Phospholipid monolayers condensed phase structures

Bayerl, T. M., Thomas, R. K., Penfold, J., Rennie, A. and Sackmann, E. (1990). Specular reflection of neutrons at phospholipid monolayers. Changes of mono-layer structure and headgroup hydration at the transition from expanded to the condensed phase. Biophys. J. 57 1095-1098. 

In addition to fatty acids and phospholipids, steroids form another elass of surfactants that are often subjected to monolayer studies. As an example a /r(aj)-isotherm for cholesterol is shown in fig. 3.13. Up to a molecular area of 0.50 nm the spread molecules hardly interact with each other. The limiting area in the condensed phase is ca. 0.40 nm per molecule, which is compatible with an orientation of the cholesterol molecules in the monolayer as indicated in the inset. It is historically interesting that establishing this cross-section has contributed to solving the structure of sterols. 

Using optieal microscopie teehniques (fluorescence microscopy, Brewster angle microseopy and microscopic ellipsometry) the formation, size and shape of domains in the LE-LC coexistence region of phospholipid monolayers have been studied extensively. Furthermore, structures within condensed domains and phases have been visualized, the contrast resulting from the optical anisotropy caused by long-range tilt orientational order. 

A consideration often overlooked in BAM studies is the possible influence of the compression rate on the domain structures. In the case of A-acylamino acid monolayers, the associations due to amide-amide hydrogen bonding are very strong and promote rapid domain growth and also make it unlikely that relaxation to an equilibrium domain shape can occur on any realistic experimental timeframe. Domain shape relaxation kinetics are noted to be dependent on the strength of intermolecular forces for example, dendritic condensed-phase domains formed in a phospholipid monolayer required 5 h to relax to equilibrium shapes and, for the phospholipid DMPE, compression rates as slow as 0.2 per molecule per minute were needed to observe equilibrium domain shapes. Examination of the variation of domain structure with time after then-formation or with compression rates are not commonly reported however, it is advisable to consider examining these variables when carrying out BAM experiments. 

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