Zeta-potential measurements as a tool to quantify the effect of charged drugs on the surface potential of egg phosphatidylcholine liposomes

The binding of charged drugs to neutral phosphatidylcholine membranes was assessed by measuring their zeta-potential values in the presence of different drug concentrations. This methodology was applied to the study of the concentration effects of two nonsteroidal antiinflammatory drugs (NSAIDs). Results revealed an intense membrane charging that was proportional to the amount of negatively charged drug in the media. A mathematical formalism was adapted and an analytical expression derived to calculate directly surface potentials from zeta-potential data. The membrane loading state, expressed as the number of molecules per unit area, was calculated for the negative and for the neutral forms of the drugs. An approach was also developed that allows the determination of the maximum number of molecules per unit area by fitting a binding isotherm to the dependence of the number molecules per unit area with the drug concentration. The calculation of the maximum mol lipid/drug ratio can also be estimated and related to the binding stoichiometry, as well as to the maximum lipid loading capacity. Furthermore, the concentration profiles for both drugs can be established in terms of the distance to the liposome surface. The developed methodology allowed for the simultaneous determination of partition coefficients (Kp) for the NSAIDs in lipid/aqueous media because zeta-potential values can be related to the drug concentration at the lipid/ aqueous media interface. Alternative independent methodologies were used to determine Kp: spectrophotometric and centrifugation assays. A mathematical relation was developed to compare the Kp values determined from the zeta-potential data with those obtained from the other techniques used because in the former case they are calculated on the basis of the number of molecules per unit area and in the latter on the basis of the total drug concentrations in solution, and the values of the partition coefficients obtained from all the techniques were found to be equal, within the experimental error. This methodology constitutes a more straightforward method than the other techniques used because partition coefficients for all drug forms (charged and noncharged) can be assessed with a minimum number of experimental determinations and it allows for a characterization of the electrostatic properties of neutral membranes upon binding of charged drugs.     

Intramolecular excimer formation in polymers: Pyrene Labelled Polyvinylacetate

The rate constant for intramolecular excimer formation between pyrenyl side-groups, in a polyvinylacetate chain at a mean separation of 200 bonds, has been measured as a function of molecular weight, solvent viscosity and solvent thermodynamic power. Above M = 1 × 10⁵, the rate constant is 1.4 × 10⁷ sec^?1 in low viscosity bad solvents. This value is about twenty times that for the rate constant of the analogous reaction between the two terminal groups in a chain with a mean end-to-end separation of 200 bonds. Increases of the viscosity and of the thermodynamic power of the solvent depress the rate constant, in agreement with the behaviour expected for a diffusion controlled reaction.     

DFT calculations, structural and spectroscopic studies on the products formed between IBr and N,N'-dimethylbenzoimidazole-2(3H)-thione and -2(3H)-selone

The NBO charge distribution calculated at DFT level on the [LEX](+) species [LE=N,N'-dimethylbenzoimidazole-2(3H)-thione (3) and -2(3H)-selone (4)(Scheme 1); X=I, Br] suggests that the most likely products from the reaction 3 of 4 and with IBr are the 10-X-2 charge-transfer (CT) adduct and the 10-Se-3 "T-shaped" hypervalent adduct featuring a linear Br--Se--I system, respectively. This prediction is confirmed by the synthesis, and X-ray diffraction analysis of 3.IBr (I) and 4.I(0.72)Br(1.28)(II). In particular II, is a 10-Se-3 "T-shaped" hypervalent adduct containing an almost linear X--Se--X system [X--Se--X 179.07(3) degrees, X=I(0.36)/Br(0.64)], which is roughly perpendicular to the average plane of the benzoimidazole moiety. The FT-Raman spectra of I and II agree very well with their structural features. In particular, the complexity of the FT-Raman spectrum of II reflects the disorder in the X-ray crystal structure of this compound.     

The long-lived molecular ion of propionitrile: An ion cyclotron resonance study

Evidence has been reported that primary loss of H and of HCN from the molecular ions of propionitrile, isobutyronitrile and butyronitrile in the mass spectrometer is preferentially preceded by hydrogen migration from C-2 to C-1. Ion cyclotron double resonance spectra of proton (or deuteron-) transfer products derived from propionitrile-2-d2 and -3-d3 and a series of bases provide evidence that such migration occurs also in long-lived propionitrile molecular ions.     

Acid-base and metal ion binding properties of guanylyl(3'-->5')guanosine (GpG-) and 2'-deoxyguanylyl(3'-->5')-2'-deoxyguanosine [d(GpG)-] in aqueous solution

The acidity constants of guanylyl(3'-->5')guanosine (GpG(-)) and 2'-deoxyguanylyl(3'-->5')-2'-deoxyguanosine [d(GpG)(-)] for the deprotonation of their (N1)H sites were measured by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO(3)). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg(2+), Ni(2+), or Cd(2+) (= M(2+)) and (GG-H)(2-), and in the case of Mg(2+) also of (GG-2H)(3-), where GG(-) = GpG(-) or d(GpG)(-). The stability constants of the M(GG)(+) complexes were estimated. The acidity constants of the H(dGuo)(+) and dGuo species (dGuo = 2'-deoxyguanosine) and the stability constants of the corresponding M(dGuo)(2+) and M(dGuo-H)(+) complexes were also measured. Comparison of these and related data allows the conclusion that N7 of the 5'G unit in GG(-) is somewhat more basic than the one in the 3'G moiety; the same holds for the (N1)(-) sites. On the basis of comparisons with the stability constants measured for the dGuo complexes, it is concluded that M(2+) binding of the GG dinucleoside monophosphates occurs predominantly in a mono-site fashion, meaning that macrochelate formation is not very pronounced. Indeed, it was a surprise to find that the stabilities of the complexes of dGuo or (dGuo-H)(-) and the corresponding ones derived from GG(-) are so similar. Consequently, it is suggested that in the M(GG)(+) and M(GG-H) complexes the metal ion is mainly located at N7 of the 5'G unit since this is the more basic site allowing also an outer-sphere interaction with the C6 carbonyl oxygen and because this coordination mode is also favorable for an electrostatic interaction with the negatively charged phosphodiester bridge. It is further suggested that Mg(2+) binding (which is rather weak compared to that of Ni(2+) and Cd(2+)) occurs mainly in an outer-sphere mode, and on the basis of the so-called Stability Ruler it is concluded that the binding properties of Zn(2+) to the GG species are similar to those of Ni(2+) and Cd(2+).     

Production of glucose and bioactive aglycone by chemical and enzymatic hydrolysis of purified oleuropein fromOlea Europea

Pure-grade oleuropein, a bitter, hypotensive, phenolic glucoside, was obtained from organic extracts of olive plant leaves by two Chromatographic steps. The purified compound was characterized by spectroscopic NMR and FAB-MS methods. The glucoside underwent chemical and enzymatic hydrolysis. Aglycone was characterized by spectroscopic methods (¹H-NMR and FAB-MS). Glucose was measured by enzymatic methods. The enzymatic hydrolysis of oleuropein was carried out by a soluble β-glucosidase. The reaction was characterized in terms of kinetic parameters, optimal pH value, activation energy, inhibition constant by glucose, and thermal stability. Preliminary experiments were also performed in a continuousflow ultrafiltration membrane reactor. The cut-off of the membrane was lower than the molecular-weight of the enzyme, thus determining β-glucosidase confinement within the reactor. Under these conditions, β-glucosidase had a good long-term stability. This is an encouraging result in view of possible industrial applications.     

Nucleophilic attack on bent molybdenum and tungsten metallocene complexes

Reactions of some [M(η⁵-C₅H₅)₂L₂]ⁿ⁺ complexes (M  Mo, W; L = ligand; n = 0, 1, 2) with NaBH₄ and LiAlH₄ are reported. New neutral and cationic hydride derivatives of the type [M(η⁵-C₅H₅)₂HL]^m+ (m = 0, 1) are described, in particular the six halohydride complexes [M(η⁵-C₅H₅)₂HX]. The deuteration studies were carried out, but the results do not lead to definite conclusions about the mechanism.     

Ultrasound Speeds and Molar Isentropic Compressions of Aqueous Binary Mixtures of Diethylamine from 278.15 to 308.15 K

Ultrasound speeds in aqueous binary mixtures of diethylamine (DEA) were measured across the entire composition range at five temperatures between 278.15 and 308.15 K. Isentropic compressibilities, κ _S , were calculated from the ultrasound speed and density data. The excess molar isentropic compressions, K _S,m ^E, were estimated and their variation with the mole fraction of the amine were fitted by the Redlich–Kister equation. Excess partial molar isentropic compressions, K _S,i ^E, were then obtained, allowing separation of the role of each component in the mixing process. Interesting insights are gained from the analysis of the temperature and composition dependence of K _S,i ^E, principally in the water-rich region. A comparison of the limiting values of this property with those of the limiting excess partial molar isobaric expansion, E _P,i ^E,∞, previously published, clearly shows the different sensitivity of these two differential thermodynamic properties to the mixing process. The different behavior of the temperature dependence of K _S,i ^E,∞ in the systems, water + DEA, and water + 2-diethylaminoethanol (DEEA), are also analyzed and interpreted in terms of changes in the solute configuration, the degree of hydrolysis and solute-solvent interactions.     

The reaction of benzothiazole sulfenamide with (TMS)3SiH: An example of degenerate-branched chain process

The reaction of sulfenamide 3 with (TMS)₃SiH initiated by the decomposition of AIBN at 76 °C has been studied in some detail. The reaction is a rare example of a radical chain-branching process. The two main products are dialkylamine 4 and the thiosilane 5. It is also established that 2-mercaptobenzothiazole (2) is formed in a substantial yield as one of the by-products. The mechanism of this chain autocatalytic reaction is complex due to a mix of different radical chain reactions and some discussion is provided. The amine obtained in a quantitative yield can arise from two independent routes of attack of (TMS)₃Si radical on sulfenamide 3. The minor route affords thiol 2 that can act as a catalyst for the major route during the reaction course and then gives a salt with secondary amine, which precipitates upon cooling. The origin of autocatalysis is discussed in some detail.