Lipid hydrocarbon

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

Figure 2 Snapshot from an MD simulation of a multilamellar liquid crystalline phase DPPC bilayer. Water molecules are colored white, lipid polar groups gray, and lipid hydrocarbon chains black. The central simulation cell containing 64 DPPC and 1792 water molecules, outlined m the upper left portion of the figure, is shown along with seven replicas generated by the periodic boundary conditions. (From Ref. 55.)...

By contrast, low temperature and low Mg concentration causes the redistribution of as much as 15% of the Ca -ATPase mass from the lipid hydrocarbon region into the cytoplasm [140,196,198], These effects of temperature and Mg concentration on the structure of the lipid phase and on the transmembrane disposition of Ca -ATPase are manifested in a slower rate of EiP formation and a longer lifetime of E]P at near zero °C temperatures and at low Mg concentrations [196,198],... 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

It should be noted that in forming this dimeric channel structure all the hydrogen bonds are parallel to the channel axis and that the inner surface is lined with the polar polypeptide groups. In addition the various lipophilic side chains coat the outer wall of the structure and are thus in contact with the lipid hydrocarbon chains. The resulting gramicidin A channel is a most efficient means of ion transport with approximately 107 sodium ions traversing the channel per second, under conditions of 1 M NaCl, 100 mV applied potential and a temperature of 25 °C 225). The detailed mechanism by which this can be achieved is under active study 226). 

Information concerning myelin structure is also available from electron microscope studies, which visualize myelin as a series of alternating dark and less dark lines (protein layers) separated by unstained zones (the lipid hydrocarbon chains) (Figs 4-4 to 4-7). There is asymmetry in the staining of the protein layers. The less dark, or intraperiod, line represents the closely apposed outer protein... 

Interpretation of the Calorimetric Results. There is little doubt that the transition observed in M. laidlawii membranes arises from the lipids since it occurs at the same temperature in both intact membranes and in water dispersions of membrane lipids. It is reasonable to conclude that in both membranes and membrane lipids the lipid hydrocarbon chains have the same conformation. The lamellar bilayer is well established for phospholipids in water (I, 20, 29) at the concentration of lipids used in these experiments. In the phase change the hydrocarbon core of the bilayer undergoes melting from a crystalline to a liquid-like state. Such a transition, like the melting of bulk paraffins, involves association between hydrocarbon chains and would vanish or be greatly perturbed if the lipids were apolarly bound to protein. We can reasonably conclude that most of the lipids in M. laidlawii membranes are not apolarly bound to protein. 

For biological membranes the situation is more complex. The results from erythrocyte ghosts and lipids (13) suggest nonpolar association through lipid hydrocarbon chains. As a test of the technique a membrane in which the bilayer conformation has been demonstrated by some independent technique is desirable. I have argued earlier from calorimetric evidence that the membrane of M. laidlawii is such a system. NMR spectra of M. laidlawii membranes taken in this laboratory do not, however, show discernible hydrocarbon proton resonance. We must consider, then, why the two techniques of calorimetry and NMR do not agree. 

Recent investigations provide new insight on the structural chemistry of dissolved organic matter (DOM) in freshwater environments and the role of these structures in contaminant binding. Molecular models of DOM derived from allochthonous and autochthonous sources show that short-chain, branched, and alicyclic structures are terminated by carboxyl or methyl groups in DOM from both sources. Allochthonous DOM, however, had aromatic structures indicative of tannin and lignin residues, whereas the autochthonous DOM was characterized by aliphatic alicyclic structures indicative of lipid hydrocarbons as the source. DOM isolated from different morphoclimatic regions had minor structural differences. 

To summarize the data on chain length effects, we have represented the transfection activity as a function of the total number of carbon atoms in the lipid hydrocarbon chains (Fig. 16). It is evident from this figure that transfection typically exhibits a maximum at an average chain length of about 14 carbon atoms. These data indicate that the most frequently used cationic lipid derivatives with dioleoyl chains do not appear to be the most appropriate choice of carriers in transfection studies. 

The transfection efficiency of cationic lipids displays a well-expressed dependence on the lipid hydrocarbon chain structure. Lipids with chain length of about 14 carbon atoms are generally the most effective. 

Cholesterol can modify both the hydrophobic attraction between lipid hydrocarbon chains and electrostatic interactions between lipid polar groups. The influence it has on the location of 9HP reflects this dual effect At low temperature, the "spacer" effect of cholesterol allows the ketone to gain access directly to the lipid-water interface. At high temperatures, a more disordered hydrocarbon core favors the solubilization of the guest molecule. 

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

The influence of cholesterol concentration at various temperatures on the thermotropic phase behavior and organization of saturated PE bilayers has been studied by combining DSC, FT-IR, and 31P-NMR. It was found that incorporation of low levels of cholesterol into the bilayer caused a progressive reduction in the temperature, enthalpy, and overall cooperativity of the lipid hydrocarbon chain melting transition... 

No influence of halothane on the lipid hydrocarbon chain conformations was found although the overall geometry changed slightly as a consequence of a small lateral expansion, accompanied by a small contraction of the bilayer thickness. Very... 

Subsequently, the ion channel activity of 34 was studied using planar bilayer methods.50c Peptide 34 was either introduced to the bulk KC1 solution or mixed with the lipid sample prior to bilayer formation. In the first method, incorporation proved difficult, possibly due to the high tendency of 34 to form aggregates. In the second method the stability of the bilayer was perturbed and single-channels were not obtained. It was postulated that this was due to the adsorption of the peptide onto the surface of the membrane due to electrostatic interactions of the crown ethers with the polar head groups. However as soon as the peptide was incorporated successfully into the bilayer, i.e. peptide parallel to the lipid hydrocarbons, singlechannel events were recorded indicative of 34 functioning in a unimolecular fashion. 

The central stmctural feature of almost all biological membranes is a continuous and fluid lipid bilayer that serves as the major permeability barrier of the cell or intracellular compartment (1) and as a scaffold for the attachment and organization of other membrane constituents (2, 3). In particular, peripheral membrane proteins are bound to the surface of lipid bilayers primarily by electrostatic and hydrogen-bonding interactions, whereas integral membrane proteins penetrate into, and usually span, the lipid bilayer, and are stabilized by hydrophobic and van der Waal s interactions with the lipid hydrocarbon chains in the interior of the lipid bilayer as well as by polar interactions... 

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