An Alternative Approach to Quantify Partition Processes in Confined Environments: The Electrochemical Behavior of PRODAN in Unilamellar Vesicles

Herein, we investigate the behavior of the electroactive molecular probe 6‐propionyl‐2‐dimethyl amino naphthalene (PRODAN) in large unilamellar vesicles (LUV) formed with the phospholipid 1,2‐di‐oleoyl‐sn‐glycero‐3‐phosphatidylcholine (DOPC) by using cyclic voltammetry (CV). The CV studies in pure water confirm our previous spectroscopic results that PRODAN self‐aggregates due to its low water solubility. Moreover, the electrochemical results also reveal that the PRODAN aggregated species are non‐electroactive within the studied electrochemical potential region. In DOPC LUV media, the redox behavior of PRODAN shows how the LUV bilayer interacts with PRODAN aggregated species to form PRODAN monomer species. Moreover, the electrochemical response of PRODAN allows us to propose a model for explaining the electrochemical experimental results and—in conjunction with our measurements—for calculating the value of the partition constant (K_p) of PRODAN between the water and LUV bilayer pseudophases. This value coincides with that obtained through an independent technique. Moreover, our electrochemical model allows us to calculate the diffusion coefficient (D) for the DOPC LUV, which coincides with the D value obtained through dynamic light scattering (DLS). Thus, our data clearly show that electrochemical measurements could be a powerful alternative approach to investigate the behavior of nonionic electroactive molecules embed in a confined environment such as the LUV bilayer. Moreover, we believe that this approach can be used to investigate the behavior of non‐optical molecular drugs embedded in bilayer media.     

New insights on the behavior of PRODAN in homogeneous media and in large unilamellar vesicles

The behavior of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) was studied in homogeneous media and in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization, and time-resolved spectroscopies. In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (pi) and the hydrogen bond donor ability (alpha) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (beta) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands gives a linear correlation with the well-known polarity parameter E(T30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes, 2.27 and 0.65 ns, while the aggregate has a lifetime of 14.6 ns. Using steady-state anisotropy measurements, the calculated volumes of the aggregate and the monomer are 5590 and 222 mL mol(-1), respectively. In DOPC LUVs, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant (K(p)) value is large enough that only at [DOPC] below 0.15 mg/mL PRODAN in water can be detected. PRODAN dissolved in LUVs at [DOPC] > 1 mg/mL exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipid tails. These environments were characterized by their fluorescence lifetimes (tau), showing that PRODAN in the polar microenvironment has a tau value of approximately 4 ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yields values similar to that of methanol and tetrahydrofuran.     

Interface water dynamics and porating electric fields for phospholipid bilayers

Lipid bilayers, normally a barrier to charged species and large molecules, are permeabilized by electric fields, a phenomenon exploited by cell biologists and geneticists for porating and transfecting cells and tissues. Recent molecular simulation studies have advanced our understanding of electroporation, but the relative contributions of atomically local details (interface water and headgroup dipole and counterion configurations) and medium- and long-range electrostatic gradients and changes in membrane structural shifts to the initiating conditions and mechanisms of pore formation remain unclear. Molecular dynamics simulations of electroporation in several lipid systems presented here reveal the effects of lipid hydrocarbon tail length and composition on the magnitude of the field required for poration and on the location of the initial sites of field-driven water intrusion into the bilayer. Minimum porating external fields of 260 mV nm(-1), 280 mV nm(-1), 320 mV nm(-1), and 380 mV nm(-1) were found for 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC), 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC), and 1,2-dioleoyl- sn-glycero-3-phosphatidylcholine (DOPC) bilayers, respectively, and correlated most strongly with the bilayer thickness. These phospholipid systems share several common features including a wide, dynamic distribution of the headgroup dipole angle with the bilayer normal ranging from 0 to 155 degrees that is only slightly shifted in a porating electric field, and similar electric field-induced shifts in water dipole orientation, although the mean water dipole moment profile at the aqueous-membrane interface is more sensitive to the electric field for DOPC than for the other phospholipids. The location of pore initiation, at the anode- or cathode-facing leaflet, varies with the composition of the bilayer and correlates with a change in the polarity of the localized electric field at the interface.     

Ultrathin spin-coated dioleoylphosphatidylcholine lipid layers in dry conditions: a combined atomic force microscopy and nanomechanical study

Atomic force microscopy (AFM) has been used to study the structural and mechanical properties of low concentrated spin-coated dioleoylphosphatidylcholine (DOPC) layers in dry environment (RH ≈ 0%) at the nanoscale. It is shown that for concentrations in the 0.1-1 mM range the structure of the DOPC spin-coated samples consists of an homogeneous lipid monolayer ～1.3 nm thick covering the whole substrate on top of which lipid bilayer (or multilayer) micro- and nanometric patches and rims are formed. The thickness of the bilayer structures is found to be ～4.5 nm (or multiples of this value for multilayer structures), while the lateral dimensions range from micrometers to tens of nanometer depending on the lipid concentration. The force required to break a bilayer (breakthrough force) is found to be ～0.24 nN. No dependence of the mechanical values on the lateral dimensions of the bilayer structures is evidenced. Remarkably, the thickness and breakthrough force values of the bilayers measured in dry environment are very similar to values reported in the literature for supported DOPC bilayers in pure water.     

Reverse micellar aggregates: effect on ketone reduction. 2. Surfactant role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.     

Inhibited phenol ionization in reverse micelles: confinement effect at the nanometer scale

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.     

Catalytic Activity of Hexokinase in Reversed Micelles

Reversed micelles can control the size of water pools and the physical property of water by changing W(0)(=[water]/[surfactant]). Hexokinase (HK) activity seems to be easily affected by the microenvironment in the neighborhood of the enzyme because it is assumed that HK binds to the outer mitochondrial membrane by insertion of its hydrophobic NH(2) tail. The catalytic activity of HK was examined in reversed micelles in order to study the effect of the microenvironment in the neighborhood of HK on the activity. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT), hexadecyltrimethyl ammonium chloride (HTAC), and octaoxyethylene dodecyl ether (C(12)E(8)) were used as anionic, cationic, and nonionic surfactants, respectively. HK activity was obtained by measuring ATP and ADP amounts with HPLC. The high electrostatic inner surfaces of AOT and HTAC reversed micelles were not favorable for HK to exhibit the catalytic activity, but the activity in HTAC reversed micelles was 2-3 times higher than that in AOT reversed micelles and the activities in both reversed micelles revealed an optimum at W(0)=10. The phenomenon was discussed in connection with the location of HK, nonuniform distribution of substrates, and the size and physical properties of the water pools. On the other hand, HK activity was much higher in C(12)E(8) reversed micelles than in AOT and HTAC reversed micelles and increased with the concentration of C(12)E(8). This suggests that HK activity is easily revealed in hydrated ethylene oxide chains. In conclusion, it was demonstrated that HK activity depends on the microenvironment such as the electrostatic field, the physical properties of water, and the hydrophobicity. Copyright 2001 Academic Press.     

Real structure of formamide entrapped by AOT nonaqueous reverse micelles: FT-IR and 1H NMR studies

Noninvasive techniques such as FT-IR and (1)H NMR spectroscopy have been employed to investigate the solubilization of formamide, FA, and its aqueous solution, FA-water, by sodium 1,4-bis(2-ethylhexyl)sulfosuccinate, AOT, in heptane or isooctane reverse micelles, respectively. Partially deuterated FA (FADH) was used in the FT-IR experiments and nu(OD), n(ND) were analyzed. Also, the nu(C=O) band of FA was investigated. For AOT, the changes of the SO(3)(-) group's symmetric, nu(s), and asymmetric, nu(a), bands were also studied. The results are showing that FA is interacting strongly with the Na+ counterions of the surfactant through electrostatic interactions maintaining their hydrogen bond network present in the FA bulk. Accordingly, partially deuterated FA is "frozen" inside the aggregates and it is possible to detect, by FT-IR technique, the cis and trans isomers. Curve fitting of the nu(OD) (in the FA-water mixture) band requires use of two peaks because the band is asymmetric, not because the solubilizate molecules are present in layers of different structure. The chemical shifts of the (1)H bound to N and C of FA were studied by (1)H NMR. The comparison of the chemical shift of AOT in reverse micelles with FA and the FA-water mixture in the polar core of the aggregate shows that there is a strong preferential solvation of Na+ by FA (through electrostatic interaction) and the AOT's sulfonate group by water (through hydrogen bond interaction).     

Headgroup hydration and mobility of DOTAP/DOPC bilayers: a fluorescence solvent relaxation study

The biophysical properties of liposome surfaces are critical for interactions between lipid aggregates and macromolecules. Liposomes formed from cationic lipids, commonly used to deliver genes into cells in vitro and in vivo, are an example of such a system. We apply the fluorescence solvent relaxation technique to study the structure and dynamics of fully hydrated liquid crystalline lipid bilayers composed of mixtures of cationic dioleoyltrimethylammoniumpropane (DOTAP) and neutral dioleoylphosphatidylcholine (DOPC). Using three different naphthalene derivatives as fluorescent dyes (Patman, Laurdan and Prodan) allowed different parts of the headgroup region to be probed. Wavelength-dependent parallax quenching measurements resulted in the precise determination of Laurdan and Patman locations within the DOPC bilayer. Acrylamide quenching experiments were used to examine DOTAP-induced dye relocalization. The nonmonotonic dependence of dipolar relaxation kinetics (occurring exclusively on the nanosecond time scale) on DOTAP content in the membrane was found to exhibit a maximum mean solvent relaxation time at 30 mol % of DOTAP. Up to 30 mol %, addition of DOTAP does not influence the amount of bound water at the level of the sn(1) carbonyls, but leads to an increased packing of phospholipid headgroups. Above this concentration, elevated lipid bilayer water penetration was observed.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Microheterogeneous Systems

The quantum yield (Φ_Δ) of singlet oxygen (O₂(¹Δ_g) production by 9H‐fluoren‐9‐one (FLU) is very sensitive to the nature of the solvent (0.02 in a highly polar and protic solvent, such as MeOH, to 1.0 in apolar solvents). This high sensitivity has been used for probing the interaction of FLU with micellar media and microemulsions based on anionic (sodium dodecyl sulfate, SDS; bis‐(2‐ethylhexyl)sodium sulfosuccinate, AOT), cationic (cetyltrimethylammonium chloride, CTAC) and nonionic (Triton X‐100, TX) surfactants. Values of Φ_Δ of FLU vary in a wide range (0.05–1.0) in both microheterogeneous media and neat solvent, and provide information on the microenvironment of FLU, i.e., on its localization within organized media. In ionic and nonionic micellar media, as well as in four‐component microemulsions, FLU is, to various extents, exposed to solvation by the polar and protic components of the microheterogeneous systems (water and/or butan‐1‐ol) in the micellar interfacial region (Φ_Δ=0.05–0.30). In contrast, in AOT reverse micelles (consisting of AOT as surfactant, cyclohexane as hydrophobic component, and water), FLU is located in the hydrophobic continuous pseudophase, and is totally separated from the micellar water pools (Φ_Δ≈1.0).     

Structure and dynamics of water at the interface with phospholipid bilayers

We have performed two molecular-dynamics simulations to study the structural and dynamical properties of water at the interface with phospholipid bilayers. In one of the simulations the bilayer contained neutral phospholipid molecules, dioleoylphosphatidylcholine (DOPC); in the second simulation the bilayer contained charged lipid molecules, dioleoylphosphatidylserine (DOPS). From the density profile of water we observe that water next to the DOPS bilayer is more perturbed as compared to water near the DOPC bilayer. Using an energetic criterion for the determination of hydrogen bonding we find that water molecules create strong hydrogen bonds with the headgroups of the phospholipid molecules. Due to the presence of these bonds and also due to the confinement of water, the translational and orientational dynamics of water at the interface are slowed down. The degree of slowing down of the dynamics depends upon the location of water molecules near a lipid headgroup.     

Fluorescence behaviour of 4-[5-(2-phenyloxazolyl)] benzenesulfonic acid and N-hexyl-4-[5-(2-phenyloxazolyl)] benzenesulfonamide in homogeneous micellar and microemulsion systems

The fluorescence and excitation spectra of 4-[5-(2-phenyloxazolyl)]benzenesulfonic acid (PPOS) and N-hexyl-4-[5-(2-phenyloxazolyl)] benzenesulfonamide (PPOSA) were investigated in homogeneous solutions of varying polarities (hexane, heptane, butanol and water) and in aqueous micellar systems of anionic surfactants (bis(2-ethylhexyl)sulfosuccinate, sodium salt (AOT), sodium dodecyl sulfate (SDS)) and cationic surfactants (benzyldimethylhexadecylammonium chloride (CDBA), hexadecyltrimethylammonium chloride (CTAB)). These compounds were also investigated in different oil-in-water (o/w) and water-in-oil (w/o) microemulsions of the system composed of SDS, n-butanol, cyclohexane and water. The results revealed that the two probes exhibit pronounced spectral changes in response to the changes in the polarity of the medium, and in hydrophobic—hydrophilic interactions. The spectral behaviour of PPOS and PPOSA in micellar systems indicates that these two probes are incorporated at the interface of the cationic micelles. In microemulsions, however, the probes exhibit different Stokes shifts compared with those found for homogeneous solutions, indicating different salvation processes of both the ground and the excited states.     

On the possibility that cyclodextrins' chiral cavities can be available on AOT n-heptane reverse micelles. A UV-visible and induced circular dichroism study

The formation of reverse micelles (RMs) of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) in n-heptane including two different beta-cyclodextrin (beta-CD) derivatives (hydroxypropyl-beta-CD, hp-beta-CD, and decenyl succinyl-beta-CD, Mod-beta-CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate, while beta-CD cannot. Using UV-vis and induced circular dichroism (ICD) spectroscopy and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene), it was possible to determine that Mod-beta-CD is located with its cavity at the oil side of the AOT RM interface, while for hp-beta-CD the cavity is inside the RM water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when dissolved in the AOT RMs with hp-beta-CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. The solvatochromic behavior of the dye depends not only on the polarity of the media but also on other specific solvent properties. A Kamlet-Taft analysis shows that the MO absorption spectrum shifts to longer wavelength with an increase in the solvent polarity-polarizability (pi*) and the hydrogen donor ability (alpha) of the medium. MO appears to be almost 3 times more sensitive to the pi* parameter than to the alpha parameter. In addition, from the MO absorption spectral changes with the hp-beta-CD concentration, the association equilibrium constants in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is available for a guest in an organic medium such as the RMs; therefore, we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.     

Comparison between two anionic reverse micelle interfaces: the role of water-surfactant interactions in interfacial properties

The water/sodium bis(2-ethylhexyl) phosphate (NaDEHP) reverse micelle (RM) system is revisited by using, for the first time, molecular probes to investigate interface properties. The solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) and 6-propionyl-2-(N,N-dimethyl)aminonaphthalene (PRODAN) in the water/NaDEHP/toluene system is studied, and the results are compared with those obtained in water/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/toluene RM media. The results demonstrate that the micropolarity, microviscosity, interfacial water structure, molecular probe partition, and intramolecular electron-transfer processes are dramatically altered for NaDEHP RM interfaces in comparison to the AOT systems. Because of organic nonpolar solvent penetration into the interface, NaDEHP RM media offer an interface with lower micropolarity and microviscosity than AOT media. Also, the interfacial water in the NaDEHP system shows enhanced water-water hydrogen-bond interaction in comparison with bulk water. The AOT RM interface represents a unique environment for PRODAN to undergo dual emission.     

Observation of the pi...H hydrogen-bonded ternary complex, (C(2)H(4))(2)H(2)O, using matrix isolation infrared spectroscopy

FTIR absorption spectra of water-containing ethene:Ar matrices, with compositions of ethene up to 1:10 ethene:Ar, have been recorded. Systematically increasing the concentration of ethene reveals features in the spectra consistent with the known 1:1 ethene:water complex, which subsequently disappear on further increase in ethene concentration. At high concentrations of ethene, new features are observed at 3669 and 3585 cm(-1), which are red-shifted with respect to matrix-isolated nu(3) and nu(1) O-H stretching modes of water and the 1:1 ethene:water complex. These shifts are consistent with a pi...H interaction of a 2:1 ethene:water complex of the form (C(2)H(4)...H-O-H...C(2)H(4)). The analogous (C(2)D(4))(2)H(2)O complex shows little shifting from positions associated with (C(2)H(4))(2)H(2)O, while the (C(2)H(4))(2)D(2)O isotopomer shows large shifts to 2722.3 and 2617.2 cm(-1), having identical nu(3)(H(2)O)/nu(3)(D(2)O) and nu(1)(H(2)O)/nu(1)(D(2)O) values when compared with monomeric water isotopomers. Features at 3626.1 and 2666.2 cm(-1) are also observed and are attributed to (C(2)H(4))(2)HDO. DFT calculations at the B3LYP/6-311+G(d,p) level for each isotopomer are presented, and the predicted vibrational frequencies are directly compared with experimental values. The interaction energy for the formation of the 2:1 ethene:water complex from the 1:1 ethene:water complex is also presented.     

Dye–Surfactant Interaction: Modulation of Photophysics of an Ionic Styryl Dye

The interesting modulation of multibond rotation–induced intramolecular charge transfer photophysics of 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide in different micelles due to different contributions of twisted intramolecular charge transfer (TICT) and hydrogen bonding deactivation channels have been reported in this paper. 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide enters into all the micelles in different positions from the water solution due to active hydrophobic force and electrostatic field, as revealed from the shift and intensity of intramolecular charge transfer (ICT) band. The presence of mechanically trapped water with the addition of salt and inherent thermodynamic water controls the ICT emission. Analysis of spectral data before and after the addition of salt confirms the orientation of 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide in cationic and anionic micelles.     

Complexation of polycations to anionic liposomes: composition and structure of the interfacial complexes

Poly(N-ethyl-4-vinylpyridinium bromide) (a polycation with a degree of polymerization of 1100) was adsorbed onto liposomes composed of egg lecithin with a 0.05-0.20 molar fraction (nu) of anionic headgroups provided by cardiolipin (a doubly anionic lipid). According to electrophoretic mobility data, this led to total charge neutralization of the liposomes, whereupon the liposomes adopted a positive charge as additional polymer continued to adsorb. Although the liposomes aggregated at the charge-neutralization point, they disassembled into individual liposomes after becoming positively charged. The degree of polymer adsorption was shown to reach a limit. Thus, by measuring the free polymer content in a liposome suspension, it was possible to determine the polymer concentration at which the liposome surface became saturated with polymer. Beyond this point, an electrostatic/steric barrier at the surface suppressed further adsorption. Dynamic light scattering studies of liposomes with and without adsorbed polymer allowed calculation of the polymer film thickness which ranged from 22 to 35 nm as the molar fraction of cardiolipin (nu) increased from 0.05 to 0.20. The greater the content on the anionic lipid in the bilayer, the thicker the polymer film. The maximum number of polymer molecules adsorbed onto the liposomes was estimated: 1-2 molecules for nu = 0.05; 3 molecules for nu = 0.1; 4- molecules for nu = 0.15; and 6 molecules for nu = 0.2. The polymer appears to lie on the liposome surface, rather than embedding into the bilayer, because addition of NaCl easily dislodges the polymer from the liposome into the bulk water.     

Incorporation of substituted acrylamides to the lamellar mesophase of Aerosol OT

The structure and stability of the lamellar liquid crystal formed by the surfactant sodium bis-2ethylhexyl sulfosuccinate (AOT) in water is perturbed by small amounts of the substituted acrylamides N-isopropyl, N,N-diethyl, N-acryloylmorpholine, and N,N-dimethyl methacrylamide, as revealed by small angle X-ray scattering (SAXS), deuterium NMR, and microscopy. These molecules are water soluble and stay mostly in the water layers between lamellae, but a small fraction of them (5-19%) are incorporated into the AOT bilayers, thereby producing dramatic changes. Both, the degree of anisotropy in the water molecules hydrating AOT (quadrupolar splitting in (2)H NMR) and the long period spacing between lamellae (SAXS), decrease with addition of this molecules at low concentrations, which is attributed to the lower average headgroup density at the AOT/water interface when the acrylamide is incorporated. The strength of these perturbations depends on the acrylamide, and goes in parallel with the hydrophobic character of the alkyl side groups in its molecule, which suggests that the acrylamides incorporated to the bilayer enter into contact with the lipophilic tails of the AOT molecule. An interaction with the hydrated heads of AOT is also suggested in the particular case of N-isopropylacrylamide. On increasing the molecule concentration an incipient melting of the lamellar phase towards an isotropic solution takes place, first at the microscopic level, then macroscopic. Near this phase transition, the ordered domains lose the random orientation prevailing at lower acrylamide concentrations, and adopt a preferred orientation, perpendicular to the magnetic field.     

基因载体材料聚乙烯亚胺与磷脂酰胆碱脂质体的相互作用对膜结构的影响

采用动态光散射、荧光光谱、zeta电位测定和等温滴定量热技术分析了分子量分别为25000,10000和1800的聚乙烯亚胺(PEI)与二油酰磷脂酰胆碱(DOPC)脂质体的相互作用及其对脂质体膜内环境极性和膜通透性的影响.结果表明,PEI通过氨基与DOPC的磷脂基团和胆碱基团产生氢键或范德华作用,从而与脂质体结合形成复合物;低浓度PEI(0.075 mg/mL)导致DOPC脂质体的聚集和表面电位的增加,但未引起脂质体膜融合和表面电位反转;进一步增加PEI的浓度对脂质体表面电位的影响很小,而结合在表面的PEI分子链之间的排斥作用阻碍了脂质体聚集.PEI分子与DOPC脂质体的结合降低了脂质分子碳氢链的堆积密度和脂质体膜内环境的疏水性,从而增强了钙黄绿素和槲皮素在脂质体膜中的通透性.PEI与DOPC脂质体的相互作用具有明显的分子尺寸效应,增大PEI的分子量可以增强与脂质体的相互作用及对脂质体膜结构的影响.     

Study of association thermodynamics between crystal violet and sodium bis(2-ethylhexyl)sulfosuccinate and kinetics of basic fading of crystal violet in microemulsions

The thermodynamics of the association between 4,4′,4″-tris(dimethyl-amino)triphenylmethyl chloride (crystal violet or CV) and sodium bis(2-ethylhexyl)-sulfosuccinate (aerosol OT or AOT) in water/AOT/n-decane microemulsion and the kinetics of the basic hydrolysis of CV in a water-in-oil microemulsion were investigated by UV–vis spectroscopic measurements. An association model of CV and AOT was used to analyze the experimental data to obtain the association constants at various temperatures. By taking the association into account, the “actual” rate constants and the activation energies of the basic hydrolysis of CV in the media of water/AOT/oil were obtained. The difference in thermodynamics and kinetics between the two media of water/AOT/n-decane and water/AOT/isooctane is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 294–300, 2008