first irrefutable observation of a discriminating interaction between enantiomeric surfactants in monolayers was most probably made by Filippus Johannes Zeelen (84) in a detailed study concerning the synthesis, monolayer behavior, and photochemistry of a series of A -stearoylamino acid derivatives that were employed to model the conformation and photochemical decomposition of proteins. Although Zeelen was able to demonstrate significant differences in the force-area curves obtained from racemic and optically active forms of several of these derivatives, publication of this work in 1956 was... 

Zeelen found the extent of chiral discrimination to be dependent on the type of monomolecular phase that was formed. Thus, racemic and optically active samples displayed identical force-area curves (Fig. 14) when both formed liquid-expanded films, but owed considerably different curves (Fig. 15) under conditions where both samples formed a more highly condensed monolayer. 

The monolayer behavior of A-stearoyltyrosine (Fig. 16) was more complex. Under conditions (0.0liV HCl, 22 C) where the racemic material formed a condensed film having a limiting molecular area of 39 2 A, the force-area curve of L-(+)-A-stearoyltyrosine exhibited a liquid-expanded film at large areas (ca. 100-45 per molecule) followed by a transition beginning at 16.5 dynes/cm surface pressure to a condensed phase having a smaller limiting molecular area of 34 2 A . However, both these latter samples exhibited only the liquid-expanded phase on distilled water alone. 

In her initial investigation, Lundquist studied the monolayer behavior of racemic and optically active forms of both tetracosan-2-ol and its acetate derivative on 0.0 lA aqueous HCl over a considerable range of temperature (77). In each case, it was possible to demonstrate chiral discrimination between pure enantiomers versus the racemic substance. Furthermore, the extent of enantiomer discrimination was significantly temperature dependent, being enhanced at lower temperatures and frequently disappearing at higher ones. Under favorable conditions of temperature, however, the appearance of the force-area curves could be very sensitive to the optical purity... 

The force-area curves for racemic and (5)-(+)-2-tetracosanyl acetate recorded with a barrier speed of 5 cm/min are shown in Figures 17 and 18, respectively. Again, both enantiomers showed identical monolayer behavior. The film balance behavior of the racemic acetate was indistinguishable from that of the pure enantiomers at temperatures above about 27°C however, below this temperature the force-area curves differed markedly even at low surface pressures, which indicates that racemic compound formation occurs at relatively large areas per molecule. 

Encouraged by the results from static surface tension measurements, we next ran force-area curves on 3N, 6N, and ION acids. As Figure 33 indicates, there is pronounced discrimination between the racemic and enantiomeric monolayers, and those differences are sharply dependent on the subphase acidity, as had been impUed by the surface tension behavior. 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

The states produced by compression of monolayers are not necessarily at thermodynamic equilibrium, but may be metastable. In some cases, this is manifested clearly by different force-area curves being produced at different rates of compression. Slow reorganization of monolayer molecules is also apparent as hysteresis when films are repeatedly compressed and expanded. In chiral monolayers, the rates of molecular reorganization may be stereospecific as well as the thermodynamic behavior. 

There are several ways of classifying the force/area curves of different organic surfactants. A convenient one for the present purposes is shown in Fig. 1, where four types of idealised monolayer behaviour are depicted. 

Airington and Patterson [39] spread the fluoroalcohol H(CF2)iqCH20H on water and determined the area occupied per molecule from the force-area curve of the spread monolayers (Fig. 4.5). By extrapolating the upper part of the curve to zero pressure, a close-packed area of 29 A was obtained. The fluorocarboxylic acid H(CF2)i2COOH was spread on water and on 0.01 hydrochloric acid (Fig. 4.6). The force-area curve obtained for hydrochloric acid agrees with the curve shown for the fluoroalcohol in Fig. 4.5. When the fluorocarboxylic acid film on hydrochloric acid was recompressed, the curve was duplicated. The compression of the fluorocarboxylic acid film on water gave a smaller area per molecule than obtained on acid. A second compression gave even a smaller area (19 A"). 

The work described here strengthens the idea that when the a-helix is present in a polypeptide monolayer, the transition in the pressure—area curve arises from the collapse of the monolayer under the action of surface forces and that at least in some cases this is in the nature of a phase transition and proceeds in an orderly manner. The transition is in fact rather analogous to the development of the tertiary structure in a protein under the action of hydrophobic forces. We can therefore now... 

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. 

Multipl3fing the length I of the film by its width b = 14.0 cm we obtain its area. Dividing this by the number of moles we obtain NA and so A. Dividing the force on the float by its effective width 13.8 cm we obtain (f. Values of these quantities are given in table 2. We plot q> against A in fig. 1. Extrapolating the steep part of the curve to = 0 we find for the close-packed monolayer A = 20.9 A . 

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