Anchor-lipid monolayers at the air-water interface; prearranging of model membrane systems

Model membrane systems are gaining more and more interest both for basic studies of membrane-related processes as well as for biotechnological applications. Several different model systems have been reported among which the tethered bilayer lipid membranes (tBLMs) form a very attractive and powerful architecture. In all the proposed architectures, a control of the lateral organization of the structures at a molecular level is of great importance for an optimized preparation. For tBLMs, a homogeneous and not too dense monolayer is required to allow for the functional incorporation of complex membrane proteins. We present here an alternative approach to the commonly used self-assembly preparation. Lipids are spread on the air-water interface of a Langmuir film balance and form a monomolecular film. This allows for a better control of the lateral pressure and distribution for subsequent transfer to solid substrates. In this paper, we describe the properties of the surface monolayer, in terms of surface pressure, structure of the lipid molecule, content of lipid mixtures, temperature, and relaxations features. It is shown that a complete mixing of anchor-lipids and free lipids can be achieved. Furthermore, an increase of the spacer lengths and a decrease of the temperature lead to more compact films. This approach is a first step toward the fully controlled assembly of a model membrane system.     

Rapid proton-coupled electron-transfer of hydroquinone through phenylenevinylene bridges

We describe the synthesis of two oligo(phenylene vinylene)s (OPVs) with a hydroquinone moiety and a thiol anchor group: 4-(2',5'-dihydroxystyryl)benzyl thioacetate and 4-[4'-(2' ',5' '-dihydroxystyryl)styryl]benzyl thioacetate. Monolayers on gold of these molecules were examined by electrochemical techniques to determine the electron transfer kinetics of the hydroquinone functionality (H2Q) through these delocalized tethers ("molecular wires") as a function of pH. Between pH 4 and 9, rate constants were ca. 100-fold faster than for the same H2Q functionality confined to the surface via alkane tethers. Also, in this same pH range rate constants were independent of the length of the OPV bridge. These new electroactive molecules in which the hydroquinone functionality is wired to the gold surface by means of OPV tethers should be useful platforms for constructing bioelectronic devices such as biosensors, biofuel cells, and biophotovoltaic cells with a fast response time.     

Substrate- and time-dependent photoluminescence of quantum dots inside the ultrathin polymer LbL film

The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties.     

Interaction of fibrinogen with nanosilica

Interaction of human plasma fibrinogen (HPF) with fumed nanosilica A-300 in a phosphate buffer solution (PBS) was studied using ¹H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial water in the temperature range of 210–273 K, TSDC (90 < T < 265 K), adsorption, FTIR, and UV spectroscopy methods. An increase in concentration of HPF in the PBS leads to a decrease in amounts of structured water (frozen at T < 273 K) because of coagulation of HPF molecules. Addition of nanosilica to the HPF solution strongly reduces the amounts of structured water because of adsorption interaction of HPF molecules with silica nanoparticles, self-association of HPF molecules, formation of denser packed hybrid agglomerates with HPF and silica, and lastly, because of conformational changes of HPF. A monolayer adsorption capacity of A-300 corresponds to 156 mg of HPF per gram of silica. The FTIR and UV spectra show that the HPF adsorption on silica leads to structural changes of the protein molecules. These changes and formation of hybrid HPF/A-300 aggregates can increase the rate of clotting that is of importance on nanosilica application as a component of tourniquet preparations.      

Development and prevalidation of a method for phenol determination by solid-phase spectrophotometry

A simple, sensitive and rapid solid-phase spectrophotometric procedure was developed for the determination of traces of phenol with 4-aminoantipyrine as a reagent (AAP-SPS), and the optimal experimental conditions were established. This method was performed by sorption and direct absorbance measurements of the product phenol-AAP sorbed on the anion-exchanger Dowex 1-X4 (0.2 g) at 495 nm (absorption maximum) and 700 nm (non-absorption wavelength). The sensitivity offered by the AAP-SPS procedure was higher by a factor of 40 compared with the respective conventional spectrophotometric method. Metrological characteristics were established using a prevalidation strategy. The AAP-SPS procedure is characterized by a linear calibration function in the working range of 0.05–0.50 μmol, low standard deviation of procedure (±0.012), low limit of determination (0.021 μmol), and favorable random (±0.85 to ±11.27%) and systematic deviations (−4.55 to +11.50%). Moreover, the accuracy of the system investigated by the recovery test is acceptable (99–102%). Favorable working and performance characteristics make the new SPS method ideal for phenol monitoring in pharmaceutical preparations as well as other matrices.     

Synthesis and structure of 2,4,6,8-tetramethyl-3,7-dithia-2,4,6,8-tetraazabicyclo[3.3.0]octane 3,3,7,7-tetraoxide

Condensation of N,N′-dimethylsulfamide with glyoxal gave 2,4,6,8-tetramethyl-3,7-dithia-2,4,6,8-tetraazabicyclo[3.3.0]octane 3,3,7,7-tetraoxide, a sulfur-containing analog of 2,4,6,8-tetramethyl-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione (Mebicar). The product structure was studied by X-ray analysis.     

Thermoresponsive reversible behavior of multistimuli pluronic-based pentablock copolymer at the air-water interface

Surface behavior of the pH- and thermoresponsive amphiphilic ABCBA pentablock copolymer has been studied with respect to the environmental conditions. We demonstrate that the pentablock copolymer poly((diethylaminoethyl methacrylate)-b-(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)-b-(diethylaminoethyl methacrylate)) possesses reversible temperature changes at the air-water interface in a narrow pH range of the water subphase. Significant diversity in the surface morphology of pentablock copolymer monolayers at different pH and temperatures observed were related to the corresponding reorganization of central and terminal blocks. Remarkable reversible variations of the surface pressure observed for the Langmuir monolayers at pH 7.4 in the course of heating and cooling between 27 and 50 degrees C is associated with conformational transformations of terminal blocks crossing the phase line in the vicinity of the lower critical solution temperature point. The observed thermoresponsive surface behavior can be exploited for modeling of the corresponding behavior of pentablock copolymers adsorbed onto various biointerfaces for intracellular delivery for deeper understanding of stimuli-responsive transformations relevant to controlled drug and biomolecules release and retention.     

Mechanism of the cyclization of dimethyl diethynyl silane with selenium tetrabromide: Computational and structural studies, and monitoring

The structure of 2,4-dibromo-2-dibromomethyl-3,3-dimethyl-1-selena-3-silacyclopentene-4, formed by regioselective electrophilic addition of SeBr₄ to dimethyl diethynyl silane, has been determined using X-ray analysis technique. Quantum chemistry methods were used to study elementary stages of the reaction. It was found that the first stage consisted of SeBr₄ conversion into bimolecular complex Br₂?SeBr₂, initiated by dimethyl diethynyl silane. Possible formation of five-membered and six-membered heterocycles involves different cyclization mechanisms. The formation of only five-membered heterocycle is explained by kinetically preferable ring closure through four-center transition state. The conclusions obtained by calculations were confirmed by monitoring of the reaction using ¹H NMR method.     

Click Functionalization of a Dibenzocyclooctyne‐Containing Conjugated Polyimine

A conjugated poly(phenyl‐co‐dibenzocyclooctyne) Schiff‐base polymer, prepared through polycondensation of dibenzocyclooctyne bisamine (DIBO‐(NH₂)₂) with bis(hexadecyloxy)phenyldialdehyde, is reported. The resulting polymer, which has a high molecular weight (M_n>30 kDa, M_w>60 kDa), undergoes efficient strain‐promoted alkyne–azide cycloaddition reactions with a series of azides. This enables quantitative modification of each repeat unit within the polymer backbone and the rapid synthesis of a conjugated polymer library with widely different substituents but a consistent degree of polymerization (DP). Kinetic studies show a second‐order reaction rate constant that is consistent with monomeric dibenzocyclooctynes. Grafting with azide‐terminated polystyrene and polyethylene glycol monomethyl ether chains of varying molecular weight resulted in the efficient syntheses of a series of graft copolymers with a conjugated backbone and maximal graft density.