Phospholipid membranes, characteristics

In a very thoughtful investigation of solvent systems to model membrane characteristics, Leahy et al. (1989, 1992) have argued that two receptors sited in different tissues (or membranes) could exist in environments that are very different in hydrogen bonding character one may be surrounded by amphiprotic groups (as in a protein) or by proton donors the other may be surrounded by proton acceptors (as in a phospholipid membrane). 

During the interaction between drug and phospholipid molecules, one or several of these parameters can change in a manner characteristic of both the drug and the receptor phospholipid membrane. 

The conventional model developed to explain cell membrane characteristics influencing drug permeability is routinely referred to as the fluid-mosaic model (Figures 2.1 and 2.2). In this model the main components, for our purposes, are a phospholipid (e.g., sphingomyelin and phosphatidylcholine) bilayer (8 nm), with polar moieties at both the external and internal surfaces, and with proteins periodically traversing the phospholipid plane perpendicularly. 

However, the ordered structure of the phospholipid membrane is not highly conducive to the presence of numerous pores. Therefore, high lipid solubility is a predominant characteristic that favors membrane absorption of a chemical. It is an important feature for oral absorption into the body as well as for distribution within the body, since the body is basically a series of polar, aqueous media chambers separated by phospholipid barriers containing polar groups. 

Liposomes have aroused interest in a great variety of areas from biochemistry and molecular biology to cosmetics and food technology. One of the most salient applications of liposomes has been promoted by their high similarity to natural cell membranes, for which they are extensively used as substitutes in medical and pharmaceutical research. Since their inception, liposomes have often been used as models for studying the nature of cell membranes, the structure and functions of which they can mimic quite closely. One example is the determination of membrane distribution coefficients of drugs with a view to estimate their ability to penetrate cells. Interactions between analytes and phospholipid membranes depend on the characteristics of both the analytes and the membrane. 

The manner in which protons diffuse is a reflection of the physical properties of the environment, the geometry of the diffusion space, and the chemical composition of the surface that defines the reaction space. The biomembrane, with heterogeneous surface composition and dielectric discontinuity normal to the surface, markedly alters the dynamics of proton transfer reactions that proceed close to its surface. Time-resolved measurements of fast, diffusion-controlled reactions of protons with chromophores and fluorophores allow us to gauge the physical, chemical, and geometric characteristics of thin water layers enclosed between phospholipid membranes. Combination of the experimental methodology and the mathematical formalism for analysis renders this procedure an accurate tool for evaluating the properties of the special environment of the water-membrane interface, where the proton-coupled energy transformation takes place. 

It turned out to be true that proteinoids, without any lipids, also form bimolecular membranes.Despite the fact that black proteinoid membranes are not as long-lived in the ultrathin state as phospholipid membranes, they last long enough to be examined. Those rich in hydrocarbon-rich amino acid side chains mostly display properties characteristic for BLMs. The same polymers are among those that most readily combine with lecithin. 

Binder H, Zschomig O (2002) The effect of metal cations on the phase behavior and hydration characteristics of phospholipid membranes. Chem Phys Lipids 115 39 1... 

Changes in the composition of the cytoplasmatic membrane. The compounds of the membranes such as lipids or proteins can influence the membrane characteristics and, therefore, the adaption to solvents. Mainly phospholipids in the membrane bilayer determine the partitioning of solutes and especially the resistance to... 

Fig. 6.9 Characteristic structures of biological membranes. (A) The fluid mosaic model (S. J. Singer and G. L. Nicholson) where the phospholipid component is predominant. (B) The mitochondrial membrane where the proteins prevail over the phospholipids...

The basic characteristic of the membrane structure is its asymmetry, reflected not only in variously arranged proteins, but also in the fact that, for example, the outside of cytoplasmatic (cellular) membranes contains uncharged lecithin-type phospholipids, while the polar heads of strongly charged phospholipids are directed into the inside of the cell (into the cytosol). 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

An important characteristic of mammalian 15-LOX is its capacity to oxidize the esters of unsaturated acid in biological membranes and plasma lipoproteins without their hydrolysis to free acids. Jung et al. [19] found that human leukocyte 15-LOX oxidized phosphatidylcholine at carbon-15 of the AA moiety. Soybean and rabbit reticulocyte 15-LOXs were also active while human leukocyte 5-LOX, rat basophilic leukemia cell 5-LOX, and rabbit platelet 12-LOX were inactive. It was suggested that the oxygenation of phospholipid is a unique property of 15-LOX. However, Murray and Brash [20] showed that rabbit reticulocyte... 

---