A micelle model for the sedimentation behavior of bovine beta-casein

The monomer-single polymer model of G.A. Gilbert (Disc. Faraday Soc. 20 (1955) 68) for moving boundary sedimentation has been used by Payens and colleagues to explain the observed results for bovine caseins, and by Harrington and colleagues to explain the observed results for myosin fibrils. Electron microscope pictures of Buchheim and Schmidt have subsequently revealed micellar beta-casein in the form of slightly elongated or spherical particles having a bimodal size distribution, but with a broad range of particle sizes, at concentrations not too far above the critical micelle concentration. The equilibrium properties of a broadly distributed micellar system can be fitted by the shell model developed by one of us, and in the present article, the shell model is extended to predict the moving boundary sedimentation behavior of such a system. The observed sedimentation patterns, as well as the critical concentration predictions of the monomer-single polymer Gilbert sedimentation model, are satisfactorily described with the present model, based on a continuous distribution of intermediates between monomers and the largest possible spherical micelles. For one example considered, the predicted frequency distribution of molecular weight is in qualitative agreement with the frequency distribution of particle volume found by Buchheim and Schmidt.     

The critical micelle condition revisited.

Several simple alternatives have been examined as a possible basis for micellar size distributions which are internally consistent with the experimentally required concept of a threshold concentration, the critical micelle condition. Among these are the two-state system, monomer in equilibrium with a single high polymer, indefinite self-association, and continuous self-association with an arbitrary upper limit beyond which all further association is absolutely prohibited. Of these, only the last is a possible choice, although lacking experimental support. This report considers in deeper detail a thermodynamic model for micelle distributions, the so-called shell model of the present author, which is in basic agreement with an earlier statistic-mechanical study [Hoeve CA & Benson GC (1957), Colloid Polym Sci, 252, 56] in predicting the possibility of broad distributions of micellar species. A self-consistent distribution model which predicts a critical micelle condition must also predict a location with respect to degree of polymerization having a minimum concentration. Thus, the molar distribution function for the shell model satisfies this requirement, whereas the equivalent concentration distribution function fails to do so, as would the statistical models for multiple ligand binding. Moreover, the position of this minimum is predicted to change with concentration.     

Layer-by-layer Assembly of Poly(lactic-co-glycolic acid)-b-poly(l-lysine) Copolymer Micelles

Biocompatible, biodegradable polyionic micelles were used as a building component for layer-by-layer (LbL) assembly that can produce drug-loaded nanolayers. To prepare the polycationic micelles, poly(lactic-co-glycolic acid)-b-poly(l-lysine) [PLGA-b-P(Lys)] copolymers were synthesized. In an aqueous phase, PLGA-b-P(Lys) copolymers were self-assembled to form spherical micelles with the inner core of poly(lactic-co-glycolic acid) (PLGA) and the cationic outer shell of P(Lys). The micelles were characterized by zeta potential, dynamic light scattering, and nuclear magnetic resonance. PLGA-b-P(Lys) micelles showed the positive zeta potential values in a broad range of pH (3–11), indicating the high stability of the polyionic micelles with the outer shell of positive charges. Cationic polymeric micelles were coated on the surface via electrostatic interactions with the oppositely charged polyelectrolyte, poly(sodium 4-styrenesulfonate). Formation of multiple micelle layers was monitored using quartz crystal microbalance in situ, and the surface topology of the layers was characterized by atomic force microscopy ex situ, as the number of micelle layer was increased. The multiple micelle layers were stable, and the thickness of micelle layer grew as the number of LbL coating increased. The approach described in this work can be used for the development of the biocompatible, biodegradable, drug-loaded bioactive nanocoatings.     

Ketal cross-linked poly(ethylene glycol)-poly(amino acid)s copolymer micelles for efficient intracellular delivery of doxorubicin

A biocompatible, robust polymer micelle bearing pH-hydrolyzable shell cross-links was developed for efficient intracellular delivery of doxorubicin (DOX). The rationally designed triblock copolymer of poly(ethylene glycol)-poly(L-aspartic acid)-poly(L-phenylalanine) (PEG-PAsp-PPhe) self-assembled to form polymer micelles with three distinct domains of the PEG outer corona, the PAsp middle shell, and the PPhe inner core. Shell cross-linking was performed by the reaction of ketal-containing cross-linkers with Asp moieties in the middle shells. The shell cross-linking did not change the micelle size and the spherical morphology. Fluorescence quenching experiments confirmed the formation of shell cross-linked diffusion barrier, as judged by the reduced Stern-Volmer quenching constant (K(SV)). Dynamic light scattering and fluorescence spectroscopy experiments showed that shell cross-linking improved the micellar physical stability even in the presence of micelle disrupting surfactants, sodium dodecyl sulfate (SDS). The hydrolysis kinetics study showed that the hydrolysis half-life (t(1/2)) of ketal cross-links was estimated to be 52 h at pH 7.4, whereas 0.7 h at pH 5.0, indicating the 74-fold faster hydrolysis at endosomal pH. Ketal cross-linked micelles showed the rapid DOX release at endosomal pH, compared to physiological pH. Confocal laser scanning microscopy (CLSM) showed that ketal cross-linked micelles were taken up by the MCF-7 breast cancer cells via endocytosis and transferred into endosomes to hydrolyze the cross-links by lowered pH and finally facilitate the DOX release to inhibit proliferation of cancer cells. This ketal cross-linked polymer micelle is promising for enhanced intracellular delivery efficiency of many hydrophobic anticancer drugs.     

Effect of cross-linking on the performance of micelles as drug delivery carriers: a cell uptake study

Poly(polyethylene glycol methyl ether methacrylate-co-methacrylic acid)-block-poly(methyl methacrylate) P(PEGMEMA-co-MAA)-b-PMMA block copolymer were prepared via RAFT (reversible addition-fragmentation chain transfer) polymerization and subsequently self-assembled into micelles as a drug delivery carrier for albendazole (ABZ). For comparison, the micelles were additionally cross-linked to study the effect of shell-cross-linking on the biological activity. The hydrodynamic diameter of cross-linked and un-cross-linked micelles was approximately 40 nm in both cases. While the cross-linked micelle was stable even in good solvents for both blocks, the un-cross-linked micelle was found to lose its integrity in cell growth media. Crosslinking had a major effect on the rate of drug release reducing it dramatically from 50% (uncrosslinked) to around 20% (crosslinked) over a 30 h incubation period. Both drug delivery systems were tested on human prostate cancer cells (PC-3, DU-145) and human ovarian cancer cells (OVCAR-3, A-2780). No toxic effects were measured with the unloaded micelle while the ABZ loaded un-cross-linked micelle lead to IC(50) values between 0.2 and 0.9 μM depending on the cell line. The IC(50) dropped to values between 0.006 and 0.06 μM, depending on cell line, once the micelles were stabilized by cross-linking. Three treatment cycles with ABZ for one day, followed by two days incubation in media using ABZ-loaded drug carriers led to complete cell death even at low concentrations in the case of the cross-linked micelle only. Cellular uptake has been studied using fluorescently labeled micelles and Nile red as model drug, showing cell uptake above the CMC but no micelle uptake below the CMC. Additional biological studies, such as colony formation assay and tubulin disorganization tests, were also performed to gain more insight into the effect of cross-linking of the shell of the micelle. In conclusion, shell-cross-linking is highly recommended, even for glassy micelles, for an efficient cellular uptake at low concentrations.     

Probing the micelle/water interface by rapid laser-induced proton pulse

The laser-induced pH jump (Gutman, M. and Huppert, D.J. (1979) Biochem. Biophys. Methods 1, 9–19) has a time resolution capable of measuring the diffusion-controlled rate constant of proton binding. In the present study we employed this technique for measuring the kinetics of protonation-deprotonation of surface groups of macromolecules.The heterogeneous surface of proteins excludes them from serving as a simple model, therefore we used micelles of a neutral detergent (Brij 58) as a high molecular weight structure. The charge was varied by the addition of a low concentration of sodium dodecyl sulfate and the surface group with which the protons react was an adsorbed pH indicator (bromocresol green or neutral red).The dissociation of a proton from adsorbed bromocresol green is slower than that from free indicator. This effect is attributed to the enhanced stabilization of the acid form of the indicator in the pallisade region of the micelle. The $\text{p}\text{K}$ shift of bromocresol green adsorbed on neutral micelles is thus quantitatively accounted for by the decreased rate of proton dissociation. Indicators such as neutral red, which are more lipid soluble in their alkaline form, do not exhibit such decelerated proton dissociation in their adsorbed state nor a $\text{p}\text{K}$ shift on adsorption to neutral micelles.The protonation of an indicator is a diffusion-controlled reaction, whether it is free in solution or adsorbed on micelles. By varying the electric charge of the micelle this rate can be accelerated or decelerated depending on the total charge of the micelle. The micellar charge calculated from this method was corroborated by other measurements which rely only on equilibrium parameters.The high time resulation of the pH jump is exemplified by the ability to estimate the diffusion coefficient of protons through the hydrated shell of the micelle.     

Hydration of alkylammonium salt micelles--influence of bromide and chloride counterions

The micellization process of dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) was studied. Nuclear magnetic resonance method was used. The 1H NMR and 13C NMR spectra were taken at higher and lower concentrations than the critical micelle concentrations (CMC) of the compounds studied. Chemical shifts were analysed. The studies performed were prompted by earlier calorimetric measurements which showed that there were significant qualitative and quantitative differences in the micellization process of the compounds studied. Namely, DTAB micelle dissociation was found to be an endothermic process while that of DTAC was exothermic. The differences found must be the result of differentiated influence of bromide and chloride counterions on the micellization process, including the phenomenon of micelle hydration. The objective of the work was to check whether cationic surfactant counterions can influence the micelle hydration process. Indeed, DTAB and DTAC, as monomers, exhibit similar hydrophobic hydration, but DTAB micelles are more hydrated than DTAC ones. It seems that the differences found in micellization of both salts studied may be attributed to different physicochemical properties of bromide and chloride ions, such as their mobilities and radii of their hydrated forms. Moreover, the effect of anions on the water structure must be taken into account. It is important whether the anions can be classified as water ordering kosmotropes, that hold the first hydration shell tightly, or water disordering chaotropes, that hold water molecules in that shell loosely.     

Benzyl alcohol penetration into micelles, dielectric constant of the binding site, partition coefficient and high-pressure squeeze-out

The absorbance maximum, lambda max, of a local anesthetic, benzyl alcohol, is shifted to longer wavelengths when solvent polarity is decreased. The shift was approximately a linear function of the dielectric constant of the solvent. This transition in electronic spectra according to the microenvironmental polarity is used to analyze benzyl alcohol binding to surfactant micelles. A facile method is devised to estimate the micelle/water partition coefficient from the dependence of lambda max of benzyl alcohol on surfactant concentrations. The effective dielectric constants of the sodium decyl sulfate, dodecyl sulfate and tetradecyl sulfate micelles were 29, 31 and 33, respectively. The partition coefficient of benzyl alcohol between the micelles and the aqueous phase was 417, 610 and 1089, respectively, in the mole fraction unit. The pressure dependence of the partition coefficient was estimated from the depression of the critical micelle concentration of sodium dodecyl sulfate by benzyl alcohol under high pressure up to 200 MPa. High pressure squeezed out benzyl alcohol molecules from the micelle until about 120 MPa, then started to squeeze in when the pressure was further increased. The volume change of benzyl alcohol by transfer from the aqueous to the micellar phase was calculated from the pressure dependence of the partition coefficient. The volume change, estimated from the thermodynamic argument, was 3.5 +/- 1.1 cm3.mol-1 at 298.15 K, which was in reasonable agreement with the partial molal volume change determined directly from the solution density measurements, 3.1 +/- 0.2 cm3.mol-1. Benzyl alcohol apparently solvates into the micelles close to surface without losing contact with the aqueous phase.     

PFG-NMR investigations of the binding of cationic neuropeptides to anionic and zwitterionic micelles

The mechanism by which peptides bind to micelles is believed to be a two-phase process, involving (i). initial electrostatic interactions between the peptide and micelle surface, followed by (ii). hydrophobic interactions between peptide side chains and the micelle core. To better characterize the electrostatic portion of this process, a series of pulse field gradient nuclear magnetic resonance (PFG-NMR) spectroscopic experiments were conducted on a group of neuropeptides with varying net cationic charges (+1 to +3) and charge location to determine both their diffusion coefficients and partition coefficients when in the presence of detergent micelles. Two types of micelles were chosen for the study, namely anionic sodium dodecylsulfate (SDS) and zwitterionic dodecylphosphocholine (DPC) micelles. Results obtained from this investigation indicate that in the case of the anionic SDS micelles, peptides with a larger net positive charge bind to a greater extent than those with a lesser net positive charge (bradykinin > substance P > neurokinin A > Met-enkephalin). In contrast, when in the presence of zwitterionic DPC micelles, the degree of mixed-charge nature of the peptide affects binding (neurokinin A > substance P > Met-enkephalin > bradykinin). Partition coefficients between the peptides and the micelles follow similar trends for both micelle types. Diffusion coefficients for the peptides in SDS micelles, when ranked from largest to smallest, follow a trend where increasing net positive charge results in the smallest diffusion coefficient: Met-enkephalin > neurokinin A > bradykinin > substance P. Diffusion coefficients when in the presence of DPC micelles, when ranked from largest to smallest, follow a trend where the presence of negatively-charged side chains results in the smallest diffusion coefficient: bradykinin > Met-enkephalin > substance P > neurokinin A.     

The Mendelian inheritance of rare flesh and shell colour variants in the black‐lipped pearl oyster (Pinctada margaritifera)

Pinctada margaritifera is French Polynesia's most economically important aquaculture species. This pearl oyster has the specific ability to produce cultured pearls with a very wide range of colours, depending on the colour phenotypes of donor oysters used. Its aquaculture is still based on natural spat collection from wild stocks. We investigated three rare colour variants of P. margaritifera – orange flesh, and red and white shell colour phenotypes – in comparison with the wild‐type black flesh and shell commonly found in this species. The study aimed to assess the geographic distribution and genetic basis of these colour variants. Colour frequencies were evaluated during transfer and graft processes of pearl oyster seed captured at collector stations. Among the collection locations studied, Mangareva Island showed the highest rate of the orange flesh phenotype, whereas Takaroa and Takume atolls had relatively high rates of red and white shell phenotypes respectively. Broodstocks were made of these rare colour variants, and crosses were performed to produce first‐ and second‐generation progenies to investigate segregation. The results were consistent with Mendelian ratios and suggest a distinct model with no co‐dominance: (i) a two‐allele model for flesh trait, whereby the orange allele is recessive to the black fleshed type, and (ii) a three‐allele model for shell trait, whereby the black wild‐type allele is dominant to the red coloration, which is dominant to the white shell. Furthermore, the proposed model provides the basis for producing selected donor pearl oyster lines through hatchery propagation.     

Shell-cross-linked micelles containing cationic polymers synthesized via the RAFT process: toward a more biocompatible gene delivery system

Block copolymers poly(2-(dimethylamino) ethyl methacrylate)-b-poly(polyethylene glycol methacrylate) (PDMAEMA-b-P(PEGMA)) were prepared via reversible addition fragmentation chain transfer polymerization (RAFT). The polymerization was found to proceed with the expected living behavior resulting in block copolymers with varying block sizes of low polydispersity (PDI <1.3). The resulting block copolymer was self-assembled in an aqueous environment, leading to the formation of pH-responsive micelles. Further stabilization of the micellar system was performed in water using ethylene glycol dimethacrylate and the RAFT process to cross-link the shell. The cross-linked micelle was found to have properties significantly different from those of the uncross-linked block copolymer micelle. While a distinct critical micelle concentration (CMC) was observed using block copolymers, the CMC was absent in the cross-linked system. In addition, a better stability against disintegration was observed when altering the ionic strength such as the absence of changes of the hydrodynamic diameter with increasing NaCl concentration. Both cross-linked and uncross-linked micelles displayed good binding ability for genes. However, the cross-linked system exhibited a slightly superior tendency to bind oligonucleotides. Cytotoxicity tests confirmed a significant improvement of the biocompatibility of the synthesized cross-linked micelle compared to that of the highly toxic PDMAEMA. The cross-linked micelles were taken up by cells without causing any signs of cell damage, while the PDMAEMA homopolymer clearly led to cell death.     

Critical micelle concentrations of gangliosides

The micellar properties of mixed, bovine gangliosides and purified galactosyl-N-acetylacetylgalactosaminyl (N-acetylneuraminyl) galactosylglucosylceramide were studied by gel filtration, equilibrium dialysis, and band and boundary centrifugation in sucrose gradients. The dissociation of micelles is very slow (days) in water and required us to approach equilibrium by association of monomers rather than by the dissociation of micelles. The gangliosides were therefore first converted into very low molecular weight aggregates (1-3 molecules) by dissolving them in Me2SO. Galactosyl-N-acetylgalactosaminyl(N-acetylneuraminyl)galactosylglucosylceramide was then diluted into aqueous sucrose gradients and sedimented by the boundary centrifugation technique. This gave a sedimenting micelle and a nonsedimenting monomer concentration of (1-2) x 10-10 M (or less) which corresponds to the critical micelle concentration value. The mixed gangliosides revealed two micellar sizes (i.e., 10 and 4.5 S), the slower sedimenting species being formed from the larger one with time (days). The critical micelle concentration of the mixed gangliosides was found to be approximately 10-8 M by a gel filtration, equilibrium dialysis, and band centrifugation.     

Synthesis and characterization of mPEG-PLA prodrug micelles

Polymeric prodrugs of mPEG-PLA-haloperidol (methoxypoly(ethylene glycol)-b-poly(lactic acid)) can self-assemble into nanoscale micelle-like structures in aqueous solutions. mPEG-PLA-haloperidol was prepared and characterized using 1H and 13C NMR. The conjugation efficiency was found to be 64.8 +/- 21%. Micelles that form spontaneously upon solubilization of the mPEG-PLA and the polymeric prodrugs in water were characterized using a variety of techniques. The mPEG-PLA and prodrug micelles were found to have diameters of 28.73 +/- 1.45 and 49.67 +/- 4.29 nm, respectively, using dynamic light scattering (DLS). The micelle size and polydispersity were also evaluated with cryogenic transmission electron microscopy (cryo-TEM) and were consistent with the DLS results. Cryo-TEM and proton NMR confirmed that the micelles were spherical in shape. DLS was also used to determine the aggregation numbers of the micelles. The aggregation numbers ranged from 351 to 603. The change in aggregation number was dependent on the total drug incorporation into the micelle core. Critical micelle concentrations were determined for the various micelle/drug formulations and found to range from 3 to 14 microg/mL. Finally, drug was incorporated into the micelle core using the conjugate, free drug with a saturated aqueous phase during production, or a combination of both techniques. Drug incorporation could be increased from 3% to 20% (w/w) using the different formulations.     

Determination of the aggregation number of detergent micelles using steady-state fluorescence quenching.

The development of a simple, reliable method for determination of detergent micelle aggregation number that relies solely on measurement of steady-state fluorescence quenching is presented. The degree of steady-state fluorescence quenching of a micelle-solubilized fluorophore (pyrene) by a quencher that partitions greatly into the micelles (coumarin 153) is dependent on the micelle concentration, which can therefore be determined. The aggregation number is calculated as the micelle concentration/detergent monomer concentration (the total detergent concentration above the critical micelle concentration). For the determination to be accurate, the partition coefficient of the quencher into the micelle phase is determined and used to calculate the micellar concentration of quencher. Also, the quenching of pyrene by a coumarin 153 molecule within the same micelle must be complete, and this was confirmed by time-resolved fluorescence measurements. Aggregation numbers were determined for one cationic and several nonionic detergents and were found to be consistent with literature values. The approach presented is an improvement on a previous luminescence quenching technique (Turro, N.J., and A. Yekta. 1978. J. Am. Chem. Soc. 100:5951-5952) and can be used on cationic, anionic, and nonionic detergents with micelles ranging greatly in size and under varying conditions, such as detergent concentration, ionic strength, or temperature.     

pH-responsive three-layered PEGylated polyplex micelle based on a lactosylated ABC triblock copolymer as a targetable and endosome-disruptive nonviral gene vector

Nonviral vectors for gene therapy have recently received an increased impetus because of the inherent safety problems of the viral vectors, while their transfection efficiency is generally low compared to the viral vectors. The lack of the ability to escape from the endosomal compartments is believed to be one of the critical barriers to the intracellular delivery of noviral gene vectors. This study was devoted to the design and preparation of a novel ABC triblock copolymer for constructing a pH-responsive and targetable nonviral gene vector. The copolymer, lactosylated poly(ethylene glycol)-block-poly(silamine)-block-poly[2-(N,N-dimethylamino)ethyl methacrylate] (Lac-PEG-PSAO-PAMA), consists of lactosylated poly(ethylene glycol) (A-segment), a pH-responsive polyamine segment (B-segment), and a DNA-condensing polyamine segment (C-segment). The Lac-PEG-PSAO-PAMA spontaneously associated with plasmid DNA (pDNA) to form three-layered polyplex micelles with a PAMA/pDNA polyion complex (PIC) core, an uncomplexed PSAO inner shell, and a lactosylated PEG outer shell, as confirmed by 1H NMR spectroscopy. Under physiological conditions, the Lac-PEG-PSAO-PAMA/pDNA polyplex micelles prepared at an N/P (number of amino groups in the copolymer/number of phosphate groups in pDNA) ratio above 3 were found to be able to condense pDNA, thus adopting a relatively small size (< 150 nm) and an almost neutral surface charge (zeta approximately +5 mV). The micelle underwent a pH-induced size variation (pH = 7.4, 132.6 nm --> pH = 4.0, 181.8 nm) presumably due to the conformational changes (globule-rod transition) of the uncomplexed PSAO chain in response to pH, leading to swelling of the free PSAO inner shell at lowered pH while retaining the condensed pDNA in the PAMA/pDNA PIC core. Furthermore, the micelles exhibited a specific cellular uptake into HuH-7 cells (hepatocytes) through asialoglycoprotein (ASGP) receptor-mediated endocytosis and achieved a far more efficient transfection ability of a reporter gene compared to the Lac-PEG-PSAO/pDNA and Lac-PEG-PAMA/pDNA polyplex micelles composed of the diblock copolymers and pDNA. The effect of hydroxychloroquine as an endosomolytic agent on the transfection efficiency was not observed for the Lac-PEG-PSAO-PAMA/pDNA polyplex micelles, whereas the nigericin treatment of the cell as an inhibitor for the endosomal acidification induced a substantial decrease in the transfection efficiency, suggesting that the protonation of the free PSAO inner shell in response to a pH decrease in the endosome might lead to the disruption of the endosome through buffering of the endosomal cavity. Therefore, the polyplex micelle composed of ABC (ligand-PEG/pH-responsive segment/DNA-condensing segment) triblock copolymer would be a promising approach to a targetable and endosome disruptive nonviral gene vector.     

Polyion complex micelles of pDNA with acetal-poly(ethylene glycol)-poly(2-(dimethylamino)ethyl methacrylate) block copolymer as the gene carrier system: physicochemical properties of micelles relevant to gene transfection efficacy

An acetal-poly(ethylene glycol)-poly(2-(dimethylamino)ethyl methacrylate) (acetal-PEG-PAMA) block copolymer spontaneously associated with plasmid DNA (pDNA) to form water-soluble complexes (polyion complex micelle: PIC micelle) in aqueous solution. Physicochemical characteristics and transfection efficiency of the PIC micelles thus prepared were studied here, focusing on the residual molar mixing ratio (N/P ratio) of AMA units in acetal-PEG-PAMA to the phosphate units in pDNA. With the N/P ratio increasing to unity, acetal-PEG-PAMA cooperatively formed complex micelles with pDNA through electrostatic interaction, allowing pDNA to condense effectively. Dynamic light scattering measurements revealed that the PIC micelle at N/P > or = 3 had a constant size of approximately 90-100 nm. Eventually, acetal-PEG-PAMA/pDNA micelles underwent no precipitation even after long-term storage for more than 1 month at all N/P ratios. The PIC micelles were stable even in the presence of excess polyanions, poly(vinyl sulfate), in contrast to polyplexes based on the PAMA homopolymer, yet this stabilization effect was highly dependent on the N/P ratio to reach a plateau at N/P = 3-4. This character may be attributed to the increased hydrophobicity in the vicinity of the complexed pDNA. Furthermore, the pDNA in the micelle was adequately protected from DNase I attack. The transfection ability of the PIC micelles toward 293 cells was remarkably enhanced with an increasing N/P ratio as high as 25. The zeta-potential of the micelles with a high N/P ratio was an appreciably large positive value, suggesting a noncooperative micelle formation. This deviated micellar composition with an excess cationic nature as well as the presence of free acetal-PEG-PAMA may play a substantial role in the enhanced transfection efficiency of the PIC micelle system in the high N/P ratio (approximately 25) region.     

Interfacial adsorption of an inhalation anesthetic onto ionic surfactant micelles and its desorption by high pressure

The effects of pressure and temperature on the critical micelle concentration (CMC) of sodium dodecylsulfate (SDS) were measured in the presence of various concentrations of an inhalation anesthetic, methoxyflurane. The change in the partial molal volume of SDS on micellization, $\text{Δ}\text{V}{}_{\mathrm{<mtext>m</mtext>}}$, increased with the increase in the concentration of methoxyflurane. The CMC-decreasing power, which is defined as the slope of the linear plot between ln(CMC) vs. mole fraction of anesthetic, was determined as a function of pressure and temperature. Since the CMC-decreasing power is correlated to the micelle/water partition coefficient of anesthetic, the volume change of the transfer (ΔV_p^o) of methoxyflurane from water to the micelle can be determined from the pressure dependence of the CMC-decreasing power. The value of ΔV_p^o amounts 6.5±1.8 cm³·mol^?1, which is in reasonable agreement with the volume change determined directly from the density data, 5.5±0.6 cm³ · mol^?1. Under the convention of thermodynamics, this indicates that the application of pressure squeezes out anesthetic molecules from the micelle. The transfer enthalpy of anesthetic from water to the micelle is slightly endothermic. The partial molal volume of methoxyflurane in the micelle (112.0 cm³·mol^?1) is smaller than that in decane (120.5 cm³·mol^?1) and is larger than that in water (108.0 cm³·mol^?1). This indicates that the anesthetic molecules are incorporated into the micellar surface region, i.e., the palisade layer of the micelle in contact with water molecules, rather than into the micelle core.     

Stepwise degradation of serum low denisty lipoprotein by sodium dodecyl sulfate.

The structure of human serum low density lipoprotein (LDL) was investigated by perturbing the LDL structure with sodium dodecyl sulfate (SDS). The change in LDL structure induced by the addition of SDS was monitored by sedimentation velocity measurements, ultraviolet difference spectroscopy, fluorescence spectroscopy and proteolytic digestion of apo-LDL with subtilisin BPN' [EC 3.4.21.14]. As the concentration of SDS was increased from 0.1 mg/ml to 3 mg/ml with LDL concentrations between 2.0 mg/ml and 4.4 mg/ml, the sedimentation coefficient of LDL changed in three distinct steps. It was found by chemical analyses that not more than 30% of the total lipid was lost from LDL in the second step, whereas the final step in the change of sedimentation coefficient corresponded to the complete removal of apo-LDL from the constituent lipids of LDL. The ultraviolet difference spectrum between the native and SDS-treated LDL and the quenching of LDL fluorescence underwent about 80% of the total change while the SDS concentration was only sufficient to cause the second of the three step changes in sedimentation coefficient. SDS-polyacrylamide gel electrophoresis of apo-LDL treated with subtilisin BPN' also showed that more than 70% of apo-LDL became susceptible to proteolysis under the same conditions. These results were interpreted as indicating that the solubilization of 20 to 30% of the lipids on the surface of LDL exposed nearly 80% or more of apo-LDL to the solvent. A small portion of apo-LDL was, however, still firmly anchored to the remaining lipid micelle as long as the concentration of SDS was less than that required to cause the final step of the change in sedimentation coefficient.     

Size, shape and the distribution of organic matter in the Recent Antarctic brachiopod Liothyrella uva

The living terebratulid articulate brachiopod Liothyrella uva (Jackson 1912) was sampled from a shallow water population at Signy Island, South Orkney Islands, Antarctica. Neither shell height nor shell breadth were directly proportional to length and as a result there was a change in shell shape with size (and hence age); this change was small but statistically significant. The proportion of the total organic matter found in the shell and internal (mantle) tissues also changed with size. In small (5–7 mm length) brachiopods 70–80% of the total organic matter was located within the shell; this fraction decreased with increasing size until above about 25 mm length the proportion of organic matter in the shell was constant at 30–45%. Variability in this measure was influenced by infection with endolithic red algae. Punctal density was independent of size with a mean value of 95.7 per mm² [SE (standard error) ± 2.2], which was greater than in populations sampled from higher latitudes. With increasing shell length there was a slight increase in the size of puncta close to the shell edge. In all morphometric measures the range of variation observed was similar to that described from populations of Liothyrella from other areas of the Southern Ocean.     

A comprehensive study of the relationship between size and protein composition in natural bovine casein micelles

Casein micelles of bovine skimmed milk were fractionated by permeation chromatography on porous glass (CPG-10, 50 nm followed by CPG-10, 300 nm) at 30 degrees C. Micelles were pooled in eight eluant fractions and their size distribution was determined by electron microscopy. The composition of casein in the eight fractions was determined by quantitative hydroxyapatite chromatography. Micelle size decreased progressively with increasing elution volume, and volume-to-surface average diameter ranged from 154 nm in fraction 1 to 62 nm in fraction 8. Concurrently there was a decrease in relative proportions of alpha s- and beta-caseins and a large enrichment of kappa-casein, which changed from 4.1% total casein in fraction 1 to 12.1% total casein in fraction 8. At least half the decrease in alpha s-casein proportions was attributed to the alpha s1-casein component, but the data also suggested a decline in proportions of alpha s2-casein in the smallest micelle fractions. A plot of kappa-casein fractional content versus micelle surface-to-volume ratio gave a straight line (correlation coefficient from linear regression 0.98) from which an average kappa-casein surface coverage of 1.5 m2/mg or 47.3 nm2/molecule was obtained. If a constant surface coverage for kappa-casein is assumed, the parameters of the linear equation predict that micelle voluminosity is inversely related to micelle diameter, being approximately 30% larger in fraction 8 compared to fraction 1.