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.     

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. I. Influence of the chain length of the fatty acid ester and of electrolytes on the critical micelle concentration

The critical micelle concentration (CMC) of four synthetic phosphatidylcholines (containing two hexanoyl, heptanoyl, octanoyl or nonanoyl residues respectively) in aqueous solutions have been determined by surface tension measurements. The dependence of the CMC on the chain length is discussed on the basis of the mass action model for micelle formation. For the three higher homologues a contribution of 1.08 kT per CH₂ group to the standard free energy of micellisation is found. The change in this free energy in going from the dihexanoyl- to the diheptanoyllecithin is somewhat larger (1.2 kT per CH₂ group).The influence of high concentrations (several moles per liter) of simple electrolytes on the CMC is interpreted as a salting-out of nonpolar solutes in water. Contrary to expectations the effects of NaCl and Lil on the CMC of dioctanoyllecithin are not additive.     

Physical and chemical studies of Thiobacillus ferroxidans lipopolysaccharides.

The lipopolysaccharides (LPS) of the obligate acidophile Thiobacillus ferroxidans grown on iron, sulfur, and glucose as energy sources were examined for various physical and chemical properties. Both qualitative and quantitative variation were found among the three preparations. The LPS extracted from iron-grown cells (Fe-LPS) contained less than 3% protein compared to 18 to 25% in LPS extracted from either sulfur-grown cells (S-LPS) or glucose-grown cells (G-LPS). S-LPS showed two distinct sedimentable species, 61S and 9.3S, which could be fractionated on a column of Sepharose 4B. The relative densities of both S-LPS and G-LPS were found to be significantly greater than that of Fe-LPS. Spectral differences were noted when each LPS was reacted with a carbocyanine dye. Fe-LPS showed a single absorbance maximum at 472 nm, S-LPS displayed its maximum at 650 nm, and G-LPS showed two maxima, the first at 468 nm and the other at 655 nm. Analysis of the methyl ester derivatives of the LPS fatty aicds using gas chromatography-mass spectrometry revealed the presence of a very stable species, tentatively identified as a methoxy methyl ester with a formula of CH3-3-C10H10-COOCH3, as the major component from each LPS. beta-Hydroxymyristic acid was found only in Fe-LPS.     

Physical parameters and chemical composition of porcine pancreatic elastase II.

Some molecular properties of the elastase II preparation, homogenous in ultracentrifugation, have been determined. The molecular weight is 25 000, the sedimentation coefficient and the diffusion coefficient are 3.69-10(-13) s(-1) and 12.09-10(-7) cm2/s, respectively. The partial specific volume was 0.716 g/cm3, and the axial ratio is 1.95. Elastase II exhibited a considerably lower content of arginine, tyrosine, and valine, and a higher content of proline, serine and conjugated carbohydrates than elastase I. The N-terminal amino acid of the enzyme is leucine, and its isoelectric point was 10.7.     

X-ray diffraction studies of lecithin bilayers.

Counting the individuals in a population before and after an annual period of environmental stress allows the proportion s of the initial population surviving the period to be computed. A series of such observations over n annual periods gives a sequence s₁, s₂, …, s_n. A statistical model is formulated from axioms describing the survival process, and it is concluded that these observed values may usefully be regarded as realizations of a random variable that arises from the normal generated distribution (n.g.d.). Equations for estimating the n.g.d. parameters ζ and τ² from observed survival proportions by the method of moments and maximum likelihood are given. The distributions of parameter estimates ζ and τ² are obtained and discussed in the context of testing hypotheses comparing survival among different populations. Finally, the dependence of the n.g.d. upon parameters ζ and τ² is examined in terms of altering survival, either by population self-regulation mechanisms or man-induced controls. The intent is to provide insight into the relationship between the n.g.d. and its supporting axioms and, more generally, basic knowledge of population processes.