The effects of Ca2+ on lipid diffusion

The effects of Ca2+ on rotational and translational diffusion of lipids in multilamellar dimyristoylphosphatidylcholine (DMPC)-water systems were investigated by time-resolved phosphorescence anisotropy steady-state fluorescence polarization and fluorescence recovery after photobleaching (FRAP) experiments. Above the phase transition temperature (Tm), addition of Ca2+ caused a steady increase in the segmental motion of the phosphorescent probe, but resulted in slower diffusion of the fluorescent and lateral diffusion probes. The former result is attributed to changes in the structure of the lipid/water interface that affects the chromophore mobility on the phosphorescence time scale but does not reflect lipid motion. Below the phase transition temperature, slower diffusion of all probes were observed with increasing concentrations of Ca2+, with sudden large changes occurring at [Ca2+] approximately 500 mM. This behaviour is attributed to association of Ca2+ with the lipid phosphate groups and the exclusion of water molecules which results in tighter packing of lipids and smaller segmental motion, leading eventually to the immobilization of lipid molecules.     

Diffusion in colloidal media

When the molecules of a solute diffuse through a medium containing large colloidal particles, which absorb the diffusing molecules, the latter are transported in the diffusion flow not as free molecules, but as absorbtion compounds: solute+colloid. When the colloidal particle is much larger than the molecule of the solute, and has therefore a much smaller mobility, this results in a reduction of the apparent diffusion coefficient for the solute. The biological implications of this are discussed.     

Molecular modeling and simulation of Mycobacterium tuberculosis cell wall permeability

The low permeability of the mycobacterial cell wall is thought to contribute to the intrinsic drug resistance of mycobacteria. In this study, the permeability of the Mycobacterium tuberculosis cell wall is studied by computer simulation. Thirteen known drugs with diverse chemical structures were modeled as solutes undergoing transport across a model for the M. tuberculosis cell wall. The properties of the solute-membrane complexes were investigated by means of molecular dynamics simulation, especially the diffusion coefficients of the solute molecules inside the cell wall. The molecular shape of the solute was found to be an important factor for permeation through the M. tuberculosis cell wall. Predominant lateral diffusion within, as opposed to transverse diffusion across, the membrane/cell wall system was observed for some solutes. The extent of lateral diffusion relative to transverse diffusion of a solute within a biological cell membrane may be an important finding with respect to absorption distribution, metabolism, elimination, and toxicity properties of drug candidates. Molecular similarity measures among the solutes were computed, and the results suggest that compounds having high molecular similarity will display similar transport behavior in a common membrane/cell wall environment. In addition, the diffusion coefficients of the solute molecules across the M. tuberculosis cell wall model were compared to those across the monolayers of dipalmitoylphosphatidylethanolamine and dimyristoylphosphatidylcholine, are two common phospholipids in bacterial and animal membranes. The differences among these three groups of diffusion coefficients were observed and analyzed.     

Differences in hydration coupled to specific and nonspecific competitive binding and to specific DNA Binding of the restriction endonuclease BamHI

Using the osmotic stress technique together with a self-cleavage assay we measure directly differences in sequestered water between specific and nonspecific DNA-BamHI complexes as well as the numbers of water molecules released coupled to specific complex formation. The difference between specific and nonspecific binding free energy of the BamHI scales linearly with solute osmolal concentration for seven neutral solutes used to set water activity. The observed osmotic dependence indicates that the nonspecific DNA-BamHI complex sequesters some 120-150 more water molecules than the specific complex. The weak sensitivity of the difference in number of waters to the solute identity suggests that these waters are sterically inaccessible to solutes. This result is in close agreement with differences in the structures determined by x-ray crystallography. We demonstrate additionally that when the same solutes that were used in competition experiments are used to probe changes accompanying the binding of free BamHI to its specific DNA sequence, the measured number of water molecules released in the binding process is strikingly solute-dependent (with up to 10-fold difference between solutes). This result is expected for reactions resulting in a large change in a surface exposed area.     

Diffusional attenuation of proton echo and control of changes in the cytoskeleton hydration level

Using the pulsed NMR-method with pulse gradient of the magnetic field, a study was made of changes in diffusional attenuation (DA) of proton echo in germs of triticum grains under the influence of agents depolymerizing cytoskeleton elements or inhibiting their ATP-activity. At a short diffusion time, we observed respective alterations of NMR-population in a fraction of cell water molecules contributing to DA at large values of pulse gradient. It is shown that the presence in DA of a site with essentially smaller coefficient of self-diffusion, as compared with basic cell water fraction, is not a consequence of restricted diffusion phenomenon. The observed changes in NMR-population of the appropriate fraction of cell water molecules are interpreted as changes in the level of cytoskeleton hydratation occurring under the influence of the used agents. A hypothesis is proposed for the functional role of similar changes in the level of hydratation of cytoskeleton elements located inside plamodesmata in the mechanism of plasmodesmatal water permeability regulation.     

Coupling of Solute and Solvent Flows in Porous Lipid Bilayer Membranes

The present experiments were designed to evaluate coupling of water and nonelectrolyte flows in porous lipid bilayer membranes (i.e., in the presence of amphotericin B) in series with unstirred layers. Alterations in solute flux during osmosis, with respect to the flux in the absence of net water flow, could be related to two factors: first, changes in the diffusional component of solute flux referable to variations in solute concentrations at the membrane interfaces produced by osmotic flow through the unstirred layers; and second, coupling of solute and solvent flows within the membrane phase. Osmotic water flow in the same direction as solute flow increased substantially the net fluxes of glycerol and erythritol through the membranes, while osmotic flow in the opposite direction to glycerol flow reduced the net flux of that solute. The observed effects of osmotic water flow on the fluxes of these solutes were in reasonable agreement with predictions based on a model for coupling of solute and solvent flows within the membrane phase, and considerably in excess of the prediction for a diffusion process alone.     

A model for the simulation of an aqueous dipeptide solution

A model for the simulation of a solution of an alanine dipeptide in water is presented that combines a previous model for bulk water (ST2) with that used in conformational energy studies on small molecules and proteins. The results of a pilot molecular dynamics study indicate that the model leads to reasonable solvent–solute interactions. No evidence is found for substantial changes in the structure or dynamics of the dipeptide in solution as compared to in vacuo. Furthermore, at the elevated temperature examined, there appear to be no significant effects on the dynamics or intermolecular bonding of the water molecules in contact with the solute.     

A multiscale theoretical model for diffusive mass transfer in cellular biological media

An integrated methodology is developed for the theoretical analysis of solute transport and reaction in cellular biological media, such as tissues, microbial flocs, and biofilms. First, the method of local spatial averaging with a weight function is used to establish the equation which describes solute conservation at the cellular biological medium scale, starting with a continuum-based formulation of solute transport at finer spatial scales. Second, an effective-medium model is developed for the self-consistent calculation of the local diffusion coefficient in the cellular biological medium, including the effects of the structural heterogeneity of the extra-cellular space and the reversible adsorption to extra-cellular polymers. The final expression for the local effective diffusion coefficient is: D(Abeta)=lambda(beta)D(Aupsilon), where D(Aupsilon) is the diffusion coefficient in water, and lambda(beta) is a function of the composition and fundamental geometric and physicochemical system properties, including the size of solute molecules, the size of extra-cellular polymer fibers, and the mass permeability of the cell membrane. Furthermore, the analysis sheds some light on the function of the extra-cellular hydrogel as a diffusive barrier to solute molecules approaching the cell membrane, and its implications on the transport of chemotherapeutic agents within a cellular biological medium. Finally, the model predicts the qualitative trend as well as the quantitative variability of a large number of published experimental data on the diffusion coefficient of oxygen in cell-entrapping gels, microbial flocs, biofilms, and mammalian tissues.     

Non-polar solutes in water and in aqueous solutions of protein denaturants. Modeling of solution and transfer processes

A simple molecular model for the thermodynamic behavior of non-polar solutes in water and in aqueous solutions of protein denaturants is presented. Three contributions are considered: (i) combinatorial arising from the mixing process, (ii) interactional characterizing the molecular interactions occurring in the mixture and (iii) a contribution originating from the structural changes occurring in the first shell of water molecules around the solute. The latter is modeled assuming that water molecules in contact with the solute are involved in a chemical equilibrium between two states. The model describes well the temperature and denaturant concentration dependences of the Gibbs energies of solution and transfer for benzene, toluene and alkanes in water and aqueous solutions of urea and guanidine hydrochloride. Model parameters are physically meaningful, allowing a discussion of the molecular interactions involved. A preferential solvation of the solute by the denaturant is found. However, the non-polar solute-denaturant interaction is not specific, i.e. leading to a distinct chemical entity. Urea and guanidine hydrochloride are non-polar solubilizing agents because their interactions with the solute are less unfavorable than those between water and the solute.     

Local macromolecule diffusion coefficients in structurally non-uniform bacterial biofilms using fluorescence recovery after photobleaching (FRAP)

Pure culture Pseudomonas putida biofilms were cultivated under controlled conditions to a desired overall biofilm thickness, then employed within classical half-cell diffusion chambers to estimate, from transient solute concentrations, the effective diffusion coefficient for several macromolecules of increasing molecular weight and molecular complexity. Results of traditional half-cell studies were found to be erroneous due to the existence of microscopic water channels or crevasses that perforate the polysaccharidic gel matrix of the biofilm, sometimes completely to the supporting substratum. Thus, half-cell devices measure a composite transfer coefficient that may overestimate the true, local flux of solutes in the biofilm polysaccharide gel matrix. An alternative analytical technique was refined to determine the local diffusion coefficients on a micro-scale to avoid the errors created by the biofilm architectural irregularities. This technique is based upon the Fluorescence Return After Photobleaching (FRAP), which allows image analysis observation of the transport of fluorescently labeled macromolecules as they migrate into a micro-scale photobleached zone. The technique can be computerized and allows one to map the local diffusion coefficients of various solute molecules at different horizontal planes and depths in a biofilm. These mappings also indirectly indicate the distribution of water channels in the biofilm, which was corroborated independently by direct microscopic observation of the settling of fluorescently-labeled latex spheres within the biofilm. Fluorescence return after photobleaching results indicate a significant reduction in the solute transport coefficients in biofilm polymer gel vs. the same value in water, with the reduction being dependent on solute molecule size and shape.     

The osmotic sensitivity of netropsin analogue binding to DNA

The binding of a netropsin analogue to random sequence DNA, monitored by CD, is seen dependent on the concentration of neutral solutes. The binding free energy decreases linearly with solute osmolal concentration and the magnitude of the effect is insensitive to the chemical identity of the solute fur betaine, sorbitol, and triethylene glycol. These solutes appear to modulate binding through their effect on water activity and changes in the hydration of the drug and DNA in the complex reaction, not through a direct interaction with the reactants or the product. The dependence of binding constant on solute concentration can be interpreted as an additional binding of some 50–60 extra solute excluding water molecules by the complex. A water sensitivity of drug binding is further seen from the dependence of binding constants on the type of anion in solution. Anions in the Hofmeister series strongly affect bulk water free energies and entropies. The differences in netropsin analogue binding to DNA with Cl⁻, F⁻, and CIO are consistent with the effect observed with neutral solutes. The ability to measure changes in water binding associated with a specific DNA interaction is a first step toward correlating changes in hydration with the strength and specificity of binding. © 1995 John Wiley & Sons, Inc.     

A method for the determination of diffusion coefficients for small molecules in aqueous solution

A method is described for determining the diffusion coefficients of small solutes in limited volumes (approximately equal to 4-9 ml) of fluid. Diffusion is measured in a three-chamber diffusion cell across a central unstirred compartment. Compartments are separated by nitrocellulose membranes. The instantaneous concentration gradient and the instantaneous flux of solute into the dilute end compartment are derived from changes in the concentration of solute in the two stirred end compartments through time. The diffusion coefficient is calculated from the slope of the least-squares regression line relating the magnitude of the instantaneous solute flux to that of the instantaneous concentration gradient. The apparatus is calibrated with a solute of known diffusivity (KCl). Diffusion coefficients thus determined in water at 25 degrees C for CaCl2 (7.54 X 10(-6) cm2.s-1), Na2-ATP (7.01 X 10(-6) cm2.s-1), 2-deoxyglucose (5.31 X 10(-6) cm2.s-1), and D-Na-lactate (5.62 X 10(-6) cm2.s-1) differed by an average of 3.7% from literature values. The method described results in accurate estimates of diffusion coefficients by a simple and relatively rapid procedure.     

The size of the unstirred layer as a function of the solute diffusion coefficient.

By monitoring the concentration distribution of several solutes that are diffusing at the same time under given mixing conditions, it was established that the unstirred layer (USL) has no clearly defined boundary. For the cases of solute permeation and water movement across planar bilayer lipid membranes, respectively, experiments carried out with double-barreled microelectrodes have shown that the thickness of the USL depends on which species is diffusing. Small molecules with a larger diffusion coefficient encounter an apparently thicker USL than larger molecules with a smaller diffusion coefficient. The ratio of the USL thicknesses of two different substances is equal to the third root of the ratio of the respective diffusion coefficients. This experimental finding is in good agreement with theoretical predictions from the theory of physicochemical hydrodynamics.     

The short-term growth response to salt of the developing barley leaf

Recent results concerning the short-term growth response to salinity of the developing barley leaf are reviewed. Plants were grown hydroponically and the growth response of leaf 3 was studied between 10 min and 5 d following addition of 100 mM NaCl to the root medium. The aim of the experiments was to relate changes in variables that are likely to affect cell elongation to changes in leaf growth. Changes in hormone content (ABA, cytokinins), water and solute relationships (osmolality, turgor, water potential, solute concentrations), gene expression (water channel), cuticle deposition, membrane potential, and transpiration were followed, while leaf elongation velocity was monitored. Leaf elongation decreased close to zero within seconds following addition of NaCl. Between 20 and 30 min after exposure to salt, elongation velocity recovered rather abruptly, to about 46% of the pre-stress level, and remained at the reduced rate for the following 5 d, when it reached about 70% of the level in non-stressed plants. Biophysical and physiological analyses led to three major conclusions. (i) The immediate reduction and sudden recovery in elongation velocity is due to changes in the water potential gradient between leaf xylem and peripheral elongating cells. Changes in transpiration, ABA and cytokinin content, water channel expression, and plasma membrane potential are involved in this response. (ii) Significant solute accumulation, which aids growth recovery, is detectable from 1 h onwards; growing and non-growing leaf regions and mesophyll and epidermis differ in their solute response. (iii) Cuticular wax density is not affected by short-term exposure to salt; transpirational changes are due to stomatal control.     

THE INFLUENCE OF ELECTROLYTES ON THE ELECTRIFICATION AND THE RATE OF DIFFUSION OF WATER THROUGH COLLODION MEMBRANES

1. When pure water is separated by a collodion membrane from a watery solution of an electrolyte the rate of diffusion of water is influenced not only by the forces of gas pressure but also by electrical forces. 2. Water is in this case attracted by the solute as if the molecules of water were charged electrically, the sign of the charge of the water particles as well as the strength of the attractive force finding expression in the following two rules, (a) Solutions of neutral salts possessing a univalent or bivalent cation influence the rate of diffusion of water through a collodion membrane, as if the water particles were charged positively and were attracted by the anion and repelled by the cation of the electrolyte; the attractive and repulsive action increasing with the number of charges of the ion and diminishing inversely with a quantity which we will designate arbitrarily as the "radius" of the ion. The same rule applies to solutions of alkalies. (b) Solutions of neutral or acid salts possessing a trivalent or tetravalent cation influence the rate of diffusion of water through a collodion membrane as if the particles of water were charged negatively and were attracted by the cation and repelled by the anion of the electrolyte. Solutions of acids obey the same rule, the high electrostatic effect of the hydrogen ion being probably due to its small "ionic radius." 3. The correctness of the assumption made in these rules concerning the sign of the charge of the water particles is proved by experiments on electrical osmose. 4. A method is given by which the strength of the attractive electric force of electrolytes on the molecules of water can be roughly estimated and the results of these measurements are in agreement with the two rules. 5. The electric attraction of water caused by the electrolyte increases with an increase in the concentration of the electrolyte, but at low concentrations more rapidly than at high concentrations. A tentative explanation for this phenomenon is offered. 6. The rate of diffusion of an electrolyte from a solution to pure solvent through a collodion membrane seems to obey largely the kinetic theory inasmuch as the number of molecules of solute diffusing through the unit of area of the membrane in unit time is (as long as the concentration is not too low) approximately proportional to the concentration of the electrolyte and is the same for the same concentrations of LiCl, NaCl, MgCl₂, and CaCl₂.     

Patterns of accumulation resulting from Taxes and changes in motility of micro-organisms

Summary An analysis is made of the development of patterns of accumulation of micro-organisms, as governed by tactic responses, changes in motility, and the effects of diffusion.When a soluble crystal is placed in a suspension of micro-organisms, the first manifestation is the development of a clear zone surrounding the crystal. This effect is a physical one, produced by a transfer of momentum from the solute molecules to the organisms.As the solute spreads and its boundary moves more slowly, the organisms are distributed in patterns which depend upon the occurrence of tactic responses and the influence of the solute upon motility. A congregation in the zone occupied by the solute can correspond either to a positive chemotaxis or to an inhibition of motility by the solute. Conversely, a withdrawal from the solute can signal either a negative chemotaxis or an enhancement of motility by the solute.The proper interpretation of the patterns described in Figs. 10 and 11 requires a microscopic study of individual organisms, in which the effect of the reagent upon motility is noted.     

Effect of temperature on the modification of spectral properties of tryptophan aqueous solutions induced by water previously treated with laser radiation

It was shown that preliminary exposure of a solvent (water) to low-intensity laser radiation reduces the tryptophan fluorescence intensity, and this fluorescence quenching effect is retained throughout the temperature range explored (from 8 up to 50 degrees C). The effects found are interpreted as resulting from changes in solvent properties induced by the action of electromagnetic radiation on interaction of water molecules with solute.     

Mechanistic insights into the physiological functions of cell adhesion proteins using single molecule force spectroscopy

Intercellular adhesion molecules play an important role in regulating several cellular processes such as a proliferation, migration and differentiation. They also play an important role in regulating solute diffusion across monolayers of cells. The adhesion characteristics of several intercellular adhesion molecules have been studied using various biochemical assays. However, the advent of single molecule force spectroscopy as a powerful tool to analyze the kinetics and strength of protein interactions has provided us with an opportunity to investigate these interactions at the level of a single molecule. The study of interactions involving intercellular adhesion molecules has gained importance because of the fact that qualitative and quantitative changes in these proteins are associated with several disease processes. In this review, we focus on the basic principles, data acquisition and analysis in single molecule force spectroscopy experiments. Furthermore, we discuss the correlation between results obtained using single molecule force experiments and the physiological functions of the proteins in the context of intercellular adhesion molecules. Finally, we summarize some of the diseases associated with changes in intercellular adhesion molecules.     

A molecular basis for an irreversible thermodynamic description on non-electrolyte permeation through lipid bilayers

The non-electrolyte permeability of liposomal membranes has been investigated according to the concepts of irreversible thermodynamics. A strong interaction between the permeation of solute and water was observed. This solute-solvent interaction can be fully described by assuming that a number of water molecules will copermeate with each molecule of solute. This number of copermeating water molecules is independent of the nature of the permeant and of temperature, but depends on the osmotic concentration of impermeants inside the liposomes.