Acyl chains, monolayer films

Absorbance spectra monolayer on mica, 244/ thickness of mica sheet, 241 Absorptivity ratios, definition, 28 Acyl chains, monolayer films, 198 Adatom(s) definition, 210 surface migration, 211214... 

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

Another set of studies from this laboratory examined the assembly of symmetric and asymmetric phosphatidylcholines at the CCfr/water interface [50]. In these studies, a series of saturated symmetric and asymmetric chain PCs were examined. Symmetric PCs with 16 or fewer carbons per acyl chain and highly asymmetric PCs were found to produce relatively disordered films at the CCU/water interface as measured by VSFS. However, the longest chain PCs studied, 1,2-distearoyl-sn-PC (C18 C18), l-stearoyl-2-palmitoyl-sn-PC (C18 C16) and l-palmitoyl-2-stearoyl-sn-PC (C16 C18), formed well-ordered crystalline phase monolayers at room temperature. These results have been explained in terms of enhanced chain-chain interactions among the longer, nearly symmetric hydrocarbon chains that reduce the intercalation of solvent. 

IRRAS has become an important tool for studying Langmuir monolayers and LB films. Much work has been done, in particular on the acyl chain conformational order in monolayers of single chain amphiphiles and phospholipids as a function of surface pressure and on the occurrence of phase transitions . Examples of IRRAS studies on LB films can be found Reviews of the applications of IRRAS are given by Dluhy et al. and Mendelsohn et al. ). 

Lipkowski and coworkers [328] have employed STM to study the spreading of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcho-line vesicles into the film at the Au(lll) electrode surface. During the initial stage, phospholipid molecules were adsorbed flatly with the acyl chains oriented parallel to the surface and assembled into an ordered monolayer similar to that formed by alkanes. Later, the molecules reoriented and the monolayer was transformed into a hemimicellar film. 

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