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PubMed2007

Nanostructures by self-assembling peptide amphiphile as potential selective drug carriers.

The self-assembling behavior, at physiological pH, of the amphiphile peptide (C18)(2)L5CCK8 in nanostructures is reported. Stable aggregates presenting a critical micellar concentration of 2 x 10(-6) mol kg(-1), and characterized by water exposed CCK8 peptide in beta-sheet conformation, are obtained. Small angle neutron scattering experiments are indicative for a 3D structure with dimensions > or =100 nm. AFM images confirm the presence of nanostructures. Fluorescence experiments indicating the sequestration of pyrene, chosen as drug model, and the anticancer Doxorubicin within the nanostructures are reported.

Antineoplastic Agents↗

PubMed2005

Salt effects on the phase behavior, structure, and rheology of chromonic liquid crystals.

Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10 degrees C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.

Cromolyn Sodium↗

PubMed2001

The formation of empty shells upon pressure induced decapsidation of turnip yellow mosaic virus.

The stability of turnip yellow mosaic virus (TYMV) was investigated under pressure, using solution neutron small angle scattering. Dissociation products were characterized by analytical ultracentrifugation and electron microscopy. At pH 6.0, TYMV remained unaffected by pressure, up to 260 Megapascals (MPa), the highest pressure reached in these experiments. At pH 8.0, TYMV remained unaffected by pressure up to 160 MPa, but decapsidated irreversibly above 200 MPa, giving rise to more and more empty shells upon increasing pressure. The organization of these empty shells was similar to that of the capsid of native virions, apart from the presence of a hole corresponding to the loss of a group of 5-8 coat protein subunits, through which the RNA may have escaped. At variance with other small isometric viruses, the capsid of TYMV never dissociated under pressure into subunits or small aggregates of subunits. This exceptional behavior of TYMV is probably due to the importance of van der Waals contacts and hydrogen bonds in the stability of its capsid.

Capsid↗

PubMed1994

Structure of dodecyl sulfate-protein complexes at subsaturating concentrations of free detergent.

Earlier neutron small-angle scattering experiments had revealed the low resolution structure of the complex between sodium dodecyl sulfate (SDS) and the single polypeptide (452 amino acid residues) of a water-soluble enzyme. The saturated complex consists of three globular micelles which are connected by short flexible polypeptide segments. New experiments, described here, were performed at subsaturating concentrations of free SDS in equilibrium with the complex. The data show a decrease in stoichiometry from one bound dodecyl sulfate (DS) anion per two amino acid residues near the critical micelle concentration (CMC) to one per four residues at half the CMC. At 0.3 CMC, a two-micelle complex is formed by the recombination of the small amino-terminal micelle with the middle one; and the center-to-center distance between the carboxyl-terminal micelle and the middle one decreases from 7.5 to 6.2 nm. These structural data allow us to better understand earlier results obtained with high-performance agarose gel chromatography of the same SDS-protein complexes.

Journal Article↗

PubMed1992

Structure of the proteolytic fragment F34 of calmodulin in the absence and presence of mastoparan as revealed by solution X-ray scattering.

The solution X-ray scattering technique has been applied to examine the conformations of the proteolytic fragment F34 (78Asp-148Lys) of calmodulin in the absence of both Ca2+ and mastoparan, in the presence of Ca2+ only, and in the presence of both Ca2+ and mastoparan. The radius of gyration and the molecular weight for the F34 fragment increased by 1.1 +/- 0.3 A and 19%, respectively, upon binding of both 2 mol of Ca2+/mol to the F34 fragment and mastoparan to form the tertiary complex. A smaller change was found for the Ca(2+)-saturated F34 fragment in the absence of mastoparan (0.3 +/- 0.3 A) without any change of the molecular weight. The analysis based on the small-angle scattering data showed that the F34 fragment in the presence of Ca2+ alone preserved the tertiary structure of the globular domain in the crystal to a great extent. Further analyses based on a two-domain model showed that the center-to-center distance between F34 and mastoparan is about 12.7 A, if the structure of the F34 fragment in the presence of mastoparan resembles that in the absence of mastoparan and if mastoparan in the complex retains an alpha-helical conformation. The modeling studies using their crystal structure coordinates have been made on the basis of the solution X-ray scattering data. The combined results support a model proposed by Persechini and Kretsinger [Persechini, A., & Kretsinger, R. H. (1988) J. Cardiovasc. Pharmacol. 12 (Suppl. 5), S1-S12], although the center-to-center distance between mastoparan and the F34 fragment is shorter by about 5 A than that in their model.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2006

Self-assembly of surfactant vesicles that transform into viscoelastic wormlike micelles upon heating.

Unilamellar vesicles are observed to form in aqueous solutions of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), when 5-methyl salicylic acid (5mS) is added at slightly larger than equimolar concentrations. When these vesicles are heated above a critical temperature, they transform into long, flexible wormlike micelles. In this process, the solutions switch from low-viscosity, Newtonian fluids to viscoelastic, shear-thinning fluids having much larger zero-shear viscosities (e.g., 1000-fold higher). The onset temperature for this transition increases with the concentration of 5mS at a fixed CTAB content. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles to micelles is a continuous one, with the vesicles and micelles coexisting over a narrow range of temperatures. The tunable vesicle-to-micelle transition and the concomitant viscosity increase upon heating may have utility in a range of areas, including microfluidics, controlled release, and tertiary oil recovery.

Cetrimonium↗

PubMed2003

Size and shape of the repetitive domain of high molecular weight wheat gluten proteins. II. Hydrodynamic studies.

This study describes the hydrodynamic properties of the repetitive domain of high molecular weight (HMW) wheat proteins, which complement the small-angle scattering (SANS) experiments performed in the first paper of this series. The sedimentation coefficients, s(0), and diffusion coefficients, D(0), were obtained from the homologous HMW proteins dB1 and dB4 that were cloned from the gluten protein HMW Dx5, and expressed in Escherichia coli. Monodisperse conditions for accurate determination of s(0) and D(0), were obtained by screening a series of buffers using dynamic light scattering. For the first time, hydrodynamic parameters were obtained on monodisperse samples that enabled the determination of the monomeric size and shape. The hydrodynamic values determined on dB1 and dB4 were used to test the worm-like chain (WLC) model that was proposed in the SANS studies. The successful matching of two separately obtained hydrodynamic parameters of dB1 and dB4 using the WLC model provides further evidence for the WLC model. The small discrepancy between the hydrodynamic and scattering data, possibly coming from the excluded volume effect, was compensated by a solvation layer of 1-2 water molecules thick around the protein in the WLC model. The solvation of the central domain is much higher than those of the terminal domains of the HMW subunits. This difference emphasizes the dual role of HMW wheat gluten proteins in water-binding and aggregation.

Biopolymers↗

PubMed1998

Interaction of gangliosides with proteins depending on oligosaccharide chain and protein surface modification.

By using neutron and synchrotron x-ray small-angle scattering techniques, we investigated the process of the complexation of gangliosides with proteins. We treated monosialoganglioside (G(M1)), disialoganglioside (G(D1a)), and a mixture of G(M1)/G(D1a). Proteins used were bovine serum albumins whose surfaces were modified with different sugars (deoxy-D-galactose, deoxy-L-fucose, deoxymaltitol, and deoxycellobiitol), which were used as model glycoproteins in a membrane. We found that the complexation of gangliosides with albumins greatly depends on the combination of ganglioside species and protein surface modification. With a varying protein/ganglioside ratio in a buffer solution at pH 7, the complexation of G(M1) or G(D1a) with albumins modified by monosaccharides appears to be less destructive for ganglioside aggregate structures in forming large complexes; the complexation of G(D1a) with the albumins modified by disaccharides induces the formation of complexes with a dimeric structure; and the complexation of G(M1) with albumins modified by disaccharides, to form small complexes, is very destructive. The present results show a strong dependence of the interaction between ganglioside and protein on the characteristics of the ganglioside and protein surface, which would relate to a physiological function of gangliosides, such as a function regulating the receptor activity of glycoproteins in a cell membrane.

Animals↗

PubMed1981

Structural dimorphism of bile salt/lecithin mixed micelles. A possible regulatory mechanism for cholesterol solubility in bile? X-ray structure analysis.

The three-dimensional structure of bile salt/lecithin mixed micelles in 0.15 M saline was derived from X-ray small-angle scattering measurements under various conditions. Two essentially different types of micelles were detected. At bile salt:lecithin molar ratios lower than approximately 2:1, lamellar particles similar to a lecithin bilayer arrangement were found. The thickness of the bilayer is 5.1 nm for mixed micelles having a molar ratio of 1:1. The lateral dimensions of the micelle were found strongly dependent upon molar ratio, increasing as lecithin content increases. In addition, it appeared that under certain incompletely defined conditions vesicular particles having diameters in the region of more than 100 nm occur. A molecular model of this type of micelle has been derived by means of a thorough interpretation of the electron density distribution across the plane of the bilayer. This model is generally consistent with the "mixed-disk" model of Mazer et al. [Mazer, N. A., Kwasnick, R. F., Carey, M. C., & Benedek, G. B. (1977) Micellization, Solubilization, Microemulsions, Proc. Int. Symp. 1, 383-402] and thus differs from that proposed by Small [Small, D. M. (1967) Gastroenterology 52,607-610] and Dervichian [Dervichian, D. G. (1968) Adv. Chem. Ser. No. 84, 78-87]. At molar ratios exceeding 2:1, a different type of micelle structure was found. This is a highly isometrical particle of globular shape, probably having a centrosymmetric arrangement of the molecular constituents. At a 3:1 bile salt:lecithin molar ratio, the mean physiological ratio for human gallbladder bile, the diameter of this micelle is 6.2 nm. Provided that other bile constituents have no significant influence on the structure, it may reasonably be expected that native gallbladder bile contains micelles of this latter type. The prevalent balance between the two micellar forms may thus regulate the capacity of bile to transport cholesterol.

Bile Acids and Salts↗

PubMed2006

Clouding phenomenon and SANS studies on tetra-n-butylammonium dodecylsulfate micellar solutions in the absence and presence of salts.

Clouding phenomenon in aqueous micellar solutions of an anionic surfactant tetra-n-butylammonium dodecylsulfate (TBADS) has been observed as a function of surfactant concentration. Small-angle neutron scattering (SANS) experiments in these systems show clustering of micelles as the temperature approaches the cloud point (CP). The individual micelles and the clusters of micelles coexist at CP. The clustering of micelles depends on the surfactant concentration and temperature. It is proposed that clustering is due to depletion of H-bonded water present around the butyl chains at the micellar surface. This is associated with entropy gain which is considered to be the major thermodynamic factor related to micellar aggregation. The structures (clusters) that emerge depend on the relative lengths of the alkyl chains of the counterion and can be tuned by the temperature.

Micelles↗

PubMed2006

Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles.

Small-angle neutron scattering (SANS) from cationic wormlike micellar solutions composed of hexadecyltrimethylammonium bromide (CTABr) and hexadecylpyridinium bromide (CPyBr) in deuterated water was studied at 40 degrees C as a function of surfactant and salt concentrations. Two scattering functions of semiflexible chains incorporating excluded volume effects, with and without the intermicellar interactions, were used in SANS data model fitting. Two needed changes were made in the well-accepted models. Extensive and systematic SANS data analysis suggests the robustness of these corrected scattering functions when the intermicellar interactions are included. The influence of the headgroups and ionic strength on the contour length and micellar flexibility of these two systems was demonstrated on the basis of the quantitative structural information obtained from the model fitting. Micellar flexibility was found to depend on surfactant concentration, even when intermicellar interactions were taken into account, despite predictions to the contrary.

Cations↗

PubMed1997

Temperature-induced structural changes in the myosin thick filament of skinned rabbit psoas muscle.

By using synchrotron radiation and an imaging plate for recording diffraction patterns, we have obtained high-resolution x-ray patterns from relaxed rabbit psoas muscle at temperatures ranging from 1 degree C to 30 degrees C. This allowed us to obtain intensity profiles of the first six myosin layer lines and apply a model-building approach for structural analysis. At temperatures 20 degrees C and higher, the layer lines are sharp with clearly defined maxima. Modeling based on the data obtained at 20 degrees C reveals that the average center of the cross-bridges is at 135 A from the center of the thick filament and both of the myosin heads appear to wrap around the backbone. At 10 degrees C and lower, the layer lines become very weak and diffuse scattering increases considerably. At 4 degrees C, the peak of the first layer line shifts toward the meridian from 0.0047 to 0.0038 A(-1) and decreases in intensity approximately by a factor of four compared to that at 20 degrees C, although the intensities of higher-order layer lines remain approximately 10-15% of the first layer line. Our modeling suggests that as the temperature is lowered from 20 degrees C to 4 degrees C the center of cross-bridges extends radially away from the center of the filament (135 A to 175 A). Furthermore, the fraction of helically ordered cross-bridges decreases at least by a factor of two, while the isotropic disorder (the temperature factor) remains approximately unchanged. Our results on the order/disordering effects of temperature are in general agreement with earlier results of Wray [Wray, J. 1987. Structure of relaxed myosin filaments in relation to nucleotide state in vertebrate skeletal muscle. J. Muscle Res. Cell Motil. 8:62a (Abstr.)] and Lowy et al. (Lowy, J., D. Popp, and A. A. Stewart. 1991. X-ray studies of order-disorder transitions in the myosin heads of skinned rabbit psoas muscles. Biophys. J. 60:812-824). and support Poulsen and Lowy's hypothesis of coexistence of ordered and disordered cross-bridge populations in muscle (Poulsen, F. R., and J. Lowy. 1983. Small angle scattering from myosin heads in relaxed and rigor frog skeletal muscle. Nature (Lond.). 303:146-152.). However, our results added new insights into the disordered population. Present modeling together with data analysis (Xu, S., S. Malinchik, Th. Kraft, B. Brenner, and L. C. Yu. 1997. X-ray diffraction studies of cross-bridges weakly bound to actin in relaxed skinned fibers of rabbit psoas muscle. Biophys. J. 73:000-000) indicate that in a relaxed muscle, cross-bridges are distributed in three populations: those that are ordered on the thick filament helix and those that are disordered; and within the disordered population, some cross-bridges are detached and some are weakly attached to actin. One critical conclusion of the present study is that the apparent order <--> disorder transition as a function of temperature is not due to an increase/decrease in thermal motion (temperature factor) for the entire population, but a redistribution of cross-bridges among the three populations. Changing the temperature leads to a change in the fraction of cross-bridges located on the helix, while changing the ionic strength at a given temperature affects the disordered population leading to a change in the relative fraction of cross-bridges detached from and weakly attached to actin. Since the redistribution is reversible, we suggest that there is an equilibrium among the three populations of cross-bridges.

Actin Cytoskeleton↗

PubMed2006

Structural variations in a family of orthodialkoxyarenes organogelators.

A series of low mass organic gelators (LMOGS) 1 to 6 whose chemical structures have in common an orthodialkoxyarene feature was prepared in order to compare the shape of their fibrillar network as investigated by small-angle neutron scattering (SANS) experiments. All members of the family exhibit a pronounced tendency to bundle formation by merging isolated fibers in extended packets of average diameter >500 A. Variations of the 2D packing symmetry are observed from hexagonal to square orderings with close derivatives of the reference member 2,3-didecyloxyanthracene (DDOA). Networks in which the fraction of isolated fibers is significant enough to allow for their SANS identification are those for 6,7-dichloro-2,3-didecyloxyanthracene (Cl(2)DDOA) and 2,3-dodecyloxy-9,10-anthraquinone (5). For this latter, the monodispersity of the cross-sections (thickness=74 A) is remarkable and the rectangular shape (b/a approximately 0.12) accounts for a merging mechanism into anisotropic bundles (ribbons).

Anisotropy↗

PubMed2006

Conformation of the HIV-1 Gag protein in solution.

A single multi-domain viral protein, termed Gag, is sufficient for assembly of retrovirus-like particles in mammalian cells. We have purified the human immunodeficiency virus type 1 (HIV-1) Gag protein (lacking myristate at its N terminus and the p6 domain at its C terminus) from bacteria. This protein is capable of assembly into virus-like particles in a defined in vitro system. We have reported that it is in monomer-dimer equilibrium in solution, and have described a mutant Gag protein that remains monomeric at high concentrations in solution. We report that the mutant protein retains several properties of wild-type Gag. This mutant enabled us to analyze solutions of monomeric protein. Hydrodynamic studies on the mutant protein showed that it is highly asymmetric, with a frictional ratio of 1.66. Small-angle neutron scattering (SANS) experiments confirmed its asymmetry and yielded an R(g) value of 34 A. Atomic-level structures of individual domains within Gag have previously been determined, but these domains are connected in Gag by flexible linkers. We constructed a series of models of the mutant Gag protein based on these domain structures, and tested each model computationally for its agreement with the experimental hydrodynamic and SANS data. The only models consistent with the data were those in which Gag was folded over, with its N-terminal matrix domain near its C-terminal nucleocapsid domain in three-dimensional space. Since Gag is a rod-shaped molecule in the assembled immature virion, these findings imply that Gag undergoes a major conformational change upon virus assembly.

Circular Dichroism↗

PubMed2005

Highly stable phospholipid unilamellar vesicles from spontaneous vesiculation: A DLS and SANS study.

Spontaneously formed unilamellar vesicles (ULV) composed of short- and long-chain phospholipids, dihexanoyl phosphorylcholine (DHPC) and dimyristoyl phosphorylcholine (DMPC), respectively, were doped with a negatively charged lipid, dimyristoyl phosphorylglycerol (DMPG), and studied with small-angle neutron scattering (SANS) and dynamic light scattering (DLS). Upon dilution, the spontaneous formation of vesicles was found to take place from bilayered micelles, or so-called "bicelles". SANS and DLS data show that ULV with narrow size distributions are highly stable at low lipid (C(lp) < 0.50 wt %) and NaCl salt (C(s)) concentrations. ULV size was found to be independent of both C(lp) and C(s) when they were below 0.33 and 0.5 wt %, respectively. Surface charge and salinity were found to be important factors in preparing ULV of a certain size. This observation is not in complete agreement with previous experimental results and cannot be completely explained with current theoretical predictions based on equilibrium calculations for catanionic surfactant mixtures. ULV size is found to be invariant over a wide range of temperatures, both below and above the phase-transition temperature, T(M), of DMPC, and was stable for periods of weeks and months, even after sonication.

Glycerophosphates↗

PubMed2005

Multilamellarity, structure and hydration of extruded POPC vesicles by SANS.

The small-angle neutron scattering (SANS) data of 12 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) dispersions at low lipid concentration (1 mg per 100-mg heavy water) prepared by 5, 9 and 29 extrusions through filters of pores with 50, 100, 200 and 400 nm diameter are presented. They were analyzed within a theory that permits the determination of both structural and hydration parameters of the bilayers as well as the portions of multilamellar vesicles in dispersions with negligible long-range order between the vesicles. The scattering length density profile across the bilayers is approximated by assuming a central hydrocarbon core surrounded by a water-accessible coat. It is modeled by two different forms of functions. In the boat model, the scattering length density of the coat changes linearly from core to water, whereas in the strip model it is constant across the water-accessible coat. It was found that the boat model reflects the reality better than the strip model. The decrease of the multilamellar vesicle portions, either with increasing the number of extrusions at same filter size and with decreasing the filter size, was characterized quantitatively.

Lipid Bilayers↗

PubMed2006

Charge stoichiometry inside polyelectrolyte-protein complexes: a direct SANS measurement for the PSSNa-lysozyme system.

We study by small angle neutron scattering and UV titration how the ratio of negative to positive charges, [-]/[+](intro), acts on the structure of complexes formed by short negatively charged polyelectrolyte chains (PSS) and globular positively charged proteins (lysozyme). The range of [-]/[+](intro) lies between 0.65 and 3.33. In all ratios, dense primary complexes are formed with radii around 10 nm. The species composition and the water content of the primary complexes are precisely obtained by the systematic use of the contrast matching of (deuterated) polymer or protein in SANS, yielding the compactness and the inner charge ratio [-]/[+](inner). The primary complexes have (i) an inner charge ratio [-]/[+](inner) close to 1 whatever [-]/[+](intro), (ii) a high total volume fraction (0.25-0.4), (iii) a constant radius (75 A) for [-]/[+](intro) 1, and (iv) a shell of PSS chains when [-]/[+](intro) > 1. Moreover, UV titration shows that there are free proteins if [-]/[+](introduced) < 1 and free PSS chains if [-]/[+](intro) is largely superior to 1. Hence, we observe that the primary complexes reach a finite size, controlled by electrostatic repulsion, and then aggregate at a higher scale with a fractal dimension of 2.1 characteristic of reaction-limited colloidal aggregation.

Electrolytes↗

PubMed2006

Charge-induced unilamellar vesicle formation and phase separation in solutions of Di-n-decylmethylamine oxide.

A double-tail amine oxide surfactant, di-n-decylmethylamine oxide (2C10MAO), was prepared, and the effects of protonation on aggregate structure were examined by small-angle neutron scattering (SANS), cryo-transmission electron microscopy (cryo-TEM), turbidity, electric conductivity, and solubilization of an oil-soluble dye at various degrees of neutralization, X, defined as the mole ratio of HCl/2C10MAO. The surfactant makes an L(2) phase in the nonprotonated state (X = 0) in water. The L(2) phase is in equilibrium with an aqueous L(1) phase. On protonation, unilamellar vesicles (ULVs) are formed over a wide range of compositions (0.05 < X< 0.4-0.5 at C = 10 mM) as observed by cryo-TEM. At X = 0.2, the ULV is stable over a wide concentration range (3 mM < or = C < 0.1 M), but an L(alpha) phase replaces the vesicle phase at C > 0.1 M. SANS results show that the mean radius of the ULV is about 25 nm and the bilayer thickness is about 2 nm, consistent with the extended configuration of the alkyl chains of the surfactant. An important contribution to the enhanced stability of the bilayer structures over the L(2) phase is suggested to be the translational entropy of the counterions. The enhanced stability of the bilayers diminishes as the counterion concentration increases either by an increase of X or by the addition of a salt. When the counterion concentration exceeds a critical value, the ULV solutions transform into the L(2) phase (or L(2)/L(1) two-phase system at low surfactant concentrations). The critical composition X is about 0.4-0.5 in water, but it is below 0.4 in D(2)O. The critical NaCl concentration is below 5 mM at X = 0.2. The stability of ULVs against multilamellar vesicles is ascribed partly to undulation forces and partly to the adjustable nature of the spontaneous curvature of amine oxide monolayers. The characteristics of the ULV of the surfactant remain the same within a temperature range 25-50 degrees C at X = 0.2. An iridescent lamellar phase and possibly an L(3) phase were observed in a very narrow X range (0 < X < 0.02) prior to the vesicle phase.

Cryoelectron Microscopy↗