Passive Transport of Ionic Drugs through Membranes with pH-Dependent Fixed Charges

We have studied both theoretically and experimentally the passive transport of ionic drugs through membranes with pH-dependent fixed charge. The system considered constitutes a simplified model for pH-controlled drug delivery through membranes of biochemical and pharmaceutical interest. The theoretical approach employed is based on the Nernst-Planck flux equations and all of the species present in the system (the neutral or ionic drug and the hydrogen and hydroxide ions) have been taken into account together with a Langmuir-type isotherm for the adsorption of the ionic drug onto the membrane surface. The membrane permeabilities of cationic, anionic, and neutral drugs through porous membranes with graft-polymerized weak polyelectrolytes have been measured as a function of the external pH. According to the nature of the grafted polyelectrolyte, the ionized membrane fixed groups can be negative or positive. For the amphoteric membrane, both fixed charge groups are present in the grafted chains. In all cases, the ionization state of the weak polyelectrolyte fixed groups changes with the local pH within the membrane. A comparison of the theoretical results with the experimental data allows one to explain qualitatively the changes of the membrane flux with the external pH and gives new physical insights into the transport problem. Copyright 2000 Academic Press.     

Chromatography in silico,basic concept in reversed-phase liquid chromatography

Basic phenomena in reversed-phase liquid chromatography have been quantitatively analyzed using a computational chemical calculation. Pyridine interacted with an ionized silica surface under neutral conditions. Alkyl-chain length affected the contact surface area with an analyte. Steric hindrance was demonstrated using a model graphitic carbon phase and unsaturated alkenes. Quantitative structure–retention relationships in reversed-phase liquid chromatography were demonstrated for phenolic compounds and acidic and basic drugs. The correlations between predicted and measured retention factors were satisfactory. Dissociation constants were derived from the atom partial charge and used to predict retention factors of partially ionized compounds.     

Membrane disordering induced by chloroform and carbon tetrachloride

We studied effects of chloroform and carbon tetrachloride on bilayer membranes of dimyristoyl-phosphatidylcholine (DMPC) and egg yolk phosphatidylcholine (Egg-PC) by birefringence, dynamic light scattering and fluorescence methods. It is shown that interference light due to the membrane birefringence considerably decreases by addition of the organohalogen compounds for both lipid membranes, indicating a significant decrease in membrane order. In addition, results of dynamic light scattering and turbidity measurements show a rupture of multilamellar DMPC vesicles induced by addition of chloroform at concentrations above 0.2 v/v%. No rupture of the vesicles is observed within the limit of solubility of carbon tetrachloride in water, but excessive addition of carbon tetrachloride (above 0.2 v/v%) induces the vesicle rupture. Chain orientational order was estimated from the interference light intensity at low concentrations of the organohalogen compounds without the occurrence of the vesicle rupture. The estimation shows a monotonic decrease in the chain order with increasing the concentration. The decreases in DMPC chain order by chloroform and by carbon tetrachloride are about 17% at 0.2 v/v% and 23% at 0.05 v/v%, respectively. The reduction in the chain order is correlated with an increase in the membrane fluidity observed by excimer fluorescence of pyrene incorporated to the membrane. Behavior of membrane disordering of Egg-PC is approximately similar with that of DMPC. This implies the strong interaction between the organohalogen compounds and the lipid chains, whether or not the bilayer has the vacancy resulted from unsaturated double bonds and different chains in length. The results of this work suggest that damages of biological membranes by chloroform and tetrachloride are not only induced by a direct attack on proteins but also by a significant membrane disorder.     

Asymmetric allylsilane-mediated carbocyclization: A highly enantiospecific synthesis of (1S, 2S)-(+)-2-methyl-3-cyclopenten-1-ol

An enantiospecific synthesis of the title compound is described which involves the TiCl₄-promoted cyclization of the chiral allylic silane having formyl group, which was obtained via the Claisen rearrangement of ($\text{R}$, $\text{E}$)-1-trimethylsilyl-1-buten-3-ol.     

Effect of proton concentration on membrane potential across a weak amphoteric polymer membrane

An amphoteric membrane consists of both positively and negatively fixed charge groups chemically bound to the polymer chains. If the external solution is changed from alkali to acid, it is possible to obtain an experimental result in which the membrane potential changes from positive to negative through the isoelectric point. It was characterized by examining the relationship between membrane potential and proton concentration (pH) obtained from both experimental and theoretical considerations. The Nernst-Planck flux equation and the Donnan equilibrium theory were also solved for a four-component system combined with the dissociation constant, in order to discuss the pH dependence of membrane potential in a weak amphoteric membrane by comparing the experimental results with the calculated results. It was proven that the calculated results substantially deviated from the theoretical results despite a similar tendency. Such a deviation was caused by the fact that the original theory disregarded the activity coefficient and the ionic mobility, which were dependent on the fixed charge concentration in a membrane. The original theoretical model was modified by adding the effect of a fixed charge group to the activity coefficient and ionic mobility. The calculated results using the modified model explained well the experimental results if the parameter called charge effectiveness, phi, was introduced into the equations. Introduction of phi into the prediction of membrane potential was already done by Kobatake et al. in a system of a strong polyelectrolyte monopolar membrane/salt aqueous solution. In this study, it was proved that phi can also be introduced into a weak amphoteric polymer membrane/salt aqueous solution system. Finally it was also concluded that the Donnan equilibrium and the Nernst-Planck flux equation were still applicable for examining the transport phenomena for the system of a weak amphoteric charged membrane and electrolyte solutions at various pH.     

Totally synthetic steroid heterocycles. Part 7.. A facile approach to 16-oxa-D-homoestrogens

In search of biologically active modified steroids, novel 16-oxa-D-homoestrone and -D-homoestradiol 3-methyl ethers were synthesized from 16-oxa-3-methoxy- D -homoestra-1,3,5(10),8,14-pentaen-17a-one. The straightforward synthesis involved stereoselective two-step reduction of the 8,14-diene system. The B/C stereoisomers were also derived from the estrapentaene. The stereostructures of these heterocyclic estrogens were determined on the basis of their spectral data.     

Computational chemical analysis of the retention of acidic drugs on a pentyl-bonded silica gel in reversed-phase liquid chromatography

A fast method to obtain a quantitative structure-retention relationship is required in chromatography for the rapid optimization of chromatographic separation conditions. Chromatographic data of acidic drugs are analyzed by a computational chemical method to simulate chromatographic simulation. The direct interaction between a model phase and a drug is calculated as an energy value using the molecular mechanics calculation of CAChe. Computational chemistry using a model adsorbent is a new method for quantitative analysis of retention in reversed-phase liquid chromatography. The correlation coefficient is 0.878 (n = 19) between the retention factors of acidic drugs and interaction energy values of the final structure (DeltaFS) between an acidic drug and model pentyl-bonded phase.     

Metal ion-catalyzed Diels-Alder and hydride transfer reactions. Catalysis of metal ions in the electron-transfer step

Rates of Diels-Alder cycloaddition of anthracenes with p-benzoquinone and its derivatives as well as rates of hydride-transfer reactions from 10-methyl-9,10-dihydroacridine to the same series of p-benzoquinones are accelerated significantly in the presence of metal ions in acetonitrile. An extensive comparison of the catalytic effects of metal ions in electron transfer from one-electron reductants (cobalt tetraphenylporphyrin and decamethylferrocene) to p-benzoquinones with those in the Diels-Alder reactions of the quinones as well as the hydride-transfer reactions has revealed that the catalysis of metal ions in each case is ascribed to the 1:1 and 1:2 complexes formed between the corresponding semiquinone radical anions and metal ions. The transient absorption and ESR spectra of the semiquinone radical anion-metal ion complexes are detected directly in the electron-transfer reduction of p-benzoquinone derivatives in the presence of metal ions. The catalytic reactivities of a variety of metal ions in each reaction are well correlated with the energy splitting values of pi(g) levels because of the complex formation between O(2)(.-) and M(n+), which are derived from the g(zz) values of the ESR spectra of the O(2)(.-)-M(n+) complex.     

Chemiluminescence of NS radical produced by the reaction of active nitrogen with sulfur vapor and sulfur chloride

Kinetics of NS (B² Π → X²Π) emission produced by the reaction of active nitrogen with sulfur vapor or sulfur chloride (S₂Cl₂) has been studied as a function of the total pressure and of the concentrations of atomic nitrogen and sulfur. The B ²Π state of NS is selectively formed by the three-body recombination reaction of N ans S atoms, N(⁴S) + S(³P) + M →. NS(B ²Π) + M. The highest vibrational level of the B ²Π state observed is ν′ = 12, which gives a lower bound for the dissociation energy of 4.93 eV.     

X-Ray Diffraction Studies of Bacteriorhodopsin. Determination of the Positions of Mercury Label at Several Engineered Cysteine Residues

Abstract— The single cysteine-containing bacteriorhodopsin mutants F27C, L100C, T170C, F171C and I222C were labeled with p -chloromercuribenzoic acid, which specifically reacts with sulfhydryl groups. These cysteines should be located at the cytoplasmic ends of the transmembrane helices A, C, F or G. We determined the positions of the bound mercury atoms by X-ray diffraction of purple membrane films, with better than 1 Å accuracy. The determined mercury positions were compared with the structural model from cryoelectron microscopy (N. Grigorieff, T. A. Ceska, K. H. Downing, J. M. Baldwin and R. Henderson, J. Mol. Biol 259, 393-421, 1996). Given that the distance between the mercury and the Cα atom of the cysteine in the xy plane must be shorter than 4.5 Å and that the mercury atom is located at the δ position, the positions obtained for the mercury labels agree with their expected positions from the structural model. The present results give a rationale for detecting structural changes upon illumination as shifts occur in the mercury label position.