Sticky ions in biological systems

Evaluation of Hofmeister Effects on the Kinetic Stability of Proteins

Dissolved salts are known to affect properties of proteins in solution including solubility and melting temperature, and the effects of dissolved salts can be ranked qualitatively by the Hofmeister series. We seek a quantitative model to predict the effects of salts in the Hofmeister series on the deactivation kinetics of enzymes. Such a model would allow for a better prediction of useful biocatalyst lifetimes or an improved estimation of protein-based pharmaceutical shelf life. Here we consider a number of salt properties that are proposed indicators of Hofmeister effects in the literature as a means for predicting salt effects on the deactivation of horse liver alcohol dehydrogenase (HL-ADH), alpha-chymotrypsin, and monomeric red fluorescent protein (mRFP). We find that surface tension increments are not accurate predictors of salt effects but find a common trend between observed deactivation constants and B-viscosity coefficients of the Jones-Dole equation, which are indicative of ion hydration. This trend suggests that deactivation constants (log k(d,obs)) vary linearly with chaotropic B-viscosity coefficients but are relatively unchanged in kosmotropic solutions. The invariance with kosmotropic B-viscosity coefficients suggests the existence of a minimum deactivation constant for proteins. Differential scanning calorimetry is used to measure protein melting temperatures and thermodynamic parameters, which are used to calculate the intrinsic irreversible deactivation constant. We find that either the protein unfolding rate or the rate of intrinsic irreversible deactivation can control the observed deactivation rates.

Hemolysis of erythrocytes and erythrocyte membrane fluidity changes by new lysosomotropic compounds

This work contains the results of studies on the influence of newly synthesized lysosomotropic substances (lysosomotropes) on human erythrocytes. Six homologous series of the compounds differing in the alkyl chain length and counterions were studied. They were found to hemolyse erythrocytes and to change their osmotic resistance. The observed hemolytic effects were dependent both on the compound's structure (polar head dimension and alkyl chain length of compound) and its form (the kind of the counterion). In parallel, the influence of lysosomotropes on fluidity of the erythrocyte membrane was studied. Three different fluorescent probes were used; 1,6-diphenyl-1,3,5-hexatriene (DPH), 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene, p-toluenesulfonate (TMA-DPH) and 6-dodecanoyl-2-dimethylaminonaphthalene (laurdan). Their anisotropy (DPH and TMA-DPH) or general polarization (laurdan) values after incorporation into ghost erythrocyte membranes were measured. The results obtained show that fluidity changes accompanied the effects observed in hemolytic experiments both quantitatively and qualitatively.

Swelling of Na∕Mg-montmorillonites and hydration of interlayer cations: A Monte Carlo study

While the swelling behavior of laboratory-prepared homoionic montmorillonites has been studied extensively in numerous experimental and simulation works, far less attention has been given to much more abundant natural montmorillonites, containing a mix of monovalent and/or bivalent cations in interlayer spaces. We carried out a series of Monte Carlo simulations in order to investigate the reasons for the remarkable difference of experimental swelling patterns of a natural Na-rich/Mg-poor montmorillonite and a homoionic Na-montmorillonite. The simulations reproduced the swelling pattern of a natural montmorillonite, suggesting a mechanism of its hydration different from that of the homoionic montmorillonite. We also found that the differences in size and hydration energy of Mg2+ and Na+ ions have strong implications for the structure and the internal energy of interlayer water. This leads to a difference in the layer spacings of the simulated Mg- and Na-montmorillonites as large as approximately 2.1 A at lower water contents.

Development of a novel high-concentration galantamine formulation suitable for intranasal delivery

The goal of the current study was to develop an intranasal (IN) formulation of the acetylcholinesterase inhibitor galantamine, an important therapeutic for treating Alzheimer's disease. To allow for delivering a therapeutically relevant dose, it was necessary to greatly enhance drug solubility. Various approaches were examined to this end, including adding co-solvents, cyclodextrins, and counterion exchange. Of these, the latter, for example, replacement of bromide ion with lactate or gluconate, resulted in a dramatic drug solubility increase, more than 12-fold. NMR confirmed the molecular structure of new drug salt forms. An in vitro epithelial tissue model was used to assess drug permeability and cellular toxicity. In vitro, galantamine lactate formulations performed as well as or better than their hydrobromide (HBr) counterparts with respect to drug permeation across the epithelial membrane with minimal toxicity. In vivo studies in rats compared pharmacokinetic (PK) profiles of different formulations. The in vivo studies confirmed that IN galantamine achieves systemic blood levels comparable to those of conventional oral administration. Both the in vitro and in vivo data support the feasibility of IN administration of this important drug.

Extraction of testosterone and epitestosterone in human urine using aqueous two-phase systems of ionic liquid and salt

Based on aqueous two-phase systems (ATPS) consisting of 1-butyl-3-methylimidazolium chloride, a hydrophilic ionic liquid (IL), and K2HPO4, a new and simple extraction technique, coupled with a reversed-phase high performance liquid chromatography (RP-HPLC), was developed for the simultaneous concentration and analysis of testosterone (T) and epitestosterone (ET) in human urine. Under the optimal conditions, the extraction efficiencies for both analytes were 80-90% in a one-step extraction. The method required only 3.0 mL of urine and a single hydrolysis/deproteinization/extraction step followed by direct injection of the IL-rich upper phase into HPLC system for analysis. The method has been satisfactorily applied to the analysis of T and ET in human urine with detection limits of 1 ng/mL and linear ranges of 10-500 ng/mL for both compounds. Compared with conventional liquid-liquid extraction or solid phase extraction, this new method is much "greener" due to no use of volatile organic solvent and low consumption of IL. The proposed extraction technique opens up new possibilities in the separation of other drugs.

Role of Chloride Ions in Modulation of the Interaction between von Willebrand Factor and ADAMTS-13

The degradation of von Willebrand factor (VWF) depends on the activity of a zinc protease (referred to as ADAMTS-13), which cleaves VWF at the Tyr(1605)-Met(1606) peptide bond. Little information is available on the physiological mechanisms involved in regulation of AD-AMTS-13 activity. In this study, the role of ions on the ADAMTS-13/VWF interaction was investigated. In the presence of 1.5 m urea, the protease cleaved multimeric VWF in the absence of NaCl at pH 8.00 and 37 degrees C, with an apparent k(cat)/K(m) congruent with 3.4 x 10(4) M(-1) s(-1), but this value decreased by approximately 10-fold in the presence of 0.15 M NaCl. Using several monovalent salts, the inhibitory effect was attributed mostly to anions, whose potency was inversely related to the corresponding Jones-Dole viscosity B coefficients (ClO(4)(-) > Cl(-) > F(-)). The specific inhibitory effect of anions was due to their binding to VWF, which caused a conformational change responsible for quenching the intrinsic fluorescence of the protein and reducing tyrosine exposition to bulk solvent. Ristocetin binding to VWF could reduce the apparent affinity and reverse the inhibitory effect of chloride. We hypothesize that, after secretion into the extracellular compartment, VWF is bound by chloride ions abundantly present in this milieu, becoming unavailable to proteolysis by AD-AMTS-13. Shear forces, which facilitate GpIbalpha binding (this effect being artificially obtained by ristocetin), can reverse the inhibitory effect of chloride, whose concentration gradient across the cell membrane may represent a simple but efficient strategy to regulate the enzymatic activity of ADAMTS-13.

Anionic regulation of biological systems: the special role of chloride in the coagulation cascade

The discovery that previously unidentified allosteric properties of several proteins, such as fibrinogen and myoglobin, can be triggered by anions binding, has suggested the possibility to design a new "active" role of chloride in the modulation of a broad range of biological systems. The molecular bases of the anions binding to proteins depends by their charge density in turn regulating the ability to bind water molecules and interact with basic groups on proteins. This review reports the role of the physiologically relevant chloride, and of other anions, in the regulation of several proteins, with special attention to the coagulation cascade. Moreover, possible mechanisms of modification of plasma, intra- or extracellular chloride concentration are listed.

Precision parameters from spin-probe studies of membranes using a partitioning technique. Application to two model membrane vesicles

A new version of the ESR spin probe partitioning method is developed and applied to the study of hydration properties of dimyristoyl-phosphatidylglycerol (DMPG) and dimyristoyl-phosphatidylcholine (DMPC) vesicles as functions of salt concentration and temperature above the lipid phase transition. The small spin probe di-tert-butyl nitroxide (DTBN) is used in order to achieve motionally narrowed Electron Spin Resonance (ESR) spectra which may be analyzed with high precision. The new method relies on the use of the second harmonic display of the ESR spectrum followed by spectral line fitting. Spectral fitting yields precise ESR parameters giving detailed information on the surroundings of the spin probe in both phospholipid and aqueous phases. The nitrogen hyperfine coupling constant of DTBN arising from those probes occupying the vesicles is used to study the hydration of the vesicle surface. The hydration properties of the negatively charged vesicle surface of DMPG vesicles are affected by the addition of salt at all temperatures. In contrast, the hydration of DMPC vesicles does not change with salt concentration at the low temperatures. However, at higher temperatures the hydration properties of DMPC vesicle are affected by salt which is interpreted to be due to the faster motion of the phospholipid molecules. The partitioning of the spin probe increases with salt concentration for both DMPG and DMPC vesicles, while water penetration decreases simultaneously. The spin probe in the phospholipid bilayer exhibits anisotropic motion and the extent of the anisotropy is increased at the higher salt concentrations.

Terahertz pulsed spectroscopy as a new tool for measuring the structuring effect of solutes on water

Absorption spectra of aqueous solution of ''chaotropes'' (structure maker) and ''kosmotropes'' (structure breaker) have been recorded in the mid-infrared (MIR) and terahertz (THz) spectral region. A different impact of the two groups of solutes on the absorption spectrum of water was found in the recorded THz spectra. A concentration-dependent increased absorption across the investigated THz spectral region (0.04-2 THz, 1.3-66 cm(-1), respectively) has been recorded for all studied chaotropic solutions, whereas the opposite has been obtained for kosmotrope containing solutions. In the case of ionic solutes a further increase in absorption towards higher frequencies was measured. The distinction between chaotrope and kosmotrope solutes was, as expected, also possible in the MIR spectral region. Depending on the structure-forming effect of the solute the OH stretch vibration of the water (around 3400 cm(-1)) was slightly shifted. A red shift has been observed for solution of kosmotropes, whereas a blue shift was observed in the case of solutions containing chaotropes. Compared to the MIR spectral region the structure influencing effect of solutes can be more efficiently studied in the THz spectral region, which provides information from interactions between neighboring water molecules.

Hofmeister Series: The Hydrolytic Activity of Aspergillus niger Lipase Depends on Specific Anion Effects

The specific activity of lipase A (Aspergillus niger) toward the hydrolysis of p-nitrophenyl acetate (p-NPA) is shown to increase as a result of sodium salt addition according to specific ion effects of the Hofmeister series. This shows explicitly that the Hofmeister effect is due to the different specific interactions between anions and the enzymatic surface.

Specific ion effects on the growth rates ofStaphylococcus aureusandPseudomonas aeruginosa

Motivated by recent advances in the physical and chemical basis of the Hofmeister effect, we measured the rate cell growth of S. aureus--a halophilic pathogenic bacterium--and of P. aeruginosa, an opportunistic pathogen, in the presence of different aqueous salt solutions at different concentrations (0.2, 0.6 and 0.9 M). Microorganism growth rates depend strongly on the kind of anion in the growth medium. In the case of S. aureus, chloride provides a favorable growth medium, while both kosmotropes (water structure makers) and chaotropes (water structure breakers) reduce the microorganism growth. In the case of P. aeruginosa, all ions affect adversely the bacterial survival. In both cases, the trends parallel the specific ion, or Hofmeister, sequences observed in a wide range of physico-chemical systems. The correspondence with specific ion effect obtained in other studies, on the activities of a DNA restriction enzyme, of horseradish peroxidase, and of Lipase A (Aspergillus niger) is particularly striking. This work provides compelling evidence for Hofmeister effects, physical chemistry in action, in these organisms.

Extended thermodynamic approach to ion interaction chromatography. A mono- and bivariate strategy to model the influence of ionic strength

Recent breakthroughs in the theory of ion interaction chromatography (IIC) permit new analyses of the dependence of retention on different interdependent factors. The influence of the ionic strength / on the surface potential, the Donnan effect, and salting effects are taken into account to model the chromatographic behaviour of charged analytes in IIC. The most reliable experimental results found in the literature were used to test the retention equations that were developed following both a monovariate (/ changes as the concentration of H, ion interaction reagent, changes) and a bivariate (/ changes because of the simultaneous variation of H and of the background electrolyte concentrations) approach. The present extended thermodynamic model builds on the sound intuition of the electrostatic approach and proves to provide the most successful and exhaustive quantitative explanation of experimental evidence. It is also able to rationalise the less extensive agreement between the pure electrostatic approach predictions and experimental results. The adequacy of the model is supported by physically reliable estimates of the adjustable constant (ion-pair constants, deltaG degrees). Moreover statistical practice demonstrates that all the adjustable parameters (three at most) are statistically significant. A linear, zero crossing function with unit slope is obtained when k(pred) is plotted against k(exp). The mean percent error between k(pred) and k(exp) is 4.5% at most. In the absence of H the present retention equation reduces, as expected, to the relationship that describes the influence of / on the retention behaviour in reversed-phase liquid chromatography.

Redesigning the monovalent cation specificity of an enzyme

Monovalent-cation-activated enzymes are abundantly represented in plants and in the animal world. Most of these enzymes are specifically activated by K+, whereas a few of them show preferential activation by Na+. The monovalent cation specificity of these enzymes remains elusive in molecular terms and has not been reengineered by site-directed mutagenesis. Here we demonstrate that thrombin, a Na+-activated allosteric enzyme involved in vertebrate blood clotting, can be converted into a K+-specific enzyme by redesigning a loop that shapes the entrance to the cation-binding site. The conversion, however, does not result into a K+-activated enzyme.

Stewart and beyond: new models of acid-base balance

The Henderson-Hasselbalch equation and the base excess have been used traditionally to describe the acid-base balance of the blood. In 1981, Stewart proposed a new model of acid-base balance based upon three variables, the "strong ion difference" (SID), the total weak acids (ATot), and the partial pressure of carbon dioxide (Pco2). Over 20 years later, Stewart's physiochemical model still remains largely unknown. In this review, we will present both the traditional and the Stewart models of acid-base balance and then derive each using an "ion equilibrium method." Modern theories of acid-base balance may be useful toward the understanding of complex acid-base disorders.

Effects of potassium and chloride on ribosome association with the RNA of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) and other picornaviruses initiate translation of their polyprotein cap-independently at an internal site of the positive-strand viral RNA. This process is mediated by the internal ribosome entry site (IRES), a highly structured cis-acting RNA element that binds translation initiation factors and ribosomal subunits. During their life cycle, picornaviruses induce proliferation of membrane structures involved in viral replication and an increase in membrane permeability probably facilitating virus progeny release. Here, I analyze the efficiency of association of the ribosomal subunits with the FMDV IRES RNA at elevated salt concentrations. Potassium stimulates FMDV translation, whereas sodium chloride concentrations up to 150 mM neither stimulate nor interfere with FMDV translation. Even high potassium concentrations allow binding of the viral RNA to ribosomes. Chloride stimulates binding of ribosomes to the viral RNA at the stage of 48S initiation complex formation and FMDV translation at concentrations up to 150 mM. Only at elevated concentrations, binding of ribosomal subunits and translation are inhibited by chloride. However, FMDV start site selection is not influenced by potassium salts. These results indicate that the association of the viral RNA with ribosomal subunits is well adapted to high salt conditions that are induced during picornavirus infection.

The hydration structure of guanidinium and thiocyanate ions: implications for protein stability in aqueous solution

Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were determined. Each ion is shown to interact weakly with water: Guanidinium has no recognizable hydration shell and is one of the most weakly hydrated cations yet characterized. Hydration of thiocyanate is characterized by a low coordination number involving around one hydrogen-bonded water molecule and approximately two water molecules weakly interacting through "hydration bonds." The weak hydration of these denaturant ions strongly supports suggestions that a major contribution to the denaturant effect is the preferential interaction of the denaturant with the protein surface. By contrast, solute species such as many sugars and related polyols that stabilize proteins are strongly hydrated and are thus preferentially retained in the bulk solvent and excluded from the protein surface.

Interaction between N-dodecyl-N,N-dimethyl-N-benzylammonium halides and phosphatidylcholine bilayers-the effect of counterions

The interaction of N-dodecyl-N,N-dimethyl-N-benzylammonium halides (DBeAX) with two types of phospholipid vesicles (MLV and SUV) was investigated using DSC and 1H NMR. It was suggested that the benzyl group like the micellisation process (J. Colloid Interface Sci. 218 (1999) 529) changes its position when interacting with phosphatidylcholine bilayers and incorporates into the bilayer. In order to enhance counterion-water interactions, the surfactants were added either to the water phase or directly to the lipid phase (a mixed film was formed). It follows from the obtained results that for both types of liposomes and both manners in which the surfactant was added, the interaction of DBeAX with liposomes and consequent changes in the phospholipid bilayer organisation depend on the kind of counterion. Results are discussed in terms of counterion ability to modify water structure.

Theoretical analysis of adsorption thermodynamics for charged peptide residues on SAM surfaces of varying functionality

Cellular response to an implant is largely controlled by protein adsorption because cells directly interact with the adsorbed protein rather than the implant surface. Protein adsorption will occur when the change in Gibbs free energy (Delta G) of the system decreases during the adsorption process. Electrostatic interactions between charged peptide residues presented by a protein's surface and surface functional groups greatly contribute to the Delta G of protein adsorption. In this study, semiempirical molecular orbital calculations were used to theoretically determine the adsorption enthalpy between charged peptide residues [aspartic acid (-1), glutamic acid (-1), and arginine (+1)] and functionalized SAM surfaces [methyl, hydroxyl, amine (+1), and carboxylic acid (-1)]. Additional enthalpic and entropic contributions attributed to water restructuring effects were then approximated based on literature values for functional group solvation and considered along with the calculated enthalpy values to estimate the change in Delta G for each residue/surface system as a function of surface separation distance. The results predict long-range attraction and repulsion to the opposite and same-charge residue/surface systems, respectively, followed by strong short-range repulsion caused by functional group dehydration. Short-range repulsion alone was predicted for the charged residues on the methyl and hydroxyl surfaces. These results provide a theoretical quantitative description of fundamental mechanisms governing protein adsorption behavior and provide a basis for the development of a knowledge-based surface design approach to control biological response.

A theoretical analysis of the thermodynamic contributions for the adsorption of individual protein residues on functionalized surfaces

Although the denaturing of adsorbed proteins on biomaterials surfaces is believed to lead to adverse tissue reactions to implanted materials, very little is currently known of the actual mechanisms involved. These mechanisms must be understood if surfaces are to be proactively designed to control protein adsorption behavior. Concepts widely employed in rational drug design and in protein and RNA folding predictions provide a means to approach this problem. Accordingly, a theoretical analysis has been conducted to estimate the thermodynamic contributions (changes in enthalpy, entropy, and Gibbs free energy) for the adsorption of selected individual mid-chain protein residues to functionalized surfaces. Enthalpic contributions from residue-surface interactions were calculated using semi-empirical quantum mechanical-based computational chemistry methods in a simulated aqueous environment (MOPAC/COSMO), and enthalpic and entropic contributions due to water restructuring effects assumed to occur during adsorption were estimated from experimental data for functional group wetting and calculated changes in solvent accessible surface area as each protein residue approached each surface. When combined with intraprotein residue-residue interactions, the understanding of residue-surface adsorption energy relationships provides a means to begin to predict protein adsorption behavior as a function of biomaterials surface chemistry. It is recognized that several assumptions have been made in this approach that could be challenged, and that truncations necessary due to programming limitations have been applied that may neglect potentially important interactions. Therefore, it must be understood that the modeling predictions may not be directly applicable to biomaterials for surface design under actual physiologic conditions at this stage. However, this attempt at modeling fundamental components of protein adsorption is presented as an initial approach to understanding these complex events.

Salt effects on the interaction of an amphiphilic model molecule with immobilized phosphatidylcholine monolayers

The retention of hydrocortisone (used as an amphiphilic model solute) on an immobilized artificial membrane (IAM) column was investigated in relation to the mobile phase concentration of three sodium salts (representing different rankings in the Hofmeister series, i.e. perchlorate, chloride and sulfate) in order to provide insight into the nature of the solute interactions with phospholipid monolayers. The influence of the salt series on solute retention was found to follow the Hofmeister series, emphasizing the role of hydrophobic effect in the solute retention mechanism on phospholipid monolayers. Retention models based on the extended Wyman relations (preferential interaction theory) were developed to analyze more quantitatively the salt effects on the hydrocortisone retention factor. This analysis as well as additional thermodynamic study suggested that the hydrocortisone binding to IAM involved both an insertion into the hydrophobic inside governed by hydrophobic effects and contacts with the interfacial region implying interactions such as van der Waals interactions/hydrogen bonds between the solute hydroxyl groups and the polar headgroups of phospholipidmonolayers.

Dynamic hydration numbers for biologically important ions

The role of ionized groups in biological systems is determined by their affinity for water [Biophys. J. 72 (1997) 65-76]. The tightly bound water associated with biologically important ions increases their apparent size. We define the apparent dynamic hydration number of an ion here as the number of tightly bound water molecules that must be assigned to the ion to explain its apparent molecular weight on a Sephadex G-10 size exclusion column, and report the first accurate determination of tightly bound water for 23 ions of biological significance, including H(+) and HO(-). We also calculate the radius of the equivalent hydrated sphere (r(h)) for each ion. We find that the ratio of the hydrated volumes of two ions approximates the ratio of the square of the charges of the same two ions. Since the 'ionic strength' of the solution also depends upon the square of the charges on the ions, our results suggest that ionic strength effects may largely arise from local effects related to the hydrated volume of the ion--that is, from space filling, osmotic, water activity, surface tension and hydration shell overlap effects rather than from long-range electric field effects.

Enzyme activation for nonaqueous media

Highly active enzyme formulations can be prepared for use in nonaqueous media. Considerable progress has been made in the past two years on gaining an improved mechanistic understanding of enzyme function and activation in dehydrated environments. This increased fundamental understanding has led to the development of a broad array of techniques for generating active, stable, and enantioselective and regioselective tailored enzymes for synthetically relevant transformations. This, in turn, is resulting in an exponential increase in the opportunities for enzymatic processes to be developed on a commercial scale.

The effect of anions on azide binding to myoglobin: an unusual functional modulation

The effect of increasing concentrations of several anions on the azide (N(-)(3)) binding properties of sperm whale and horse ferric myoglobin has been studied. Surprisingly, a number of anions may act as heterotropic effectors, decreasing the affinity of myoglobins for N(-)(3), in the following order: ClO(-)(4)=I(-)>Br(-)>Cl(-) and SO(2-)(4), which mirrors the increase in their charge density. The largest effects were measured using ClO(-)(4) and I(-), which produce a 4-fold and 8-fold reduction of the N(-)(3) binding affinity in horse and sperm whale myoglobins, respectively. A dissociation equilibrium constant (K(d)) ranging from 150 to 250 mM was estimated for ClO(-)(4) and I(-) binding to myoglobins. In order to analyse the molecular mechanism producing the reduction of the N(-)(3) binding affinity to ferric myoglobin, the potential anionic binding sites within ferric myoglobin were investigated by a molecular modelling study using the program Grid. Analysis of the theoretical results suggests two particularly favourable binding sites: the first, next to the distal side of the haem, whose occupancy might alter the electrostatic potential surrounding the bound N(-)(3); the second, involving residues of helices B and G which are far from the haem iron atom, thus implying a long range effect on the bound N(-)(3). Based on the evidence that no significant conformational changes are found in the three-dimensional structures of N(-)(3)-free and N(-)(3)-bound myoglobin and on previous results on N(-)(3) binding to ferric myoglobin mutants in CD3 positions, we favour the first hypothesis, suggesting that the functional heterotropic modulation of monomeric myoglobin is mainly depending on a decrease of the positive charge density induced by the binding of anions to the haem distal side.

A framework based on the extended Wyman concept for analyzing the salt effects on the solute retention in high-performance affinity chromatography

The analysis of binding data of a ligand to a macromolecule in the presence of an additive can be classically formulated in terms of the linked functions of Wyman. In the case of a salt, this approach has been extended by Tanford such that the contributions of both salt and water are taken into account. In this paper, the extended Wyman theory was applied to high-performance affinity chromatography (HPAC) in order to define a general model describing the effects of the mobile-phase salts on the ligand binding. Various HPAC literature data, as well as our data concerning dansyl amino acid retention on a vancomycin stationary phase, were examined in relation to this model. From the results, this theoretical approach was considered to be adequate to describe accurately the salt dependence on solute retention. This work shows the importance of taking into account the effects of both ionic species and water in the investigation of relative contributions of the interactions involved in the ligand binding to immobilized receptor.

Determinants of protein retention characteristics on cation-exchange adsorbents

There are currently a large number of commercially available strong and weak cation-exchange adsorbents for preparative protein purification, typically prepared by coupling charged ligands to a mechanically rigid porous bead. Because of the diverse chemical nature of the base matrix (carbohydrate, synthetic polymer, inorganic) and the coupling and ligand chemistry, cation-exchange adsorbents from different suppliers can differ substantially in chemical surface properties and physical structure. The differences in chemical properties can be in ionic capacity, hydrophobicity, the presence of hydrogen bond donors/acceptors, and the nature of the charged functional groups. In order to probe the effects of these factors on protein affinity, the isocratic retention of a set of model proteins was examined on a set of cation-exchange adsorbents to obtain a quantitative assessment of retention differences between adsorbents. Two adsorbent factors were found to be the dominant determinants of overall protein retention: the anion type and the adsorbent pore size distribution. Protein retention on strong cation-exchangers was found to be greater than that on corresponding weak cation-exchangers. Protein retention was increased on adsorbents with pore size distributions that include significant amounts of pore space with dimensions similar to those of the protein solute.

Fluctuations and the Hofmeister effect

The Hofmeister effect consists in changes of protein solubility triggered by neutral electrolyte cosolutes. Based on the assumption that salts cause stochastic fluctuations of the free energy barrier profiles, a kinetic theory of this phenomenon is proposed. An exponentially correlated noise, of amplitude proportional to the salt concentration, is added to each energy level, and the time-dependence of the mean protein concentration is calculated. It is found that the theory yields the well-known Setschenow equation if the noise correlation time is low in comparison to the aggregation time constant. Experimental data on salting-in agents are well fitted, whereas, in the case of salting-out cosolutes, two independent dichotomic fluctuations are needed to fit the data. This may result from the fact that, in both cases, the low-concentration regime is dominated by salting-in electrostatic contributions, whereas, at high salt concentrations, electron donor/acceptor interactions become important; these have opposite effects. The theory offers a novel way to metricate Hofmeister effects and also leads to thermodynamic quantities, which account for the influence of salts. The formalism may also be applied to describe kinetic phenomena in the presence of cosolutes.

Cation radius effects on cell-free translation in rabbit reticulocyte lysate

The effect of monovalent cation concentrations on the translation was examined in the rabbit reticulocyte cell-free system. The translation of standard reporter gene luciferase was studied using different concentrations of LiCl, NaCl, KCl, RbCl, CsCl, NH(4)Cl, and (CH(3))(4)NCl and the acetates of Na(+), K(+), and NH4(+). Only the salts of K(+), Rb(+), and NH4(+) and to some minor extent of Cs(+) significantly supported translation. Optimum concentrations were dependent on the cation used. Optimum concentrations ranged between 40 mM (NH(4)Ac), 80 mM (KCl, NH(4)Cl), and 100 mM (RbCl, KAc). The maximum efficiency of translation depends on the ionic radius of the cation used. KCl and RbCl were superior to all other salts tested in stimulating in vitro translation. The results were confirmed, using a second reporter system, M-hirudin. Here, however, broad optima were observed with RbCl being slightly superior to KCl in supporting translation.

Enhancement of chymotrypsin-inhibitor/substrate interactions by 3 M NaCl

A series of 16 bovine pancreatic trypsin inhibitor variants mutated at the P(1) position of the binding loop and seven tetrapeptide p-nitroanilide (pNa) substrates of the general formula: suc-Ala-Ala-Pro-Aaa-pNa (where Aaa denotes either: Phe, Arg, Lys, Leu, Met, Nva, Nle) were used to investigate the influence of high salt concentration on the activity of bovine chymotrypsin. The increase of the association constant (K(a)) and the specificity index (k(cat)/K(m)) in the presence of 3 M NaCl highly depends on the chemical nature of the residue at the P(1) position. The highest increase was observed for inhibitors/substrates containing the basic side chains at this site. Surprisingly, for the remaining 13 residues the observed salt effect is not correlated with any side chain properties. In particular, there is a lack of correlation between the accessible non-polar surface area and the magnitude of the salt effect. It suggests that salt-induced increase of the K(a) and k(cat)/K(m) values is not caused by the enhancement of the hydrophobic interactions in chymotrypsin-inhibitor/substrate complex. Moreover, the increase of the K(a) and k(cat)/K(m) values occurs only in the presence of Na(+) ions, while K(+) and Li(+) ions do not change the activity of chymotrypsin. Additionally, the activities of two other proteinases: bovine trypsin and Streptomyces griseus proteinase B were tested in the presence of 3 M NaCl using their specific substrates. The activity of both enzymes was almost not affected by the presence of high NaCl concentration.

A comparison of salts for the crystallization of macromolecules

Thirty-one proteins and viruses that we knew from our own experience could be crystallized, or had been reported to have been crystallized by others, were investigated. In this experiment, each protein or virus was subjected to a crystallization screen of 12 different salts, each titrated to pH 7.2 beforehand, at concentrations ranging from 20% saturation to 90% saturation. Eight macromolecules failed to crystallize at all from any salt and were omitted from consideration. From the remaining 23 proteins, each salt was scored according to how many proteins and viruses it successfully crystallized. Among several results, one was particularly striking. Sodium malonate clearly was much more successful than any other salt, resulting in the crystallization of 19 of the 23 macromolecules, almost twice as effective as the next most successful salt, which was a draw between sodium acetate, sodium tartrate, sodium formate, and ammonium sulfate (11 of 22). The high success rate of sodium malonate in producing crystals was even more impressive when an overall unique success rate with individual macromolecules was considered.

Effect of various ions, pH, and osmotic pressure on oxidation of elemental sulfur by Thiobacillus thiooxidans

The oxidation of elemental sulfur by Thiobacillus thiooxidans was studied at pH 2.3, 4.5, and 7.0 in the presence of different concentrations of various anions (sulfate, phosphate, chloride, nitrate, and fluoride) and cations (potassium, sodium, lithium, rubidium, and cesium). The results agree with the expected response of this acidophilic bacterium to charge neutralization of colloids by ions, pH-dependent membrane permeability of ions, and osmotic pressure.

CFTR: mechanism of anion conduction

CFTR: Mechanism of Anion Conduction. Physiol. Rev. 79, Suppl.: S47-S75, 1999. - The purpose of this review is to collect together the results of recent investigations of anion conductance by the cystic fibrosis transmembrane conductance regulator along with some of the basic background that is a prerequisite for developing some physical picture of the conduction process. The review begins with an introduction to the concepts of permeability and conductance and the Nernst-Planck and rate theory models that are used to interpret these parameters. Some of the physical forces that impinge on anion conductance are considered in the context of permeability selectivity and anion binding to proteins. Probes of the conduction process are considered, particularly permeant anions that bind tightly within the pore and block anion flow. Finally, structure-function studies are reviewed in the context of some predictions for the origin of pore properties.

Cl- regulates the structure of the fibrin clot

The differences between coarse and fine fibrin clots first reported by Ferry have been interpreted in terms of nonspecific ionic strength effects for nearly 50 years and have fostered the notion that fibrin polymerization is largely controlled by electrostatic forces. Here we report spectroscopic and electron microscopy studies carried out in the presence of different salts that demonstrate that this long-held interpretation needs to be modified. In fact, the differences are due entirely to the specific binding of Cl- to fibrin fibers and not to generic ionic strength or electrostatic effects. Binding of Cl- opposes the lateral aggregation of protofibrils and results in thinner fibers that are also more curved than those grown in the presence of inert anions such as F-. The effect of Cl- is pH dependent and increases at pH > 8.0, whereas fibers grown in the presence of F- remain thick over the entire pH range from 6.5 to 9.0. From the pH dependence of the Cl- effect it is suggested that the anion exerts its role by increasing the pKa of a basic group ionizing around pH 9.2. The important role of Cl- in structuring the fibrin clot also clarifies the role played by the release of fibrinopeptide B, which leads to slightly thicker fibers in the presence of Cl- but actually reduces the size of the fibers in the presence of F-. This effect becomes more evident at high, close to physiological concentrations of fibrinogen. We conclude that Cl- is a basic physiological modulator of fibrin polymerization and acts to prevent the growth of thicker, stiffer, and straighter fibers by increasing the pKa of a basic group. This discovery opens new possibilities for the design of molecules that can specifically modify the clot structure by targeting the structural domains responsible for Cl- binding to fibrin.

Calorimetric studies of E. coli SSB protein-single-stranded DNA interactions. Effects of monovalent salts on binding enthalpy

Isothermal titration calorimetry (ITC) was used to examine the effects of monovalent salts (NaCl, NaBr, NaF and ChCl) on the binding enthalpy (DeltaHobs) for E. coli SSB tetramer binding to the single-stranded oligodeoxythymidylates, dT(pT)69 and dT(pT)34 over a wide range of salt concentrations from 10 mM to 2.0 M (25 degrees C, pH 8.1), and when possible, the binding free energy and entropy (DeltaG degrees obs, DeltaS degrees obs). At low monovalent salt concentrations (<0.1 M), the total DeltaHobs for saturating all sites on the SSB tetramer with ssDNA shows little dependence on salt concentration, but is extremely large and exothermic (DeltaHobs=-150(+/-5) kcal/mol). This is much larger than any DeltaHobs previously reported for a protein-nucleic acid interaction. However, at salt concentrations above 0.1 M, DeltaHobs is quite sensitive to NaCl and NaBr concentration, becoming less negative with increasing salt concentration (DeltaHobs=-70(+/-1)-kcal/mol in 2 M NaBr). These salt effects on DeltaHobs were mainly a function of anion type and concentration, with the largest effects observed in NaBr, and then NaCl, with little effect of [NaF]. These large effects of salt on DeltaHobs appear to be coupled to a net release of weakly bound anions (Br- and Cl-) from the SSB protein upon DNA binding. However, at lower salt concentrations (</=0.1 M), specific cation effects on DeltaHobs also are observed. Under conditions where we can determine DeltaG degrees obs, DeltaS degrees obs, and DeltaHobs (25 degrees C, pH 8.1, 0.17 to 2 M NaBr), SSB binding to dT(pT)69 is enthalpically driven with a large unfavorable entropic contribution, both of which are dependent upon [NaBr]. These studies show that weak anion binding to a protein can result in large effects of salt concentration on DeltaHobs (as well as DeltaG degrees obs and DeltaS degrees obs) for a protein-ssDNA interaction. The possibility of such effects needs to be considered in any interpretation of the thermodynamics of this and other protein-nucleic acid interactions.

Characterization of the interaction between bovine pancreatic trypsin inhibitor and thiocyanate by NMR

The interaction between Bovine Pancreatic Trypsin Inhibitor and thiocyanate was studied using NMR spectroscopy following several experimental approaches. The chemical shift variations of the BPTI protons in the absence and in the presence of increasing thiocyanate concentrations (up to 0.2 M) were significant (> 0.05 ppm) for 30 protein protons belonging to 20 residues. The largest deviation, 0.2 ppm, was observed for the amide backbone proton of Arg42 in the absence of thiocyanate and in the presence of 40 molar equivalents of thiocyanate. The influence of the presence of thiocyanate on the electrostatic potential surrounding the protein was demonstrated by NOESY spectra selective at the water frequency: the presence of SCN- favours acid catalysed exchange and disfavours base catalysis. However, a specific effect of thiocyanate was pointed out since the comparison of the chemical shifts in the presence of 40 molar equivalents of KSCN and KCl, respectively, showed much more as well as larger deviations compared to measurements in the absence of salt. A dissociation constant, KD, for a 1/1 complex between BPTI and thiocyanate was calculated from chemical shifts measurements: KD = 89 +/- 8 mM. A second value, KD = 99 +/- 10 mM, was extracted from SC15N relaxation time measurements.

Cystic fibrosis transmembrane conductance regulator (CFTR) anion binding as a probe of the pore

We compared the effects of mutations in transmembrane segments (TMs) TM1, TM5, and TM6 on the conduction and activation properties of the cystic fibrosis transmembrane conductance regulator (CFTR) to determine which functional property was most sensitive to mutations and, thereby, to develop a criterion for measuring the importance of a particular residue or TM for anion conduction or activation. Anion substitution studies provided strong evidence for the binding of permeant anions in the pore. Anion binding was highly sensitive to point mutations in TM5 and TM6. Permeability ratios, in contrast, were relatively unaffected by the same mutations, so that anion binding emerged as the conduction property most sensitive to structural changes in CFTR. The relative insensitivity of permeability ratios to CFTR mutations was in accord with the notion that anion-water interactions are important determinants of permeability selectivity. By the criterion of anion binding, TM5 and TM6 were judged to be likely to contribute to the structure of the anion-selective pore, whereas TM1 was judged to be less important. Mutations in TM5 and TM6 also dramatically reduced the sensitivity of CFTR to activation by 3-isobutyl 1-methyl xanthine (IBMX), as expected if these TMs are intimately involved in the physical process that opens and closes the channel.

The contribution of water to the selectivity of pyruvate kinase for Na+ and K+

For many years it has been known that K+ is an essential activator of pyruvate kinase [Kachmar, J. F. & Boyer, P. D. (1953) J. Biol. Chem. 200, 669-683] and that Na+ induces relatively small enhancements of activity. The effect of these two alkali metal ions on the activity of pyruvate kinase entrapped in the low water environment of reverse 'micelles formed with cetyltrimethylammonium, hexanol, n-octane and various water concentrations was studied. In reverse micelles with 3.6% water, the activity with 7 mM Na+ is more than 82 times higher than in aqueous solution with an equivalent Na+ concentration. As the concentration of water in reverse micelles is raised, the activating effect of relatively low concentrations of Na+ (or K+) decreases simultaneously to a more than 100-fold increase in the concentration of Na+ or K+ required for attaining half-maximal activation. Similar results were obtained with NH4+, Rb+ and Cs+. Therefore, the amount of water in the system is critical for observing activation by alkali metal ions. In fact, the concentration of Na+ required for half-maximal activation in standard aqueous media is higher than the concentrations that can be experimentally assayed. As evidenced from fluorescence and kinetic data, it appears that the entrapment of pyruvate kinase in reverse micelles does not produce gross structural alterations. Therefore, it is suggested that in conventional aqueous systems, the basis of the high discrimination between Na+ and K+ by pyruvate kinase is the higher energy required for desolvating Na+. Nevertheless, at all the water concentrations studied, the activities reached with K+ were higher than with Na+ which suggests that the Na+ form of the enzyme has a lower catalytic capacity than the K+-enzyme complex.