Curvature effect on the structure of phospholipid bilayers

High-resolution small-angle X-ray scattering (SAXS), complemented by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments, was used to study the effect of curvature on the bilayer structure of dioleoyl-phosphatidylcholine (DOPC) and dioleoyl-phosphatidylserine (DOPS) unilamellar vesicles (ULVs). Bilayer curvature, as a result of finite vesicle size, was varied as a function of vesicle radius and determined by DLS and SANS measurements. Unilamellarity of large DOPC ULVs was achieved by the addition of small amounts (up to 4 mol %) of the charged lipid, DOPS. A comparison of SANS data over the range of 0.02 < q <0.2 A-1 indicated no change in the overall bilayer thickness as a function of ULV diameter (620 to 1840 A). SANS data were corroborated by high-resolution (0.06 < q <0.6 A-1) SAXS data for the same diameter ULVs and data obtained from planar samples of aligned bilayers. Both the inner and outer leaflets of the bilayer were found to be indistinguishable. This observation agrees well with simple geometric models describing the effect of vesicle curvature. However, 1220-A-diameter pure DOPS ULVs form asymmetric bilayers whose structure can most likely be rationalized in terms of geometrical constraints coupled with electrostatic interactions, rather than curvature alone.     

Formation of kinetically trapped nanoscopic unilamellar vesicles from metastable nanodiscs

Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.     

Fluorescent and highly stable unimodal DMPC based unilamellar vesicles formed by spontaneous curvature

The formation of uniform and highly stable unilamellar vesicles (ULVs) and the theory behind it are ongoing tasks within the vesicle community. Herein, we report the formation of highly stable, fluorescent, and unimodal 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) based ULVs with an average size of ~100 nm, as determined by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS). The ULVs are formed by mixing a two-component powder mixture or mixed lipid film of DMPC and 5 mol % of a novel amphiphilic carbenium salt, sodium 2-didecylamino-6,10-bis(N-methyltaruino)-4,8,12-trioxatriangulenium (Na-DSA) in aqueous solution when subjected to shaking. We propose that the high stability and the unimodal size distribution of the 5% DSA ULVs confirmed by DLS studies are a product of spontaneous curvature. UV-vis absorption/emission studies reveal that the structure of DSA promotes a strong interaction between the DMPC and the DSA to take place due to the complementary charge distribution of the DSA and DMPC head groups. The strong interaction may introduce an asymmetric amphiphile composition in the inner and outer leaflet of the bilayer which drives the spontaneous curvature.     

On the effect of the solid support on the interleaflet distribution of lipids in supported lipid bilayers

The adsorption and spreading of lipid vesicles on solid supports has become a popular way to create supported lipid bilayers (SLBs), but little attention has been paid to the possible redistribution of lipid material between the two leaflets of an SLB. We use the technique of quartz crystal microbalance with dissipation monitoring (QCM-D) to follow the adsorption of prothrombin on SLBs formed from sonicated unilamellar vesicles containing mixtures of dioleoylphosphatidylcholine (DOPC) and dioleoylphospatidylserine (DOPS). The specific interaction of prothrombin with negatively charged lipids is quantified and serves as a reporter of the content of accessible DOPS in SLBs. We compare results obtained on silica and mica and find that the underlying support can induce substantial redistribution of lipid material between the two leaflets. In particular, SLBs formed on mica showed a substantially depleted amount of accessible DOPS in the presence of calcium. The mechanisms that lead to the lipid redistribution process are discussed.     

Low-pH-induced transformation of bilayer membrane into bicontinuous cubic phase in dioleoylphosphatidylserine/monoolein membranes

Cubic biomembranes, nonbilayer membranes with connections in three-dimensional space that have a cubic symmetry, have been observed in various cells. Interconversion between the bilayer liquid-crystalline (L(alpha)) phase and cubic phases attracted much attention in terms of both biological and physicochemical aspects. Herein we report the pH effect on the phase and structure of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes under a physiological ion concentration condition, which was revealed by small-angle X-ray scattering (SAXS) measurement. At neutral pH, DOPS/MO membranes containing high concentrations of DOPS were in the L(alpha) phase. First, the pH effect on the phase and structure of the multilamellar vesicles (MLVs) of the DOPS/MO membranes preformed at neutral pH was investigated by adding various low-pH buffers into the MLV suspension. For 20%-DOPS/80%-MO MLVs, at and below pH 2.9, a transition from the L(alpha) to cubic (Q(224)) phase occurred within 1 h. This phase transition was reversible; a subsequent increase in pH to a neutral one in the membrane suspension transformed the cubic phase into the original L(alpha) phase. Second, we found that a decrease in pH transformed large unilamellar vesicles of DOPS/MO membranes into the cubic phase under similar conditions. We have proposed the mechanism of the low-pH-induced phase transition and also made a quantitative analysis on the critical pH of the phase transition. This finding is the first demonstration that a change in pH can induce a reversible phase transition between the L(alpha) and cubic phases of lipid membranes within 1 h.     

A small-angle neutron and static light scattering study of micelles formed in aqueous mixtures of a nonionic alkylglucoside and an anionic surfactant

The size and shape of micelles formed in aqueous mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic sugar-based surfactant n-decyl beta-D-glucopyranoside (C(10)G) at different concentrations of added salt have been investigated with small-angle neutron and static light scattering. Rather small prolate ellipsoidal micelles form in the absence of added salt and at [NaCl] = 10 mM in D(2)O. The micelles grow considerably in length to large rods as the electrolyte concentration is raised to [NaCl] = 0.1 M. In excess of nonionic surfactant ([SDS]/[C(10)G] = 1:3) at [NaCl] = 0.1 M in D(2)O, several thousands of Angstroms long wormlike micelles are observed. Most interestingly, a conspicuously large isotope solvent effect was observed from static light scattering data according to which micelles formed at [SDS]/[C(10)G] = 1:3 and [NaCl] = 0.1 M in H(2)O are at least five times smaller than micelles formed in the corresponding samples in D(2)O.     

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.     

Contrast variation SANS investigation of composition distributions in mixed surfactant micelles

Small angle neutron scattering measurements have been performed on three systems (HFDeP-d5-C (N-1(1,1,2,2-tetrahydroperfluorodecanoyl)pyridinium-d5 chloride)/C16PC in 63 mM NaCl; HFDeP-d5-C/C12PC in 200 mM NaCl, and as an example of an ideally mixed system, SDS/SDS-d25 in 200 mM NaCl) containing micelles formed in a binary mixture of surfactants, in order to investigate the composition distribution of the mixed micelles. The experimental data were collected varying the contrast between the average scattering length density of micelles and aqueous solvent by changing the H2O/D2O ratio. Analysis of data includes a model-independent approach--the indirect Fourier transformation method and direct modeling-simultaneous fit at all contrasts by the scattering from micelles of equal size and shape with composition distribution and an effective interaction. It has earlier been shown (Almgren, M.; Garamus, V. M. J. Phys. Chem. B 2005, 109, 11348) that for micelles of equal size, independent of the composition, and with negligible intermicellar interactions, the scattered intensity at zero angle varies quadratically with the contrast, with the minimum intensity at the nominal match point proportional to sigma2, the variance of the micelle composition distribution. Within the regular solution framework, the composition distribution and its variance are uniquely defined by the value of the interaction parameter and the micelle aggregation number. At 25 degrees C, the first system gave sigma = 0.37, corresponding to a broad, bimodal composition distribution, the second sigma = 0.22, a broad distribution with a shallow minimum at the midpoint. For SDS/SDS-d25, we found sigma = 0.006 +/- 0.030, which is a smaller value than that of the binominal composition distribution expected for an ideally mixed system.     

1-辛烯Ziegler-Natta聚合的活性中心多分散性——分子量分布及动力学的研究

本文通过对TiCl_3-Al(C_2H_5)_3聚合1-辛烯产物的双峰型分子量分布曲线用Schulz函数进行拟合、分峰处理,发现这种双峰型分布中的高分子量峰部分由一种活性中心生成,低分子量峰部分则由至少两种活性中心生成.将不同转化率时聚辛烯的分子量分布分峰拟合数据与聚合速率、活性中心数等测定数据相结合,得到了高分子量和低分子量两部分产物相应的活性中心上各自的动力学参数(增长速率常数、活性中心数、链转移常数等)及各自的聚合速率曲线,从而证实了多种活性中心的存在、它们催化特性的差异及与聚合物分子量分布增宽的关系.     

1-辛烯Ziegler-Natta聚合的活性中心多分散性——分子量分布及动力学的研究

 本文通过对TiCl₃-Al(C₂H₅)₃聚合1-辛烯产物的双峰型分子量分布曲线用Schulz函数进行拟合、分峰处理,发现这种双峰型分布中的高分子量峰部分由一种活性中心生成,低分子量峰部分则由至少两种活性中心生成.将不同转化率时聚辛烯的分子量分布分峰拟合数据与聚合速率、活性中心数等测定数据相结合,得到了高分子量和低分子量两部分产物相应的活性中心上各自的动力学参数(增长速率常数、活性中心数、链转移常数等)及各自的聚合速率曲线,从而证实了多种活性中心的存在、它们催化特性的差异及与聚合物分子量分布增宽的关系.     

Micelle to vesicle transitions of N-dodecyl-1, ω-diaminoalkanes: effects of pH, temperature and salt

The self-assembly behavior of pH-sensitive amphiphiles N-dodecyl-1, 2-diaminoethane (C12N2N), N-dodecyl-1, 3-diaminopropane (C12N3N) and N-dodecyl-1, 4-diaminobutane (C12N4N) has been studied in aqueous solutions. Light scattering, viscosity and cryo-transmission electronic microscopy (cryo-TEM) results revealed that the aggregates transferred from spherical micelles to vesicles (MVT) via wormlike micelles as the pH was gradually varied from acidic to basic conditions. pH-dependent zeta potential and (1)H NMR studies confirmed these transitions. Interestingly, the formed wormlike micelles could transform into vesicles upon heating, which was studied by cryo-TEM, light scattering and viscosity techniques in detail. It is concluded that the pH and thermal MVT are a general phenomenon in all three amphiphiles investigated. Furthermore, NaCl induced a wormlike micelle to vesicle transition was also observed in C12N2N solution.     

Kinetics of low pH-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine∕monoolein

Recently, it has been well recognized that the modulation of electrostatic interactions due to surface charges can induce transitions between lamellar liquid-crystalline (L(α)) and inverse bicontinuous double-diamond cubic (Q(II)(D)) phases in biological lipids. To reveal their kinetic pathway and mechanism, we investigated the low pH-induced L(α) to Q(II)(D) phase transitions in 20%-dioleoylphosphatidylserine (DOPS)/80%-monoolein (MO) using time-resolved small-angle x-ray scattering and a rapid mixing method. At a final pH of 2.6-2.9, the L(α) phase was transformed completely into the hexagonal II (H(II)) phase within 2-10 s after mixing a low pH buffer with a suspension of multilamellar vesicles of 20%-DOPS∕80%-MO (the initial step). Subsequently, the H(II) phase slowly converted into the Q(II)(D) phase and completely disappeared within 15-30 min (the second step). The rate constants of the second step were obtained using the singular value decomposition analysis. On the basis of these data, we discuss the underlying mechanism of the kinetic pathway of the low pH-induced L(α) to Q(II)(D) phase transitions.     

Highly stable polymerizable vesicles in anionic surfactant/ammonium salt mixtures in the presence of cross-linking monomers for convenient preparation of hollow nanospheres

Spontaneous and stable vesicles are formed from vinylbenzyltrimethylammonium chloride (VBTAC) and sodium dodecyl sulfate (SDS) ranging in molar composition from 3:7 to 7:3 in the presence of 30 mol % divinylbenzene (DVB). Dynamic light scattering analysis and transmission electron microscope observations revealed that microparticles with diameters around 120 nm (30%) and 580 nm (70%) were formed. Trapping efficiency of the vesicles examined with D-(+)-glucose amounted to ca. 15%. These vesicles were capable of undergoing polymerization in the presence of water-soluble radical initiators such as 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride to fix the vesicular structure. Retention of the vesicle size and bimodal size distribution characteristics during the polymerization indicated that intravesicular polymerization prevailed over the intervesicular process. SDS was stably retained in the cross-linked vesicle and was not removed from the vesicle by exhaustive dialysis or ultrafiltration, due to the electrostatic interaction within the cross-linked polymeric framework produced from VBTAC and DVB. The resulting hollow nanospheres are readily redispersed in water without the aid of additional surfactants.     

Vibrational distributions of the CO(v) products of the C2H2 + O(3P) and HCCO + O(3P) reactions studied by FTIR emission

The C2H2 + O(3P) and HCCO + O(3P) reactions are investigated using Fourier transform infrared (FTIR) emission spectroscopy. The O(3P) radicals are produced by 193 nm photolysis of an SO2 precursor or microwave discharge in O2. The HCCO radical is either formed in the first step of the C2H2 + O(3P) reaction or by 193 nm photodissociation of ethyl ethynyl ether. Vibrationally excited CO and CO2 products are observed. The microwave discharge experiment [C2H2 + O(3P)] shows a bimodal distribution of the CO(v) product, which is due to the sequential C2H2 + O(3P) and HCCO + O(3P) reactions. The vibrational distribution of CO(v) from the HCCO + O(3P) reaction also shows its own bimodal shape. The vibrational distribution of CO(v) from C2H2 + O(3P) can be characterized by a Boltzmann plot with a vibrational temperature of approximately 2400 +/- 100 K, in agreement with previous results. The CO distribution from the HCCO + O(3P) reaction, when studied under conditions to minimize other processes, shows very little contamination from other reactions, and the distribution can be characterized by a linear combination of Boltzmann plots with two vibrational temperatures: 2320 +/- 40 and 10 300 +/- 600 K. From the experimental results and previous theoretical work, the bimodal CO(v) distribution for the HCCO + O(3P) reaction suggests a sequential dissociation process of the HC(O)CO++ --> CO + HCO; HCO --> H + CO.     

Size control of mixed egg yolk phosphatidylcholine (EggPC)/oleate vesicles

The size distribution after addition of oleate surfactant to preformed Egg yolk phosphatidylcholine (EggPC) vesicles was investigated by gel filtration chromatography combining with dynamic light scattering. Phospholipid and oleate concentration, fluorescence intensity and size of the vesicles were measured for each elution fraction. The spontaneous vesiculation of oleate at pH 8.5 was accelerated in the presence of preformed EggPC vesicles. The size distribution of newly formed vesicles was dependent on the preformed vesicular size. For example, oleate addition to large preformed vesicles (230 nm) resulted in altering of vesicles to both larger and smaller than preformed ones, while addition of oleate to small preformed vesicles (50 nm) led to the formation of only larger vesicles without exhibition of newly small vesicles. The combinations of gel filtration chromatography and dynamic light scattering could provide more detailed insight into the size change of newly formed vesicles.     

A novel scheme of heterogeneous polymerization of ethylene

The heterogeneous polymerization of ethylene initiated by radiation in tert-butyl alcohol was studied. The polymerization was carried out in a 100-ml reactor at 25–100°C and pressures of 200–300 kg/cm² in the presence of 50 ml of tert-butyl alcohol containing 7 wt-% water. The amounts of polymerized monomer, the average molecular weight of polymer formed, and the molecular weight distribution of polymer were measured at various stages of reaction and at various temperatures. The molecular weight distribution was found to be very much dependent on the reaction time and temperature. For the polymer formed at 50–60°C in the very early stages of reaction, the molecular weight distribution is unimodal, and in the intermediate stage a shoulder appears at a molecular weight higher than the first peak which increases as the polymerization proceeds; eventually a bimodal curve is formed. The bimodal distribution curves were analyzed to determine the fractions and average molecular weights of the each peak. On the basis of these data for the molecular weight distribution and kinetic behavior, a new scheme for the heterogeneous polymerization is proposed which indicates that the polymerization proceeds via propagating radicals in two different physical states, namely, loose and rigid states.     

Angle dependent total cross sections and the optical theorem

Cross sections are either represented by generalized asymptotical partial wave expansions or obtained as a spherical average of an appropriate differential cross section. In these cases it is usually assumed that the total scattering cross section, as a property of a scattering object, does not depend on the incident angles. This viewpoint is supported by common knowledge in connection with low energy scattering. However this unconscious belief is not always correct. In the present paper we will show that a non-spherical scatterer may exhibit strong dependence on the incident direction. To do this we will represent the scattering data of the most general potential, separable in ellipsoidal coordinates, in perturbed ellipsoidal (Lamé) wave functions. These functions arise when variables in the Schr?dinger equation are separated in an ellipsoidal coordinate system. The Lamé wave functions are analogous to spherical- and Bessel functions in the case of spherical symmetry. We will expand the total scattering cross section and derive the optical theorem explicitly demonstrating the incident angle dependence for such a class of potentials. As an illustration we will present and display some calculations of the total cross section versus incident direction. Unexpected behavior will be discussed and explained. We also use results from classical acoustic scattering by a triaxial ellipsoid. The general character of the ellipsoidal coordinate system is emphasized.     

Surface-dependent transitions during self-assembly of phospholipid membranes on mica, silica, and glass

Formation of supported membranes by exposure of solid surfaces to phospholipid vesicles is a much-used technique in membrane research. Freshly cleaved mica, because of its superior flatness, is a preferred support, and we used ellipsometry to study membrane formation kinetics on mica. Neutral dioleoyl-phosphatidylcholine (DOPC) and negatively charged dioleoyl-phosphatidylserine/dioleoyl-phosphatidylcholine (20% DOPS/80% DOPC) vesicles were prepared by sonication. Results were compared with membrane formation on silica and glass, and the influence of stirring, buffer, and calcium was assessed. Without calcium, DOPC vesicles had a low affinity (Kd approximately 30 microM) for mica, and DOPS/DOPC vesicles hardly adsorbed. Addition of calcium promptly caused condensation of the adhering vesicles, with either loss of excess lipid or rapid additional lipid adsorption up to full surface coverage. Vesicle-mica interactions dominate the adsorption process, but vesicle-vesicle interactions also seem to be required for the condensation process. Membranes on mica proved unstable in Tris-HCl buffer. For glass, transport-limited adsorption of DOPC and DOPS/DOPC vesicles with immediate condensation into bilayers was observed, with and without calcium. For silica, vesicle adsorption was also rapid, even in the absence of calcium, but the transition to condensed layers required a critical surface coverage of about 50% of bilayer mass, indicating vesicle-vesicle interaction. For all three surfaces, additional adsorption of DOPC (but not DOPS/DOPC) vesicles to condensed membranes was observed. DOPC membranes on mica were rapidly degraded by phospholipase A2 (PLA2), which pleads against the role of membrane defects as initial PLA2 targets. During degradation, layer thickness remained unchanged while layer density decreased, in accordance with recent atomic force microscopy measurements of gel-phase phospholipid degradation by PLA2.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Multilayer vesicles and vesicle clusters formed by the fullerene-based surfactant C60(CH3)5K

The self-assembly behavior of a fullerene-based surfactant, C60(CH3)5K, in water was studied using a combination of static and dynamic light scattering, as well as transmission electron microscopy, and compared to that of the compound C60(C6H5)5K. Both fullerene surfactant systems spontaneously assemble into large vesicles consisting of closed spherical shells formed by bilayers, with critical aggregation concentrations (CAC) lower than 10(-6) g ml(-1). At low concentrations, the aggregate sizes of C60(CH3)5K (radius R approximately 26.8 nm) and C60(C6H5)5K (R approximately 17.0 nm) were found to be substantially different from each other, showing that the change of the substituents surrounding the polar cyclopentadienide head group makes it possible to control the size of the resulting aggregates. Furthermore, the C60(CH3)5K vesicles were found to exist in two qualitatively different types of aggregation with a critical reaggregation concentration (CRC) located at 3.30 x 10(-6) g ml(-1). Above the CRC, larger aggregates were observed (R approximately 37.6 nm), showing a more complex form of supramolecular aggregation, e.g., in terms of multi-bilayer vesicles and/or of clusters of bilayer vesicles.