Lipids in aqueous solution

3 Study Fig. 17.9, what are the major differences between the phosphatidic 

5 A scientist creates some phospholipid liposomes (Fig. 17.3c) by mixing PE and PC which phospholipid will most likely occupy the inside layer  

6 Assuming everything else between two fatty acids is identical, would you expect a fatty acid with (a) 0 or 3 double bonds to have a lower melting point (b) one with a 10 carbon chain or 20 carbon chain  

7 List two types of diffusion lipid molecules can undergo in a liquid crystalline phase. 

8 Why is transmembrane flip-flop of a lipid molecule in a bilayer energetically unfavorable  

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Table 3. Microenvironmental polarity parameters ( ) and steady-state fluorescence polarization values (P) for guests bound to hybrid assemblies formed with artificial receptors and peptide lipid in aqueous solution at 30.0...

Unilamellar phosphatidylcholine vesicles can be readily prepared by sonicating dispersions of the lipid in aqueous solution at a temperature above the gel-liquid transition point. When formed in the presence of metal ions, the internal space contains encapsulated species that can subsequently undergo crystallization reactions with membrane-permeable species such as OH and H2S (Fig. 21). Alternatively, coreactants can be transported into the interior of the vesicles via ionophores sited in the lipid bilayer. The following materials have been invest -... 

The previously described measurements have been performed on lipids in aqueous solutions, but lipid bilayers also swell in some other solvents (12) and the results of such measurements compare quite well with the aqueous case. In addition, hydration (solvation) forces act between DNA polyelectrolytes (13) and polysaccharides (14). These facts make the interpretation of the forces even more complicated and it is no wonder that different approaches to explain the nature of this solvation force exist. So far no truly ab initio theory has been proposed. The existing theories include models based on the electrostatic approach, the free energy approach, and an approach based on the entropic or protrusion model. 

Micellar assemblies continuously undergo the formation and breakdown in structure and how quickly this occurs depends on the interaction parameter and also the cmc value. The cmc value for natural lipids in aqueous solution range from pmol to picomolar concentrations and this equates to lipid exchange rates of hours between aggregates. However, for block copolymers much more stable and kinetically trapped aggregates are formed that thus have significantly lower cmc values. 

Liposomes are vesicles with a bilayer lipid sheet formed by cationic lipids in aqueous solutions. When liposomes get in contact with nucleic acids they undergo a rearrangement into nucleic acid lipid complexes called lipoplexes. Lipoplexes can be actively taken up by eukaryotic cells by endocytosis, and the lipoplex is then internalized into the cell cytosol within endosomes. The endo-somal complex is finally destroyed by increasing the osmotic pressure created by the lipids buffering action and by the fusion of the lipid with the endosomal membrane. The ability of a lipid to destroy endosomes, also referred to as endosomal escape, is one of the main characteristics of a good synthetic transfection reagent, as it indicates the capability of the vector to release its nncleic acid load into cells once having crossed the cell membrane. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

It should be noted that Cypridina luciferin emits a fairly strong chemiluminescence in aqueous solutions in the presence of various lipids and surfactants, even in the complete absence of luciferase. The luminescence is especially conspicuous with cationic surfactants (such as hexadecyltrimethylammonium bromide) and certain emulsion materials (such as egg yolk and mayonnaise). Certain metal ions (especially Fe2+) and peroxides can also cause luminescence of the luciferin. Therefore, great care must be taken in the detection of Cypridina luciferase in biological samples with Cypridina luciferin. 

Hydrolysis of substrates is performed in water, buffered aqueous solutions or biphasic mixtures of water and an organic solvent. Hydrolases tolerate low levels of polar organic solvents such as DMSO, DMF, and acetone in aqueous media. These cosolvents help to dissolve hydrophobic substrates. Although most hydrolases require soluble substrates, lipases display weak activity on soluble compounds in aqueous solutions. Their activity markedly increases when the substrate reaches the critical micellar concentration where it forms a second phase. This interfacial activation at the lipid-water interface has been explained by the presence of a... 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

Results of parameter optimization and MD simulations of small model compounds have been published, including alcohols [63], alkanes [63], aromatic [64] and heteroaromatic [209] compounds and liquid amides [65], Studies of ions in aqueous solution were also performed [61, 88] and results from an MD simulation on a DPPC lipid monolayer have been reported (Harder, MacKerell, Roux, submitted). Notable from the monolayer study was the reproduction of the dipole potential across the monolayer, a value that cannot be reproduced using non-polarizable models. This exciting, unforeseen observation points to the types of results that may be obtained from polarizable macromolecular force fields that are not accessible to the present additive models. 

Second, P-gp differs from other transporters in that it recognizes its substrates when dissolved in the lipid membrane [52], and not when dissolved in aqueous solution. The site of recognition and binding has been shown to be located in the membrane leaflet facing the cytosol [53, 54], This implies that the membrane concentration of the substrate, Csm, determines activation [57]. Since the nature of a molecular interaction is strongly influenced by the solvent, the lipid membrane must be taken into account as the solvent for the SAR analysis of P-gp. Under certain conditions, the effect of additional solvents or excipients (used to apply hydrophobic substrates or inhibitors) on the lipid membrane and/or on the transporter must also be considered. Lipophilicity of substrates has long been known to play an important role in P-gp-substrate interactions nevertheless, the correlation of the octanol/water partition coefficients with the concentration of half-maximum... 

The empirical correlation (Eq. (7)) allows an estimation of the membrane concentration of substrates required for half-maximum activation of P-gp. For hydro-phobic substrates, the membrane concentration CSm, is usually much higher than the concentration in aqueous solution, Csaq, and is given by Csm = Kiw x CSaq, where Csm is given in [moles drug/mole lipid] x [moles drug/liter lipid]. Replacing the aqueous substrate concentration Csaq, in the Michaelis-Menten equation (Eq. (9)) by the membrane concentration, Csm allows comparison of the activation... 

Figure 22.1 The amphiphilic nature of phospholipids in solution drives the formation of complex structures. Spherical micelles may form in aqueous solution, wherein the hydrophilic head groups all point out toward the surrounding water environment and the hydrophobic tails point inward to the exclusion of water. Larger lipid bilayers may form by similar forces, creating sheets, spheres, and other highly complex morphologies. In non-aqueous solution, inverted micelles may form, wherein the tails all point toward the outer hydrophobic region and the heads point inward forming hexagonal shapes.

Mixtures of phospholipids in aqueous solution will spontaneously associate to form liposomal structures. To prepare liposomes having morphologies useful for bioconjugate or delivery techniques, it is necessary to control this assemblage to create vesicles of the proper size and shape. Many methods are available to accomplish this goal, however all of them have at least several steps in common (1) dissolving the lipid mixture in organic solvent, (2) dispersion in an aqueous phase, and (3) fractionation to isolate the correct liposomal population. 

Once the desired mixture of lipid components is dissolved and homogenized in organic solvent, one of several techniques may be used to disperse the liposomes in aqueous solution. These methods may be broadly classified as (1) mechanical dispersion, (2) detergent-assisted solubilization, and (3) solvent-mediated dispersion. 

Activation of PE residues with these crosslinkers can proceed by one of two routes the purified PE phospholipid may be modified in organic solvent prior to incorporation into a liposome, or an intact liposome containing PE may be activated while suspended in aqueous solution. Most often, the PE derivative is prepared before the liposome is constructed. In this way, a stable, stock preparation of modified PE may be made and used in a number of different liposomal recipes to determine the best formulation for the intended application. However, it may be desirable to modify PE after formation of the liposomal structures to ensure that only the outer half of the lipid bilayer is altered. This may be particularly important if substances to be entrapped within the liposome are sensitive or react with the PE derivatives. 

Construct a liposome by dissolving the desired lipids in chloroform to homogenize fully the mixture, drying them to remove solvent, and using any established method of forming bilayer vesicles in aqueous solution (i.e., sonication see Section 1, this chapter). 

Liposomes containing PE lipid components may be activated with these crosslinkers to contain iodoacetyl derivatives on their surface (Figure 22.29). The reaction conditions described in Chapter 5, Section 1.5 may be used, substituting a liposome suspension for the initial protein being modified in that protocol. The derivatives are stable enough in aqueous solution to allow purification of the modified vesicles from excess reagent (by dialysis or gel filtration) without... 

The following treatment has been suggested by Shiu et al. (1994) and is reproduced briefly below. The simplest, first-order approach is to take into account the effect of dissociation by deducing the ratio of ionic to non-ionic species I, the fraction ionic x and the fraction non-ionic xN for the chemical at both the pH and temperature of experimental data determination (/D, xID, xND) and at the pH and temperature of the desired environmental simulation (/E, xIE, xNE). It is assumed that dissociation takes place only in aqueous solution, not in air, organic carbon, octanol or lipid phases. Some ions and ion pairs are known to exist in the latter two phases, but there are insufficient data to justify a general procedure for estimating the quantities. No correction is made for the effect of cations other than H+. This approach must be regarded as merely a first correction for the dissociation effect. An accurate evaluation should preferably be based on experimental... 

Lopez-Nicolas JM, Bru R, Sanchez-Ferrer A and Garcia-Carmona F. 1995. Use of soluble lipids for biochemical processes linoleic acid cyclodextrin inclusion complexes in aqueous solutions. Biochem J 308 151-154. 

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