Physical chemical studies of short-chain lecithin homologues. IV. A simple model for the influence of salt and the alkyl chain length on the micellar size

A simplified association model for micellisation is presented. In this model two features are incorporated; (a) There is an optimum for the change of the standard free energy per monomer upon micellisation at a certain association number. (b) At higher association numbers this free energy change becomes constant. The resulting equations for the dependence of the average micellar weight on the concentration are used to explain the experimetitally observed effects of a salting-out agent (NaCl) and of the alkyl chain length of dihexanoyl-, diheptanoyl- and dioctanoyl-lecithin.     

Physical chemical studies of short-chain lecithin homologues. II. Micellar weights of dihexanoyl- and diheptanoyllecithin

The micellar weights of dihexanoyl- and diheptanoyllecithin in aqueous solutions are calculated from light scattering and ultracentrifugation data. A monomer-micelle assocation model is used and corrections for the thermodynamic nonideality, on the basis of rigid noninteracting particles, are applied. A few experiments on the influence of high NaCI concentrations (up to 3 M) are described. Dihexanoyllecithin forms micelles with micellar weight of 15 000 to 20 000 and with rather narrow weight distributions. Diheptanoyllecithin micelles however, have broad size distributions with micellar weights of 20 000 up to about 100 000 in the concentration range studied. Micelles are assumed to be spherical or to have sphero-cylindrical shapes depending on the molecular weights. Two models are used: (1) a compact structure, where no attention is paid to the hydrocarbon-water contact (2) micelles with as little hydrocarbon-water contact as possible.     

Physical chemical studies of short-chain lecithin homologues. 3. Phase separation and light scattering studies on aqueous dioctanoyllecithin solutions

Some phase separation phenomena in aqueous dioctanoyllecithin solutions including the effects of NaCl and Lil on the phase diagram are reported. At low electrolyte concentrations (below 0.2 M) both salts cause the upper consolute temperatures (u.c.t) to decrease, probably due to a decrease of electrostatic attraction between the lipid molecules. At higher salt concentrations the effect of Lil continues in the same direction (salting-in) but NaCl leads to an increase of the u.c.t. (salting-out).Micellar weight determinations could be performed at room temperature in homogeneous lecithin solutions containing 0.2 M Lil. An attempt was made to interpret the light scattering data with the help of the open association model (equal association constants for aggregation steps beyond a certain step) and the Flory-Huegins type of thermodynamic nonideality. The angular dependence of the light scattering points to very large and extended micelles. The radii of gyration are approximately proportional to the square root of the micellar weights.     

The critical micelle concentration of some physiologically important fatty acyl-coenzyme A's as a function of chain length

The critical micelle concentration (CMC) of a series of saturated fatty acyl-CoAs have been determined using a fluorescent titration method with 2-toluidinylnaphthalene-6-sulfonate as a probe. The CMC was found to be a function of the number of carbon atoms in the acyl chain over the range tested (12 to 18). A double bond has the effect on the CMC of decreasing the acyl chain by 1.6 carbon atoms. Knowledge that the CMC of each of these fatty acyl-CoAs is a function of the acyl chain length and the availability of a simple and appropriate method for the estimation of this property under other conditions should be of importance in designing and interpreting in vitro experiments with these compounds.     

Effect of chain length on the stability of lecithin bilayers

The shift reagent NaCl₃ was added to vesicles of synthetic, saturated (DiC₁₀-C₁₆) lecithins and egg lecithin and the accessibility of the N(CH₃)₃ groups to Na³⁺ ions was studied by NMR. Long chain lecithins, e.g. dipalmitoyl and egg lecithin form bilayers “stable” on the time scale of our experiments and practically impermeable to cations. Short chain lecithins on the other hand form short-lived vesicles surrounded by unstable bilayers which are not effective cation barriers. Ion transport across the latter lecithin bilayers may involve, besides passive diffusion, collision-induced transient rupture and resealing of bilayers coupled with ion movement.     

Noninvasive methods to determine the critical micelle concentration of some bile acid salts

In this work the critical micelle concentrations (cmc) of four bile salts, sodium cholate, sodium glycocholate, sodium deoxycholate, and sodium glycodeoxycholate, are determined and presented. Three independent noninvasive methodologies (potentiometry, derivative spectrophotometry, and light scattering) were used for cmc determination, at 25 degrees C with ionic strength adjusted to 0.10 M with NaCl. Spectrophotometric and potentiometric studies of some bile salts were also executed at various ionic strength values, thus allowing the influence of the ionic strength on the cmc value of the bile salt to be assessed. A critical comparison of the cmc values obtained with data collected from the literature is presented. Furthermore, this work makes an evaluation of the conceptual bases of different methodologies commonly used for cmc determination, since variations in the results obtained can be related mainly to different intrinsic features of the methods used (such as sensitivity or the need to include tracers or probes) or to the operational cmc definition applied. The undoubted definition of the experimental bile salt concentration that corresponds to cmc (operational cmc) is essential since in the case of these amphiphiles the formation of micelles is not as abrupt as in the case of ordinary association colloids. The biphasic nature of their aggregation leads to a "round-shaped" variation of the experimental parameters under analysis, which makes difficult the evaluation of the cmc values and can be responsible for the different results obtained.     

Lanthanide-induced phosphorus-31 NMR downfield chemical shifts of lysophosphatidylcholines are sensitive to lysophospholipid critical micelle concentration.

Lysophosphatidylcholine (lysoPC) monomers or micelles in water give rise to a narrow, isotropic phosphorus-31 NMR signal (40.6 ppm; v1/2 1.7 Hz; 32.2 MHz). Upon addition of praseodymium ions, the phosphorus signals are shifted downfield. However, the downfield shifts for the longer-chain lysophosphatidylcholines, which exist in the aggregated state, are far greater than those for the shorter-chain homologues, which exist as monomers. At a Pr3+/lysoPC molar ratio of 0.5, the signals of C12lysoPC through C18lysoPC were shifted by 12.1 ppm, whereas the signals of C6lysoPC and C8lysoPC were shifted by only 2.26 ppm. This very pronounced difference in lanthanide-induced downfield shifts between micelles and monomers can be utilized to determine with accuracy lysoPC critical micelle concentrations (CMC) from downfield shift-vs.-concentration plots. The CMC values we determined were 57 mM for C8lysoPC, 5.7 mM for C10lysoPC, and 0.6 mM for C12lysoPC. The shift reagent phosphorus-31 nuclear magnetic resonance technique particularly lends itself to the measurement of CMC values in the millimolar and high micromolar range. The method can equally be used for measuring critical micelle concentrations of short-chain phosphatidylcholines.     

Nuclear magnetic resonance studies on liposomes: Effects of steroids on lecithin fatty acyl chain mobility

Summary The effects of fourteen sterols on the NMR spectra of liposomes derived from egg yolk phosphatidylcholines were studied by continuous-wave and Fourier-transform measurements at 60 MHz. Sterols were compared for their ability to broaden the acyl methylene resonances of phosphatidylcholine, when incorporated into liposomes at 25% molar ratio. The ratio of the phosphatidylcholine peak heights (acyl methylene: cholinen-methyl) was used as a criterion of the relative condensing activity for the different sterols. This ratio was inversely proportional to the molar volume of the incorporated sterol, as measured by the parachor of the compound. Small sterols had little condensing effect, and the larger sterols such as cholesterol and ergosterol had maximum condensing effects. The study confirmed the importance of the sterol side-chain at C-17 as a requirement for sterol-phospholipid interaction.     

Significance of glycosphingolipid fatty acid chain length on membrane microdomain-mediated signal transduction

Lactosylceramide (LacCer), a neutral glycosphingolipid, is abundantly expressed on human neutrophils, and specifically recognizes several pathogenic microorganisms. LacCer forms membrane microdomains coupled with the Src family kinase Lyn on the plasma membrane, and ligand binding to LacCer activates Lyn, resulting in neutrophil functions. In contrast, neutrophilic differentiated HL-60 cells do not have Lyn-associated LacCer-enriched microdomains and lack LacCer-mediated functions. In neutrophil plasma membranes, the very long fatty acid C24:0 and C24:1 chains are the main components of LacCer, whereas plasma membrane of D-HL-60 cells mainly includes C16-LacCer species. Here, we suggest that LacCer species containing very long fatty acid chains are indispensable for the association of Lyn with LacCer-enriched microdomains and LacCer-mediated functions.     

Spin label study of detergents in the region of critical micelle concentration.

A series of spin probes was employed to examine the behavior of the detergent sodium dodecyl sulfate (SDS) at concentrations above and below the critical micelle concentration (cmc). The existence of detergent aggregates below the cmc was evidenced by the appearance of composite electron spin resonance (ESR) spectra for probes that have measurable solubility in water. The spectra were indicative of two probe populations: one in an aqueous environment and another in detergent aggregates. The ESR spectra of probes which are highly insoluble in water exhibited line broadening due to intermolecular spin exchange interactions, indicating that the probes were concentrated in detergent aggregates present below the experimental cmc. The results are discussed in terms of their significance for the study of the mechanisms of micelle formation and for the detection of detergent aggregates at very low concentrations.     

Perspectives on the therapeutic potential of short-chain fatty acid receptors

There is rapidly growing interest in the human microbiome because of its implication in metabolic disorders and inflammatory diseases. Consequently, understanding the biology of short chain fatty acids and their receptors has become very important for identifying novel therapeutic avenues. GPR41 and GPR43 have been recognized as the cognate receptors for SCFAs and their roles in metabolism and inflammation have drawn much attention in recent years. GPR43 is highly expressed on immune cells and has been suggested to play a role in inflammatory diseases such as inflammatory bowel disease. Both GPR41 and GPR43 have been implicated in diabetes and obesity via the regulation of adipose tissue and gastrointestinal hormones. So far, many studies have provided contradictory results, and therefore further research is required to validate these receptors as drug targets. We will also discuss the synthetic modulators of GPR41 and GPR43 that are critical to understanding the functions of these receptors. [BMB Reports 2014; 47(3): 173-178]