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.     

Confocal Raman microscopy for simultaneous monitoring of partitioning and disordering of tricyclic antidepressants in phospholipid vesicle membranes

Characterization of drug–membrane interactions is important in order to understand the mechanisms of action of drugs and to design more effective drugs and delivery vehicles. Raman spectra provide compositional and conformational information of drugs and lipid membranes, respectively, allowing membrane disordering effects and drug partitioning to be assessed. Traditional Raman spectroscopy and other widely used bioanalytical techniques such as differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) typically require high sample concentrations. Here, we describe how temperature‐controlled, optical‐trapping confocal Raman microscopy facilitates the analysis of drug–membrane interactions using micromolar concentrations of drug, while avoiding drug depletion from solution by working at even lower lipid concentrations. The potential for confocal Raman microscopy as an effective bioanalytical tool is illustrated using tricyclic antidepressants (TCAs), which are cationic amphiphilic molecules that bind to phospholipid membranes and influence lipid phase transitions. The interaction of these drugs with vesicle membranes of differing head‐group charge is investigated while varying the ring and side‐chain structure of the drug. Changes in membrane structure are observed in Raman bands that report intra‐ and intermolecular order versus temperature. The partitioning of drugs into the membrane can also be determined from the Raman scattering intensities. These results demonstrate the usefulness of confocal Raman microscopy for the analysis of drug–membrane systems at biologically relevant drug concentrations. Effective tools for monitoring drug–membrane interactions are crucial for rational design of new drugs. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermodynamic studies of the binding interactions of surfactin analogues to lipid vesicles Application of isothermal titration calorimetry

Isothermal titration calorimetry was applied for studying the binding interactions of cyclic and linear surfactins with different ionic charge (z= −2 and −3) and lipid chain length (n=14 and 18) to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidyl-choline (POPC) vesicles in 10mMTris buffer at pH8.5with 150mMNaCl at 25°C. Surfactin analogues interacted spontaneously (ΔG _D ^w→b < 0) with POPC vesicles. The binding reactions were endothermic (ΔH _D ^w→b > 0) and entropy-driven process (ΔS _D ^w→b > 0). Moreover, significant differences in the binding constant values (K) ranging from 6.6·10³ to 9.6·10⁴ M⁻¹ show that cyclic structure and the increase of lipid chain length are favourable on the surfactin binding affinity to POPC vesicles, whereas the rise of the number of negative charges has an opposite effect.     

Micelle-to-vesicle transition induced by organic additives in catanionic surfactant systems

A micelle-to-vesicle transition (MVT) induced by the addition of a series of apolar hydrocarbons (n-butylbenzene, n-hexane, n-octane, and n-dodecane) to the catanionic surfactant system n-dodecyltriethylammonium bromide/sodium n-dodecylsulfate (DTEAB/SDS) has been investigated for the first time by means of rheology and turbidity measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Interestingly, a MVT can take place within certain micellar regions, which are dependent on the structure and chain length of the hydrocarbon. However, these hydrocarbons are unable to induce a MVT in another catanionic surfactant system, namely, n-dodecyltriethylammonium bromide/sodium n-dodecylsulfonate (DTEAB/SDSO(3)), in which the molecular interactions are weaker than in the DTEAB/SDS system. On the other hand, polar additives, such as n-octanol and n-octylamine, exhibit much higher efficiency and activity in inducing MVT than hydrocarbons in both DETAB/SDS and DTEAB/SDSO(3). Moreover, DLS, TEM, and time-resolved fluorescence quenching (TRFQ) results demonstrate that the ratio of vesicles to micelles in the system can be actively controlled by addition of polar additives. Possible mechanisms for the above phenomena are presented, and the potential application of controllable micelle/vesicle systems in the synthesis of tailored bimodal mesoporous materials is discussed.     

Vesicle Formation from an Amphiphilic Porphyrin Derivative at the Air–Water Interface

A tetraphenyl porphyrin derivative with two C₁₆ alkyl chains covalently bound to each of the four peripheral phenyl rings through ether linkages formed multilayer clusters or vesicles at the air–water surface. More interestingly, spherical vesicles were also formed when deposited on appropriate solid surfaces, and these vesicles were stable even in dry conditions. Various microscopic images of the cast film deposited on a mica surface confirmed closed‐ended nanotube/nanorod‐type formation with necking and bulging. These narrow tubes are proposed to be intermediates for the formation of vesicles by fission at either side of the bulge. Such vesicular formation is not common when either cast or Langmuir–Blodgett films were deposited on a solid surface.     

Probing the interaction of the potassium channel modulating KCNE1 in lipid bilayers via solid‐state NMR spectroscopy

KCNE1 is known to modulate the voltage‐gated potassium channel α subunit KCNQ1 to generate slowly activating potassium currents. This potassium channel is essential for the cardiac action potential that mediates a heartbeat as well as the potassium ion homeostasis in the inner ear. Therefore, it is important to know the structure and dynamics of KCNE1 to better understand its modulatory role. Previously, the Sanders group solved the three‐dimensional structure of KCNE1 in LMPG micelles, which yielded a better understanding of this KCNQ1/KCNE1 channel activity. However, research in the Lorigan group showed different structural properties of KCNE1 when incorporated into POPC/POPG lipid bilayers as opposed to LMPG micelles. It is hence necessary to study the structure of KCNE1 in a more native‐like environment such as multi‐lamellar vesicles. In this study, the dynamics of lipid bilayers upon incorporation of the membrane protein KCNE1 were investigated using ³¹P solid‐state nuclear magnetic resonance (NMR) spectroscopy. Specifically, the protein/lipid interaction was studied at varying molar ratios of protein to lipid content. The static ³¹P NMR and T₁ relaxation time were investigated. The ³¹P NMR powder spectra indicated significant perturbations of KCNE1 on the phospholipid headgroups of multi‐lamellar vesicles as shown from the changes in the ³¹P spectral line shape and the chemical shift anisotropy line width. ³¹P T₁ relaxation times were shown to be reversely proportional to the molar ratios of KCNE1 incorporated. The ³¹P NMR data clearly indicate that KCNE1 interacts with the membrane. Copyright © 2017 John Wiley & Sons, Ltd.     

Adsorption, association and precipitation in hexadecyltrimethylammonium bromide/sodium dodecyl sulfate mixtures

The phase equilibria of surfactant aqueous mixtures, hexadecyltrimethylammonium bromide and sodium dodecyl sulfate, have been studied by polarizing microscopy, quasielastic light scattering, conductivity, potentiometric, electrophoretic, and surface tension measurements. Adsorption at the air/solution interface, association and precipitation in bulk solution strongly depended on the molar ratio and the concentration of surfactants. Catanionic vesicles coexisted with crystalline catanionic salts in a broad concentration range. The relative proportions of crystallites and vesicles varied according to the concentration and the molar ratio of the surfactants. The solid crystalline phase was progressively converted to catanionic vesicles with increasing surfactant molar ratio. At the highest excess of one of the surfactants transition from catanionic vesicles to mixed micelles occurred. The formation and stability of different phases are discussed in terms of surfactant molecular packing constraints and electrostatic interactions in the headgroup region. Surfactant tail-length asymmetry and the change of electrostatic interactions in the headgroup region from attractive to repulsive are governing factors for the transition from planar to curved bilayers. Received: 9 June 1998 Accepted: 18 August 1998     

On the carrier properties of perfluoropolyether-betaine mixed vesicles: the contribution of electron spin resonance spectroscopy

The carrier properties of mixed unilamellar vesicles of fluorocarbon-hydrocarbon surfactants built up with the ammonium salt of a perfluoropolyether and n-dodecylbetaine were investigated by electron spin resonance spectroscopy (ESR). The ESR-active lipophilic nitroxide 5-doxylstearic acid (5-DSA) was used as a model of a lipophilic drug to be carried and delivered into cell membranes. Healthy and malignant colorectal tissues were used as the target cells. Cell suspensions of living tissues were studied in physiological conditions. For the maintenance of the surgically removed tissues, McCoy’s 5A culture medium was used. 5-DSA probe was rapidly delivered from perfluoropolyether (PFPE)/betaine mixed vesicles to the membranes of both healthy and malignant colorectal cells. The analysis of the computed 5-DSA ESR line shapes ensured that no spectral differences occurred when 5-DSA was either introduced directly into the cells or through the intervention of vesicles. This well agreed for an unmodified physical status of the membranes where the probe was mainly localized.