Gel filtration chromatographic study on the self-association of surfactants and related compounds

Photophysical Properties of Coumarin 1 in Bile Salt Aggregates: An Insight into the Role of Bile Salt Structure on the Aggregation Behavior

The photophysical behavior of Coumarin 1 (Cou1), a well-known 7-aminocoumarin derivative, is very sensitive to the microenvironment in which it resides. In the present study, the effect of six bile salt variants on the photophysical behavior of Cou1 has been investigated. Dihydroxy (deoxycholates) as well as trihydroxy (cholates) bile salts with conjugated and unconjugated side chains have been chosen to get insight into the role of bile salt structure on the microenvironment of Cou1. Cou1 photophysics was found to be extremely sensitive to the aggregation process of the bile salt variants. The reduced polarity of the micellar environment stabilizes the planar intramolecular charge transferred state of Cou1, resulting in significant modulation in its photophysics in the bile salt media. The changes in the fluorescence parameters such as fluorescence intensity, emission energy, fluorescence quantum yield, anisotropy, and lifetime of Cou1 reveal that there is a distinct difference in the aggregation behavior of deoxycholates from that of cholates. The deoxycholates form micelles more or less critically similar to those of conventional surfactants, whereas the cholates self-assemble rather noncritically over a wide concentration range, thus signifying the vital role of the extra hydroxyl group in the aggregation pattern of trihydroxy bile salts. The conjugated bile salts are found to provide a relatively more compact, rigid, and hydrophobic microenvironment to Cou1 as compared to their unconjugated counterparts. Considering the significant modulation in the photophysical properties of Cou1, it has been employed as a molecular reporter for monitoring the aggregation process of bile salt variants and important information could be obtained about the effect of bile salt structure on the aggregation pattern and also about the micellar properties.

Micelle formation of bile salts and zwitterionic derivative as studied by two-dimensional NMR spectroscopy

The self-association of sodium taurodeoxycholate (NaTDC) and a zwitterionic derivative of cholic acid (CHAPS) in deuterium oxide was investigated by one- and two-dimensional nuclear magnetic resonance spectroscopy (NMR) spectroscopy. Analysis of the concentration dependence of the chemical shifts of several protons suggested that NaTDC and CHAPS form nonamers and heptamers, respectively, as well as dimer. The equilibrium constants of dimerization and the micellar aggregation numbers are close to the literature values. From the intensities of intermolecular cross-peaks in the nuclear Overhauser effect spectroscopy (NOESY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) spectra of NaTDC and CHAPS micellar solutions, partial structures of their micelles were estimated. The CHAPS micelle consists mainly of the back-to-back association, similarly to taurocholate (NaTC). However, the NaTDC micelle consists of the back-to-face association, because the face of NaTDC is rather hydrophobic. Furthermore, the back of bile molecules forms a convex plane and the face forms a concave plane. The back-to-face structure of NaTDC will be stabilized by a close contact between these planes. The chemical shift changes of several protons of CHAPS and NaTC in the micellar state are close to each other, but are different from those of NaTDC. This finding is consistent with the difference in their micellar structures.

Elasticity theory and shape transitions of viral shells

Recently, continuum elasticity theory has been applied to explain the shape transition of icosahedral viral capsids--single-protein-thick crystalline shells--from spherical to "buckled" or faceted as their radius increases through a critical value determined by the competition between stretching and bending energies of a closed two-dimensional (2D) elastic network. In the present work we generalize this approach to capsids with nonicosahedral symmetries, e.g., spherocylindrical and conical shells. One key additional physical ingredient is the role played by nonzero spontaneous curvature. Another is associated with the special way in which the energy of the 12 topologically required fivefold sites depends on the "background" local curvature of the shell in which they are embedded. Systematic evaluation of these contributions leads to a shape "phase" diagram in which transitions are observed from icosahedral to spherocylindrical capsids as a function of the ratio of stretching to bending energies and of the spontaneous curvature of the 2D protein network. We find that the transition from icosahedral to spherocylindrical symmetry is continuous or weakly first order near the onset of buckling, leading to extensive shape degeneracy. These results are discussed in the context of experimentally observed variations in the shapes of a variety of viral capsids.

Interaction of polysorbate 80 with erythropoietin: a case study in protein-surfactant interactions

PURPOSE

The cause of antibody positive pure red cell aplasia associated with the subcutaneous administration of EPREX to patients with chronic kidney failure has been determined to be due to the leaching of weakly adjuvant compounds from the uncoated rubber stoppers that were formerly used in prefilled syringes. Other researchers have suggested that polysorbate 80 micelles containing erythropoietin may be a causative factor. The purpose of this work was to repeat previously published studies in a more controlled manner and to define the precise nature of the interactions between polysorbate 80 and erythropoietin.

METHODS

The contents of EPREX prefilled syringes and laboratory-prepared, well-characterized formulations of EPREX were analyzed by size exclusion chromatography. Fractions were analyzed for the presence of erythropoietin by ELISA. EPREX formulations prepared with increasing amounts of polysorbate 80 were analyzed by light scattering.

RESULTS

Well-controlled chromatographic studies showed that when EPREX formulations containing no aggregate were analyzed by high-performance liquid chromatography, erythropoietin monomer could not be detected under the polysorbate 80 peak. Dimer and oligomers of erythropoietin coeluted under the polysorbate 80 peak as the molecular weights overlapped on the size exclusion chromatogram. Solution light scattering indicated that polysorbate 80 associates with erythropoietin in a defined stoichiometric ratio of 1:12.

CONCLUSIONS

Based on controlled studies, previous results suggesting that EPREX contains micelle-associated erythropoietin were incorrect. As with other surfactants and proteins, polysorbate 80 associates with erythropoietin in a defined stoichiometric ratio.

Ultraviolet spectroscopic estimation of microenvironments and bitter tastes of oxyphenonium bromide in cyclodextrin solutions

The UV absorbance and bitter taste of oxyphenonium bromide (OB), an antiacetylcholine drug, in cyclodextrin (CD) solutions are measured, and the local environment of the binding site and the reduction of the bitter taste intensity are quantitatively estimated from the UV data. The UV spectrum of OB is changed with the addition of alpha-, beta-, and gamma-CD, because the phenyl group of OB is included into the CD cavity. The maximum wavelength, lambda(max), senses environmental changes of OB best among several spectral characteristics. From comparison of lambda(max) between a CD solution and the reference ethanol-water and dioxane-water systems, the dielectric constant of the binding site is evaluated. This value leads us to estimate the microenvironment and structure of the binding site. The suppression of the bitter taste of 4 mM OB by CDs is in the increasing order alpha-CD < gamma-CD < beta-CD. The extent of this suppression can be quantitatively predicted from the UV absorbance by assuming that the free OB molecule alone exhibits the bitter taste, regardless of the kind and concentration of CD. Some implications and limitations of the present approach are discussed.

Partitioning of flunitrazepam into model membranes studied by temperature controlled gel filtration chromatography

The use of gel filtration chromatography with Sephadex as a separation medium was used in order to study flunitrazepam (FNTZ) partitioning into artificial model membranes consisting of dipalmitoyl-phosphatidylcholine (dpPC) vesicles, under controlled temperature conditions. In this system two phenomena are taking place simultaneously: the ligand-liposome interaction and the lipid self-aggregation to form the liposome. The liposome-FNTZ interaction was evidenced by the non-enantiography of the first derivative of FNTZ elution peak in frontal chromatography through Sephadex G-75. On the other hand, the presence of FNTZ reduced liposomes mean size and increased their size dispersion as evidenced by molecular filtration through Sephadex G-200. The dpPC-buffer FNTZ partition coefficient determined in zonal chromatography through Sephadex G-10 increased about 33% when the temperature rose above the temperature of dpPC transition from the liquid crystalline to the fluid phase. Gel filtration chromatography seems a suitable technique to study lipid liposome-FNTZ interactions at a qualitative level. In addition, this technique has the advantage over other methods of giving the possibility of observing the mutual effects exerted between the drug and the self-aggregating structure.