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PubMed1983

Binding of hydrophobic drugs to lipid bilayers and to the (Ca2+ + Mg2+)-ATPase.

Microelectrophoretic studies of the binding of a number of commonly used hydrophobic amine drugs to liposomes demonstrated the existence of relatively large surface potentials associated with binding of the protonated forms of the drugs. A theoretical treatment based on Langmuir adsorption isotherms and the Gouy-Chapman theory of the diffuse double layer allows estimation of drug-binding constants from electrophoretic mobility data. Such constants allow calculation of the charge effects arising from drug binding in more complex membrane systems, and it is shown that shifts in the apparent Ca+ affinity of the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum in the presence of hydrophobic amine drugs are readily explicable in terms of the electrostatic effects of drug binding.

Amines↗
PubMed1989

Adsorption of the cationic antitumoral drug celiptium to phosphatidylglycerol in membrane model systems. Effect on membrane electrical properties.

The binding of the cationic antitumoral drug Celiptium to the anionic phospholipid phosphatidylglycerol was studied by measuring surface potentials and surface pressures in monolayers, and by determination of electrophoretic mobility on liposomes. Surface potential and zeta potential data were interpreted in terms of the Gouy-Chapman-Stern theory of the diffuse electrical double layer. A unique drug-to-lipid adsorption constant KaD, could not be calculated. KaD was observed to increase rapidly from 10(4) M-1 to 10(6) M-1 with an increase in drug concentration from 5 x 10(-7) M to 7 x 10(-6) M. This was accompanied by a marked decrease (in absolute value) in the corresponding electrophoretic mobilities which, from negative at low drug concentrations, became positive at drug concentrations of 10(-5) M and above. This indicates that the drug-to-lipid binding cannot be accounted for by a simple Langmuir adsorption isotherm, but corresponds to a more complex process, probably of a cooperative nature. Comparison of delta V and zeta potential data shows that adsorption of Celiptium to phosphatidylglycerol not only lowers the electrical surface potential, psi 0 (in absolute value) but also markedly reduces the polarization potential, delta Vp. These observations suggest that Celiptium destabilizes the electrical properties of cell plasma membranes.

Algorithms↗
PubMed1993

Electric field-mediated glycophorin insertion in cell membrane is a localized event.

Purified soluble glycophorin, an intrinsic protein, can be back 'electroinserted' in the membrane of Chinese hamster ovary cells by submitting the cell/protein mixture to short electric field pulses. Previous studies showed that this complex between pulsed cells and proteins, which is detected only when the cell membrane is electropermeabilized, was very stable. This strongly suggested that the protein was indeed inserted in the membrane. The basic processes involved in this phenomena are studied in the present work. The association is observed at the single cell level by means of videoimmunofluorescence. Electric field-mediated insertion occurs firstly in a limited patch of the cell surface, which size is in agreement with the prediction of Electropermeabilization theory. A free diffusion of the inserted proteins then follows on the cell surface. The diffusion coefficient is computed to be less than 10(-10) cm2/s as observed for transmembranous proteins. This slow process gives an homogeneous distribution of the inserted protein.

Animals↗
PubMed1995

Determination of binding constants of Ca2+, Na+, and Cl- ions to liposomal membranes of dipalmitoylphosphatidylcholine at gel phase by particle electrophoresis.

The zeta potentials of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) liposomes were measured at a gel phase as a function of CaCl2 concentration (0-200 mM) in a solution containing different NaCl concentrations (0-200 mM). The data obtained were analyzed with the diffuse double layer theory including the Graham theory. The intrinsic binding constants of ions to DPPC membranes and the distance of the shear plane have been determined independent of both the concentration of CaCl2 and that of NaCl. The values of the constants were 37 M-1 for Ca2+, 0.28 M-1 for Cl, and 0.25 M-1 for Na+; the distance was 0.24 nm under the assumption of binding stoichiometry of Ca2+/DPPC = 1:1.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1977

Importance of glycerol and fatty acid residues on the ionic properties of phosphatidylglycerols at the air-water interface.

Ionic properties of didodecanoylphosphatidylglycerol (C12PG), didodecanolyphosphatidyl-l'-propanol (C12PP), di-(12-methyl, 13-methyl)-pentadecanoylphosphatidylglycerols (C15PG) and dihexadecanoylphosphatidylglycerol (C16PG) have been studied at the air-water interface using titration experiments at constant ionic strength and film expansion experiments at constant pH, with Li+, Na+, K+ and Cs+ in the subphase. For each lipid, the apparent pK in the surface is strongly dependent on the subphase salt concentration and differs from expected intrinsic pK in the bulk. Discrimination between alkaline cations is observed. These results can be accounted for by strong surface potentials, which are satisfactorily calculated by using the Gouy and Chapman theory of the diffuse double layer. The comparison of C12PP and PG expansion data shows the importance of the glycerol residue of PG ionic properties, favouring penetration of cations in the films. Lipids in the liquid-crystalline state, such as C12-and C15PG, do not interact with alkaline cations as does C16PG in the gel phase. In particular, film condensations bring about a clear-cut discrimination between Na+ and K+. Results are discussed with regard to cation penetration and the structure of water at the interface. The importance on membrane functions of these strong surface potentials generated by PG monolayers is suggested.

Air↗
PubMed1988

Kinetics of antigen-antibody reactions at solid-liquid interfaces.

The kinetics of antigen-antibody reactions is reviewed with special attention paid to the specific properties at solid-liquid interfaces. Theories of possible diffusion limitation in forward reaction rates are compared to experiments. It is found that the intrinsic forward reaction rate in the bimolecular antigen-antibody reaction is normally not limited by diffusion either in solution or at the solid-liquid interface. However, reactions at the solid-liquid interface can be diffusion limited due to depletion of reactants close to the surface. This effect depends on geometry, intrinsic reaction rate and surface concentration of receptor molecules. Normally cell surface reactions are not diffusion limited whereas reactions at artificial surfaces often are limited by diffusion. When not limited by diffusion it is also found that the intrinsic forward and reverse reaction rates are lower for surface reactions compared to reactions in solution. Antigen-antibody reactions at solid-liquid interfaces can often be considered as practically irreversible and limited by mass transport or steric interactions.

Antigen-Antibody Reactions↗
PubMed1983

The influence of hydrophobic ions and dipolar molecules on the electrostatic barrier in biomembranes.

To calculate the electric field inside a membrane the aqueous phase can be approximated by a conductor since the dielectric constant of water is much larger than that of the membrane. Then, using the method of image charges, ions adsorbed inside the membrane can be considered as dipoles and dipolar molecules adsorbed inside the membrane may similarly be regarded as sets of two similarly oriented dipoles. The microscopic interactions and, therefore, the spatial correlations of the adsorbed species can then be obtained. Together with the Gouy theory for the diffuse double layer these results allow the determination of the adsorbed phase--aqueous phase equilibrium. From the densities and spatial correlations of the adsorbed ions and dipolar species, their influence upon the electrostatic barrier as experienced by an ion translocating the membrane can be calculated. Changes observed in the relaxation time and initial conductance of translocating hydrophobic ions in voltage-pulse experiments on bilayer membranes are predicted using this model of the electrostatic barrier. In addition, an equation giving the surface tension as a function of the (non-ideal) adsorption of hydrophobic ions and dipoles is derived.

Adsorption↗
PubMed1981

Interaction of some charged amphiphilic drugs with phosphatidylcholine vesicles. A spin label study of surface potential effects.

Adsorptions of amphiphilic drugs (propranolol, alprenolol, metoprolol and tetracaine) to phosphalidylcholine vesicles in media of different pH and NaCl concentrations have been studied. A positively charged spin label amphiphile, N,N-di-methyl-N-nonyl-N-tempoylammoniumbromide, was used to follow the variation in the surface potential by ESR. Competition experiments between the probe molecule and the drugs were carried out. A spin-labeled analogue of propranolol was also employed. We have analysed the results in terms of the theory for the diffuse double layer (Gouy-Chapman) and treated various equilibrium models. A weak, specific adsorption of chloride ions was introduced. For the charged forms of the drugs simulations of the experiments by numerical solution of the system of equations in a satisfactory way furnished intrinsic binding constants, independent of surface potential effects. The common electrostatic surface potential is mainly ruling the competition, and not the number of surface vacancies.

Journal Article↗
PubMed1986

The bacteriorhodopsin model membrane system as a prototype molecular computing element.

The quest for more sophisticated integrated circuits to overcome the limitation of currently available silicon integrated circuits has led to the proposal of using biological molecules as computational elements by computer scientists and engineers. While the theoretical aspect of this possibility has been pursued by computer scientists, the research and development of experimental prototypes have not been pursued with an equal intensity. In this survey, we make an attempt to examine model membrane systems that incorporate the protein pigment bacteriorhodopsin which is found in Halobacterium halobium. This system was chosen for several reasons. The pigment/membrane system is sufficiently simple and stable for rigorous quantitative study, yet at the same time sufficiently complex in molecular structure to permit alteration of this structure in an attempt to manipulate the photosignal. Several methods of forming the pigment/membrane assembly are described and the potential application to biochip design is discussed. Experimental data using these membranes and measured by a tunable voltage clamp method are presented along with a theoretical analysis based on the Gouy-Chapman diffuse double layer theory to illustrate the usefulness of this approach. It is shown that detailed layouts of the pigment/membrane assembly as well as external loading conditions can modify the time course of the photosignal in a predictable manner. Some problems that may arise in the actual implementation and manufacturing, as well as the use of existing technology in protein chemistry, immunology, and recombinant DNA technology are discussed.

Bacteriorhodopsins↗
PubMed2005

Assessment of supporting electrolyte contributions in electrochemically modulated liquid chromatography.

This paper reports the results of an investigation on the role of the supporting electrolyte in separations using electrochemically modulated liquid chromatography (EMLC) with a porous graphitic carbon stationary phase. With respect to the identity of the supporting electrolyte, the elution strength of the electrolyte anion increased as F- < OH- < BF4- < ClO4- < PF6- for injections of negatively charged aromatic molecules, whereas a 10-fold increase in electrolyte concentration induced a 60% change in retention for the same solutes. Furthermore, both the concentration and composition of the supporting electrolyte affected retention in a manner that varied with the charge of the analyte and applied potential. This behavior is explained using Gouy-Chapman diffuse double layer theory, coupled with comparisons of this theory with closely related models for ion-pair chromatography. Insights into the retention mechanism reveal that an ion-exchange mechanism controls the retention of negatively charged solutes at applied potentials removed from the potential of zero charge (PZC). At potentials close to the PZC, the electrostatic model is less effective with the predominant retention mechanism likely involving hydrophobic interactions with the carbonaceous stationary phase. The combined effects of these findings are demonstrated by using a temporal gradient in supporting electrolyte concentration to optimize an EMLC separation.

Chromatography, Liquid↗
PubMed2004

Physicochemical properties and mechanism of drug release from ethyl cellulose matrix tablets prepared by direct compression and hot-melt extrusion.

The objective of this research project was to determine the physicochemical properties and investigate the drug release mechanism from ethyl cellulose (EC) matrix tablets prepared by either direct compression or hot-melt extrusion (HME) of binary mixtures of water soluble drug (guaifenesin) and the polymer. Ethyl cellulose was separated into "fine" or "coarse" particle size fractions corresponding to 325-80 and 80-30 mesh particles, respectively. Tablets containing 30% guaifenesin were prepared at 10, 30, or 50 kN compaction forces and extruded at processing temperatures of 80-90 and 90-110 degrees C. The drug dissolution and release kinetics were determined and the tablet pore characteristics, tortuosity, thermal properties and surface morphologies were studied using helium pycnometry, mercury porosimetry, differential scanning calorimetry and scanning electron microscopy. The tortuosity was measured directly by a novel technique that allows for the calculation of diffusion coefficients in three experiments. The Higuchi diffusion model, Percolation Theory and Polymer Free Volume Theory were applied to the dissolution data to explain the release properties of drug from the matrix systems. The release rate was shown to be dependent on the ethyl cellulose particle size, compaction force and extrusion temperature.

Cellulose↗
PubMed2004

Development of a robust microtiter plate-based assay method for assessment of bioactivity.

A microtiter plate-based assay was developed for the quantitative monitoring of bioactive compound production in Streptomyces hygroscopicus fermentation samples. The method reported demonstrates the successful application of the theories of disk diffusion based methods of bioactivity assessment, to a microtiter assay for high throughput analysis. The assay method facilitates the generation of the dose-response curve of test organisms (Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae) to a bioactive compound. Using this dose-response curve, the method facilitates definition of three distinct Minimum Inhibitory Concentration (MIC) values for use in the characterisation of the bioactive attributes of a sample. The assay uses established standard procedures to facilitate adaptation of the assay for use with a wider range of test microorganisms. Errors due to the assumption of a linear relationship between turbidity and biomass concentration are also reduced, due to incorporation of a step to convert turbidity to biomass concentration, for use in the calculation of bioactivity.

Anti-Bacterial Agents↗
PubMed1974

Phospholipid flip-flop and the distribution of surface charges in excitable membranes.

There is now good evidence that most of the lipids in a biological membrane are arranged in the form of a bilayer. Charged lipids in the membrane of an excitable cell are subject to a significant driving force, the gradient of the intramembrane potential, which will tend to redistribute the lipids between the two halves of the bilayer by a "phospholipid flip-flop" mechanism. We have calculated, by combining the Boltzmann relation from statistics and the Gouy equation from the theory of the diffuse double layer, the steady-state distribution of charged lipids in the bilayer. This distribution is completely determined, within the framework of the model, by three experimentally accessible variables; the percentage of charged lipid in the bilayer as a whole, the resting potential and the ionic strength. The known values for the percentage of anionic phospholipids in squid axons (10-15%), the membrane potential (50-100 mV) and ionic strength (0.5 M) imply that the charge density and double layer potential at the outer surface of the nerve will be substantially greater than the charge density and double layer potential at the inner surface, in agreement with the best available evidence from physiological measurements.

Mathematics↗
PubMed1978

Fluorescence changes in isolated broken chloroplasts and the involvement of the electrical double layer.

We studied the effects of a variety of cations on chlorophyll fluorescence yield of broken chloroplasts prepared under carefully controlled ionic conditions. In the absence of light-induced electron transport and associated proton pumping, two types of cation-induced chlorophyll fluorescence changes could be distinguished in broken chloroplasts. These are termed "reversible" and "irreversible" fluorescence yield changes. Reversible fluorescence yield changes are characterized by antagonistic effects of monovalent and divalent cations and are prevented by the presence of 5 mM Mg2+ in the suspending media. Reversible-type fluorescence yield changes show little or no dependence on the structure, lipid solubility, or coordination number of the cation, but depend strictly on the net positive charge carried by the ion. It is proposed that these fluorescence changes are brought about through the interaction of monovalent or divalent cations with an electrical double layer at the interface of the outer surface of the thylakoid membrane and the surrounding aqueous solution. The results are interpreted in terms of the Gouy-Chapman theory of the diffuse double layer, indicating that the thylakoid outer surface bears an excess fixed negative charge density of about 2.5 muC/cm2, or approximately 1 negative charge per 640 A2 of membrane surface. Chlorophyll fluorescence quenching in isolated broken chloroplasts suspended in media containing 5 mM MgCl2 is also observed on addition of certain polyvalent cations to the medium. This type of cation-induced fluorescence change appears to be largely irreversible and may occur through specific binding of the cation to the thylakoid as a result of the high electrostatic attraction exerted by the negatively charged membrane surface.

Cations, Divalent↗
PubMed1994

The barrel-stave model as applied to alamethicin and its analogs reevaluated.

Alamethicin and its analogs from cation selective, multi-conductance channels in lipid bilayers. The conductance levels have been thought to be due to a barrel-stave structure where conducting pores (barrels) are formed by the self-assembly of a variable number of alpha-helical rods (staves). The conductance transitions were then interpreted as the addition or deletion of peptide monomers from the pore-forming complex (Sansom, M.S. 1991. Prog. Biophys. Mol. Biol. 55:139-235). Initially, pore conductances were calculated from that expected of right circular cylinders of "bulk" electrolyte. More recent theories also included the access resistance of the electrolyte outside the pore. However, they all consistently overestimated the observed conductances. The reason for the discrepancy is presented here. Previous theories ignored the effects of ion concentration gradients near the pore. Hence, they only held in the limit of small bilayer potentials (< 25 mV) and so would overestimate measurements that typically used much larger potentials (> 100 mV). This theoretical flaw is corrected by using Läuger's theory of diffusion-limited ion flow (Läuger, P. 1976. Biochim. Biophys. Acta. 455:493-509). Thus, including the effects of ion concentration gradients results in a considerable improvement in predicting pore conductances. It is found that: 1) the effects of ion concentration gradients must be included in the barrel-stave model for it to apply to the available data; 2) previously published explanations for the discrepancy between the model and the data, namely the "distorted bundle" and the "head-to-tail aggregate" hypotheses are not necessary (reviewed by Sansom, 1991).

Alamethicin↗
PubMed1998

Optimal harvesting under stochastic fluctuations and critical depensation.

We consider the derivation of the optimal harvesting strategy maximizing the expected cumulative yield from present up to extinction, under the assumption that the harvested population fluctuates stochastically and is subjected to an Allee-effect. By relying on both stochastic calculus and the classical theory of linear diffusions, we derive both the optimal harvesting thresholds at which harvesting should be initiated at full capacity and the value of the optimal strategy. In contrast to ordinary models which are absent of critical depensation, we show that the presence of an Allee-effect leads to the introduction of a lower harvesting threshold at which the population should be immediately depleted under the optimal policy. Moreover, we demonstrate that discounting increases the incentives to harvest and, therefore, increases the probability of a soon extinction of the harvested population.

Animals↗
PubMed1996

Excitation-contraction coupling in heart: new insights from Ca2+ sparks.

Ca2+ sparks, the elementary units of sarcoplasmic reticulum (SR) Ca2+ release in cardiac, smooth and skeletal muscle are localized (2-4 microns ) increases in intracellular Ca2+ concentration, [Ca2+]i, that last briefly (30-100 ms). These Ca2+ sparks arise from the openings of a single SR Ca2+ release channel (ryanodine receptor, RyR) or a few RyRs acting in concert. In heart muscle, Ca2+ sparks can occur spontaneously in quiescent cells at a low rate (100 s-1 per cell). Identical Ca2+ sparks are also triggered by depolarization because the voltage-gated sarcolemmal L-type Ca2+ channels (dihydropyridine receptors, DHPRs) locally increase [Ca2+]i and thereby activate the RyRs by Ca(2+)-induced Ca2+ release (CICR). The exquisite responsiveness of this process, reflected by the ability of even a single DHPR to activate a Ca2+ spark, is perhaps due to the large local increase in [Ca2+]i in the vicinity of the RyR that is a consequence of the close apposition of the DHPRs and the RyRs. In this review we examine our current understanding of cardiac excitation-contraction (EC) coupling in light of recent studies on the elementary Ca2+ release events or Ca2+ sparks. In addition, we further characterized Ca2+ spark properties in rat and mouse heart cells. Specifically we have determined that: (i) Ca2+ sparks occur at the junctions between the transverse-tubules and the SR in both species; (ii) Ca2+ sparks are asymmetric, being 18% longer in the longitudinal direction than in the transverse direction; and (iii) Ca2+ sparks individually do not produce measurable sarcomere shortening (< 1%). These results are discussed with respect to local activation of the RyRs, the stability of CICR, Ca2+ diffusion, and the theory of EC coupling.

Animals↗
PubMed2003

Use of mammography screening among older Samoan women in Los Angeles county: a diffusion network approach.

Minority migrant populations, such as older Samoan women, are likely to underuse preventive health services, including mammography screening. The purpose of this paper is to explore how informal (lay peers from churches) and formal (health care providers) health communication networks influence mammography screening use among older Samoan women. To do so, we apply diffusion of innovation theory and network analysis to understand how interpersonal networks may affect mammography use in this urban-dwelling, migrant population. The data come from a survey of 260 Samoan women, aged 50 years or older, who attended 39 randomly sampled Samoan churches in Los Angeles County (USA) between 1996 and 1997. Retrospective data, based over a 20-year period from this sample's year of first use of mammography screening, suggest that interpersonal networks may have accounted for the dramatic increase in the rate of adoption within the past 5 years of the survey. Using this information, we categorized women into mutually exclusive stages of mammography use and regressed these stages of mammography use on formal (had a provider referral) and informal (level of connectedness with peers in churches) health communication networks. The results indicated that being well-connected within women's informal, church-based health communication networks increased the likelihood of being in the decision (planned to have) and implementation and confirmation (had a recent mammogram) stages, but having a provider referral for a mammogram (formal networks) only increased the likelihood of being in the latter stages compared to women in the knowledge and persuasion stages. Formal and informal health communication networks influence recent use of mammography screening, but informal networks, in and of themselves, are also influential on future intention to use mammography screening.

Aged↗