Lipid-based molecular gels

On hindsight, while the use of Hpid-based organogels in tissue engineering is still rather limited, the promising experimental results presented by Lukyanova et al, coupled with the diverse and imperative functions of Hpids in cell development and differentiation, have certainly unveiled exciting possibilities for lipid-based molecular gels to be developed into potent tissue engineering scaffolds in the future. 

Figure 24.3 The simulation is based on the coordinates of H. Heller, M. Schaefer, and K. Schulten, Molecular Dynamics Simulation of a Bilayer of 200 Lipids inm the Gel and in the Liquid-Crystal Phases, Journal of Physical Chemistry, 97, 8343-8360 (1993) and taken from an interactive animated tutorial by E. Martz and A. Herraez, Lipid Bilayers and the Gramicidin Channel (http //molvis. sds.edu/bilayers/index.htm (2001) by courtesy of Professor Martz.

Both NPLC and RPLC have been used in lipid anal) is by LC. The first mode has the capability to separate different classes of lipids based on the polar head groups in a single injection, whereas the RPLC mode is mainly used for investigating the molecular diversity within a given lipid class based on both the molecular size and the degree of unsaturation of molecules. This RPLC mode is that most commonly used for the separation of triacylglycerols. For lipid-dass separations, chemically bonded phases (nitrile, diol, or polyvinylalcohol phases) produce better results than silica gel. 

This separation is based on the size of the porous, hydrophobic gels. The pore size must be greater than the pore size of the molecules to be separated. Gel-permeation cleanup (GPC) is used for cleaning sample extracts from synthetic macromolecules, polymers, proteins, lipids, steroids, viruses, natural resins, and other high molecular weight compounds. Methylene chloride is used as the solvent for separation. A 5 mL aliquot of the extract is loaded onto the GPC column. Elution is carried out using a suitable solvent, and the eluate is concentrated for analysis. 

As regards the lipids, two types of adsorbents are available, one of which is a form of silica gel and is utilized in normal-phase HPLC, and the other of which can be a silica gel bonded to a hydrophobic chain and is employed in reverse-phase HPLC. In normal-phase HPLC the phospholipids appear to be separated based on the molecular classes present (PE, PC, Sph, etc.), whereas in reverse-phase HPLC the separation is closely related to the lipophilic character of the acyl (fatty acyl, hydrocarbon chain) of the particular phospholipids. High-quality adsorbents suitable for HPLC are easily available from commercial companies. 

Figure 6.11 shows the activity of an artificial enzyme can be controlled based on the phase behavior of a lipid bilayer. The catalytic site for hydrolysis was supplied by a monoalkyl azobenzene compound with a histidine residue which was buried in the hydrophobic environment of a hpid bilayer matrix formed using a dialkyl ammonium salt. Azobenzene compound association depended on the state of the matrix bilayer. The azobenzene catalyst aggregated into clusters when the bilayer matrix was in a gel state. In contrast, the azobenzene derivative can be dispersed into the liquid crystalhne phase of the bilayer matrix above its phase transition temperature. This bilayer-type artificial enzyme catalyzed the hydrolysis of a Z-phenylalanine p-nitrophenyl ester. The activation energy for this reaction in the gel state is twice as large as that observed in the hquid crystalline state. The clustering of the catalysts upon phase separation suppress their catalytic activity, probably due to the disadvantageous electrostatic environment around the catalysts and the suppressed substrate diffusion. This activity control is unique to such molecular assembhes. 

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