Bilayer hydrophobic core

Simple considerations show that the membrane potential cannot be treated with computer simulations, and continuum electrostatic methods may constimte the only practical approach to address such questions. The capacitance of a typical lipid membrane is on the order of 1 j.F/cm-, which corresponds to a thickness of approximately 25 A and a dielectric constant of 2 for the hydrophobic core of a bilayer. In the presence of a membrane potential the bulk solution remains electrically neutral and a small charge imbalance is distributed in the neighborhood of the interfaces. The membrane potential arises from... 

Figure 7. Mechanism of the proton-translocating ubiquinol cytochrome c reductase (complex III) Q cycle. There is a potential difference of up to 150 mV across the hydrophobic core of this complex (potential barrier represented by the vertical broken line). Cytochromes hour and b N are heme groups on the same peptide subunits of complex III which can transfer electrons across the hydrophobic core. The movement of two electrons provides the driving force to transfer two protons from the matrix to the cytosol. Diffusion of UQ and UQHj, which are uncharged, in the hydrophobic core, and lipid bilayer of the inner membrane is not influenced by the membrane potential (see Nicholls and Ferguson, 1992).

As described above, some solutes such as gases can enter the cell by diffusing down an electrochemical gradient across the membrane and do not require metabolic energy. The simple passive diffusion of a solute across the membrane is limited by the thermal agitation of that specific molecule, by the concentration gradient across the membrane, and by the solubility of that solute (the permeability coefficient. Figure 41—6) in the hydrophobic core of the membrane bilayer. Solubility is... 

It is expected that the neutral species of the anesthetics can penetrate more deeply into the hydrophobic bilayer interior than the cationic ones. From the H and C NMR, we have demonstrated that the neutral species, DEC and PRC, are trapped deeply in the bilayer DEC can penetrate into the hydrophobic core of the bilayer, zone III, and PRC can penetrate into the inside of the bilayer preferentially trapping from zone II to the middle of zone III [48]. This information is valuable in the sense that it is difficult to observe the NMR signal of the neutral species in water because of the extremely low solubility. The preferential location is in accordance with the solubility in water the neutral species of DEC and PRC, sparingly soluble in water, are expected to favor the hydrophobic bilayer interior. 

We have also determined the delivery sites of alkylbenzenes by NMR. As already described in Section III.A, PrBe are deeply transported to the chain tail region in the bilayer core and the delivery site can be classified into category III [46]. Benzene, however, cannot deeply penetrate into the hydrophobic core, zone III, but is trapped preferentially at the interfacial site of the bilayer, zone II the delivery site can be classified into category II. Although benzene is generally considered to be hydrophobic, the delivery site of benzene determined by NMR is reasonable in the sense of the 7r-electrons with some affinity for the hydrophilic sites of the bilayer. Both drug and lipid sides of the H NMR spectra show that alkylbenzenes can deeply penetrate into the bilayer interior in the order PrBe > ethylbenzene > toluene > benzene, which is consistent with the sequence of the insolubility in water. 

Owing to their chemical structure, carotenes as polyterpenoids are hydrophobic in nature (Britton et al., 2004). Therefore, as it might be expected, the carotenes are bound within the hydrophobic core of the lipid membranes. Polar carotenoids, with the molecules terminated on one or two sides with the oxygen-bearing substitutes, also bind to the lipid bilayer in such a way that the chromophore, constituted by the polyene backbone is embedded in the hydrophobic core of the membrane. There are several lines of evidence for such a localization of carotenoids with respect to the lipid bilayers. 

PHOSPHOLIPIDS associate to form a bilayer consisting of a hydrophobic core (phospholipid tails) and a polar surface (phospholipid heads). 

Figure 1 (Plate 1). A molecular view of a small section of a flat lipid bilayer generated by molecular dynamics simulations. The bilayers are composed of l-stearoyl-2-docosa-hexaenoyl-5M-g]ycero-3-phosphatidylcholine lipids, i.e. the sn 1 chain is 18 C atoms long and the sn2 chain has 22 carbons, including six cis double bonds. The hydrophobic core is in the centre of the picture, and the hydrated head-group regions are both on top and bottom of the view graph. The head group is zwitterionic and no salt has been added. From [102], Reproduced by permission of the American Physical Society. Copyright (2003)...

A clue as to why the cationic N-terminal region and the hydrophobic C-terminal portion of SP are required for full histamine-releasing activity comes from studies of the electrical conductivity of black lipid membranes in the presence of peptides. Using SP, these authors [176] concluded that SP probably binds by its N-terminal region to negatively charged sites on membrane lipids, while the C-terminal portion of the molecule penetrates the hydrophobic core of the lipid bilayer, which could induce an increase in membrane permeability or a slight alteration in membrane conformation. 

Fig. 8 Proposed model for gramicidin S in a membrane according to the orientational constraints obtained from and N-NMR. The upright backbone alignment (r 80°) and slant of the /3-sheets (p -45°) are compatible with the formation of an oligomeric /3-barrel that is stabilized by hydrogen bonds (dotted lines). A The oligomer is depicted sideways from within the lipid bilayer interior (showing only backbone atoms for clarity, but with hydrophobic side chains added to one of the monomers). Atomic coordinates of GS were taken from a monomeric structure [4], and the two DMPC lipid molecules are drawn to scale (from a molecular dynamics simulation coordinate file). The bilayer cross-section is coloured yellow in its hydrophobic core, red in the amphiphilic regions, and light blue near the aqueous surface. B Illustrates a top view of the putative pore, although the number of monomers remains speculative...

The phospholipid molecules are such that in aqueous media they spontaneously form extended bilayers with a hydrophobic core. Although membrane proteins vary enormously they all form compact structures. This minimises the surface of interaction with the lipid, so that, although protein may account for 30-80% of the weight of the membrane, it does not affect the basic physical properties of the lipid bilayer. 

Flavonoids bear different degrees of hydroxylation, polymerization, and methylation that define both specific and nonspecific interactions with membrane lipids. Molecule size, tridimensional structure, and hydrophili-city/hydrophobicity are chemical parameters that determine the nature and extent of flavonoid interactions with lipid bilayers. The hydrophilic character of certain flavonoids and their oligomers endows these molecules with the ability to bind to the polar headgroups of lipids localized at the water-lipid interface of membranes. On the other hand, flavonoids with hydrophobic character can reach and cross the lipid bilayer. In this section, we will discuss current experimental evidences on the consequences of flavonoid interactions with both the surface and the hydrophobic core of the lipid bilayer. 

Coenzyme Q (ubiquinone) is an essential cofactor in the electron transport chain in which it accepts electrons from complex I and II. Coenzyme Q also serves as an important antioxidant in both mitochondria I and lipid membranes. Coenzyme Q is a lipid-soluble compound composed of a redox active quinoid moiety and a hydrophobic tail. The predominant form of coenzyme Q in humans is coenzyme Q10, which contains ten isoprenoid units in the tail, whereas the predominant form in rodents is coenzyme Q9, which has nine isoprenoid units in the tail. Coenzyme Q is soluble and mobile in the hydrophobic core of the phospholipid bilayer of the inner membrane of the mitochondria in which it transfers electrons one at a time to complex III of the electron transport chain. 

From the data presented here several conclusions may be reached regarding the effect of cholesterol on lipid bilayers. It is shown that, even if the presence of cholesterol in bilayers serves to moderate temperature-induced changes, its ability to affect the location of solubilized molecules is highly temperature dependent We have also shown, in accord with previous work (11), that the presence of cholesterol in the gel phase results in a larger separation between the lipid polar groups and this in turn allows water to penetrate into the lipid hydrophobic core. 

A hydrophobic central region that is buried in the hydrophobic core of the bilayer and... 

As with the majority of transmembrane proteins, the hydrophobic membrane-spanning region consists mainly of amino acid residues with hydrophobic side-chains that are folded in an a-helical conformation (see Topic B3). As each amino acid residue adds 0.15 nm to the length of an a-helix, a helix of 25 residues would have a length of 3.75 nm, just enough to span the hydrophobic core of the bilayer. The hydrophobic side-chains of the residues in the helix protrude outwards from the helix axis to interact via hydrophobic bonds with... 

---