Lipopolymer-phospholipid monolayer

Keywords Diffusion, Langmuir monolayer, Lipopolymer, Phospholipid, Viscoelasticity... 

Although the first section of this chapter was concerned with structural and dynamic information on monolayers of lipopolymers, before investigating lipopolymer-phospholipid mixtures, it is reasonable to consider the structural information that exists concerning pure phospholipid monolayers at the air-water interface. Film balance experiments, X-ray and neutron reflectometry, and molecular dynamics simulations have provided insight into the structural properties of these biologically... 

Another model for understanding the diffusion of lipopolymers at the air-water interface in Region II is the free area model, useful for describing the motion of phospholipids on a Langmuir monolayer and many systems where the diffusing particles can be approximated by hard spheres, disks or cylinders [38], In this model, a particle can diffuse in any direction that is free, or in other words, in any direction that is empty of another particle. As would be expected, more crowded or concentrated systems diffuse more slowly. Assuming the particles are at a constant temperature and that other energetic considerations can be described within a constant, D0, this type of diffusion can be expressed as... 

Fig. 19 a, b Viscoelastic response of the DSPE-PEG2000 monolayer as a function of amount of DMPC incorporated. Loss modulus a and storage modulus b are shown vs Aiipo, and are essentially independent of amount of phospholipids incorporated for mol% lipopolymer >40%. No viscoelastic transition occurs for mol% lipopolymer <40% [42] (reproduced with permission from the American Chemical Society)... 

The diameter, d, is calculated from the area per lipopolymer using the familiar relation Area = nr2. Substituting 650 A2 for Area gives d = 28.8 A, and from (5), L = 40.9 A. Free polymers in solution would have a value of L — R . According to (6), this solution of 30% lipopolymers where A = 65 A2 yields L/R = 1.36/1, or slightly stretched. Thus, it appears that around the density when the polymers start to interact with each other and become stretched, they cause the diffusion of lipids on the monolayer to slow down proportionally [51]. In other words, the lipopolymers start to act like obstacles instead of fellow-phospholipids. The second transition, from the second to the third diffusion regime, appears at around... 

Baekmark TR, Elender G, Lasic DD, Sackmann E (1995) Conformational transitions of mixed monolayers of phospholipids and polyethylene oxide lipopolymers and interaction forces with solid surfaces. Langmuir 11 3975-3987 (correction) (1996) Langmuir 12 4980-4980... 

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