Collapsed monolayer

Figure 3. The nucleation process of the collapsed monolayer of stearic acid on acidic subphase at 15 °C during over-compression process from (a) 19.5 to (c) 18.5 A2/molecule.

As an extension of their previous work, the Tachibana group (82, 83) studied the collapse fragments that occur when monolayers of 12-hydroxystearic acid are compressed slowly (18 A /molecule hr) at surface areas of less than 21 A /molecule, the normal cross-sectional area of a hydrocarbon chain. The collapsed monolayers were transferred from the subphase to hydrophilic supports by a horizontal lifting method for electron microscopic observation, which revealed (Fig. 30) flat platelets when the sample was racemic and twisted... 

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... 

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. 

Figure 2. Polarized IR absorption spectrum of a collapsed monolayer of poly(y-n-decyl-i.-glutamate) air dried on a silver chloride plate.

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 diffraction pattern of poly(L-methionine), Figure 4a, is much more crystalline than that of the other two polymers and in several respects resembles that of collapsed monolayers of poly(y-methyl-L-glutamate). The correspondence is shown clearly by the reciprocal lattice diagrams (Figure 5). For an extended account of the basic theory see Stokes (23) and Elliott (24). 

Figure 4. Electron diffraction patterns from collapsed monolayers, (a) Poly(i.-methionine). The outermost reflection at 1.49 A is not visible on this photograph the sharp meridional reflection is 006. (b) 1 1 poly(i.-alanine) + poly(i>-...

Figure 5. Reciprocal lattice rotation diagrams are for (left) poly(i.-methionine) arid (right) poly(y-methyl-iu-glutamate) from collapsed monolayers using elec-tron diffraction. The reciprocals of the spacings d, (A) correspond to their positions on the diffraction photographs [compare the diagram for poly(i.-methionine) with the top right hand quadrant of Figure 6a].

Electron diffraction patterns have been obtained from collapsed monolayers of poly (alanine) and poly (y-benzyl glutamate). The enantiomorphic forms of the other two polymers give patterns with very poor crystallinity, and their racemic mixtures have not therefore yet been investigated. The principal features of the diffraction pattern of poly-... 

The diflFraction pattern of poly(y-benzyl glutamate) is very diflFuse, the only sharp reflection being at 5.26 A on or close to the meridian. A reflection at about 1.50 A on the meridian is observed, but in other respects the diflFraction pattern shows none of the detail found in the x-ray diflFraction patterns of poly(y-benzyl glutamate) fibers (30) (where it should be noted the specimens had been heated to promote crystallinity). Polarized IR spectra were obtained from collapsed monolayers which showed them to be in the a-helical conformation in all cases. Poly ( -benzyl aspartate) proved particularly interesting in that the spectrum of the collapsed monolayer was similar to that of poly( -benzyl-L-... 

Support for this general picture comes from the electron diflFraction observations on the collapsed monolayer. If the sharp reflection at 5.36 A is a true meridional reflection (better orientation is necessary to be certain), it can be indexed as 005 and the 1.489 A meridional reflection as 00,18 for a hexagonal cell with c = 26.8 A. An 005 reflection would not be produced by a perfect helix, but it can probably be accounted for assuming a distorted a-helix, the distortions being caused by the unsatisfactory packing of the two forms. The reduced axial increment per residue, 1.489 A compared with 1.495 A in the enantiomorphic form, shows that the packing causes the helix to shorten slightly. 

Figure 3.47. Rheology of collapsed monolayers of long-chain fatty alcohols. Comparison between measured (—) moduli (fig. a) and loss tangents (fig. b) and those computed by a...

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... 

Cholestane (as formed after oxidation of cholesterol molecule) collapsed films ° show two step heights. These steps correspond to trilayers (length of cholestane x 3 = 50 A) and six layers (100 A). This indicates that in the collapsed state the trilayer is able to form higher order two-dimensional crystals, i.e., six layers. These analyses show that in the collapsed monolayers of different lipids the following phase equilibria (with equilibrium constant, K) exist ... 

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