Electrostatic bonds, phospholipids

Bauerle and Seelig [395] studied the structural aspects of amlodipine (weak base, primary amine pKa 9.26 [162]) and nimodipine (nonionizable) binding to phospholipid bilayers, using NMR, microcalorimetry, and zeta-potential measurements. They were able to see evidence of interactions of amlodipine with the cis double bond in the acyl chains. They saw no clear evidence for (=P—O- 1 H N—) electrostatic interactions. 

Hydrogen bonding and electrostatic interactions between the sample molecules and the phospholipid bilayer membranes are thought to play a key role in the transport of such solute molecules. When dilute 2% phospholipid in alkane is used in the artificial membrane [25,556], the effect of hydrogen bonding and electrostatic effects may be underestimated. We thus explored the effects of higher phospholipid content in alkane solutions. Egg and soy lecithins were selected for this purpose, since multicomponent mixtures such as model 11.0 are very costly, even at levels of 2% wt/vol in dodecane. The costs of components in 74% wt/vol (see below) levels would have been prohibitive. 

For acids, the membrane retention actually increases in the case of egg lecithin, compared to soy lecithin. This may be due to decreased repulsions between the negatively charged sample and negatively charged phospholipid, allowing H-bond-ing and hydrophobic forces to more fully realize in the less negatively charged egg lecithin membranes. The neutral molecules display about the same transport properties in soy and egg lecithin, in line with the absence of direct electrostatic effects. These differences between egg and soy lecithins make soy lecithin the preferred basis for further model development. 

Figure 7.50 Hydrogen bonding/electrostatics scale based on phospholipid-alkane permeability differences A log Pe versus Pe (dodecane).

Hydrogen bonding and electrostatic interactions between the sample molecules and the phospholipid bilayer membranes are thought to play a key role in the transport of such molecules. When dilute 2% wt/vol phospholipid in alkane is used in the artificial membrane [15, 23], the effect of hydrogen bonding and electrostatic effects may be underestimated. 

As with other multisubunit enzymes (e.g., allosteric enzymes), the structural integrity of a membrane-bound enzyme primarily is maintained by noncovalent interactions such as hydrogen bonding, electrostatics, and hydrophobic interactions. Hydrophobic polypeptides (or hydrophobic portions of polypeptides) apparently are used to anchor the enzymes to the membrane through interactions with phospholipids. Therefore, I would characterize the interaction between the enzyme and membrane as chemical in nature rather than as geometric. ... 

During the extraction process three types of interactions are usually disrupted, these are van der Waals forces in lipid-lipid, lipid-protein, and liquid-carbohydrate complexes electrostatic and hydrogen bonding interactions between lipids andproteins andcovalent bonding between lipids, carbohydrates, and proteins (Roby t and White, 1987). The solvent of choice depends on the type of lipid and the interactions to be disrupted. Thus, neutral lipids may be extracted with nonpolar solvents, while phospholipids and glycolipids are extracted with more polar solvent mixtures (Shahidi and Wanasundara, 1998). 

Iscoms (Immune-stimulating complexes) are stable complexes of cholesterol, phospholipid, and Quil A (derived from Quillaja saponaria) in size ranges from 40 to lOOnm. They are promising carriers for antigens in subunit vaccines. Iscoms are considered to be multi-micellar structures, shaped and stabilized by hydro-phobic interactions, electrostatic repulsion, steric factors and possibly hydrogen bonds. Protection... 

In the classical model, as well as its more contemporary variant, the Robertson unit membrane hypothesis, membranes are seen to consist of a blmolecular leaflet of phospholipid sandwiched between two layers of globular protein. The non-polar fatty acid chains of the phospholipid are oriented Inward, perpendicular to the plane of the membrane system. The polar groups of the phospholipid at the external surface of the blmolecular leaflet are presumed to bond electrostatically to the protein, affording... 

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