Protein interactions studied by SAXS: effect of ionic strength and protein concentration for BSA in aqueous solutions

We have studied a series of samples of bovine serum albumin (BSA) solutions with protein concentration, c, ranging from 2 to 500 mg/mL and ionic strength, I, from 0 to 2 M by small-angle X-ray scattering (SAXS). The scattering intensity distribution was compared to simulations using an oblate ellipsoid form factor with radii of 17 x 42 x 42 A, combined with either a screened Coulomb, repulsive structure factor, SSC(q), or an attractive square-well structure factor, SSW(q). At pH = 7, BSA is negatively charged. At low ionic strength, I < 0.3 M, the total interaction exhibits a decrease of the repulsive interaction when compared to the salt-free solution, as the net surface charge is screened, and the data can be fitted by assuming an ellipsoid form factor and screened Coulomb interaction. At moderate ionic strength (0.3-0.5 M), the interaction is rather weak, and a hard-sphere structure factor has been used to simulate the data with a higher volume fraction. Upon further increase of the ionic strength (I >or= 1.0 M), the overall interaction potential was dominated by an additional attractive potential, and the data could be successfully fitted by an ellipsoid form factor and a square-well potential model. The fit parameters, well depth and well width, indicate that the attractive potential caused by a high salt concentration is weak and long-ranged. Although the long-range, attractive potential dominated the protein interaction, no gelation or precipitation was observed in any of the samples. This is explained by the increase of a short-range, repulsive interaction between protein molecules by forming a hydration layer with increasing salt concentration. The competition between long-range, attractive and short-range, repulsive interactions accounted for the stability of concentrated BSA solution at high ionic strength.     

Ionic strength effects on the critical micellar concentration of ionic and nonionic surfactants: the binding model

We have recently investigated the aggregation behavior of zwitterionic n-dodecyl phosphocholine in the presence of high salt. As double logarithmic Corrin-Harkins plots of the critical micellar concentration versus the salt concentration were not linear, here we re-examine those data in the context of the binding model of surfactant aggregation, as previously developed by us for ionic surfactants. We have also re-examined plenty of data available in the literature on the salt-dependent aggregation of neutral surfactants. The use of double-logarithmic plots allowed us to show that the binding model is of general applicability. Indeed, it permits unified treatment of ionic and uncharged aggregation without requiring the introduction of linear terms in the salt concentration, as needed in the empirical Corrin-Harkins treatment of nonionic surfactants. The use of this model could be of help in a broad range of surfactant-based applications in the presence of high salt.     

Electrochemistry of nanopore electrodes in low ionic strength solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).     

Synthesis, oligonucleotide incorporation and base pair stability of 7-methyl-8-oxo-2'-deoxyguanosine

7-Methyl-8-oxo-2'-deoxyguanosine, an analogue of the abundant promutagen 8-oxo-2'-deoxyguanosine, was incorporated into oligonucleotides and tested for its stability in various base pairs.     

Self-diffusion and sedimentation of dextran in concentrated solutions

Frictional coefficients for dextran in water have been evaluated from (i) self-diffusion coefficients determined by pulsed-field-gradient NMR, and (ii) sedimentation coefficients in concentrated solutions. The results show that these frictional coefficients are only equal at infinite dilution and that f_s increases more rapidly than f_D^* as the concentration increases.     

Separation and analysis of radioiodine in nuclear reprocessing solutions of high ionic strength

Ion chromatography (IC) can be used to separate radioiodine from fission products and other ionic species in complex reprocessing solution matrices. A preliminary concentration and separation of the radioiodine is performed using a column filled with a platinum-coated copper bed which converts all forms of iodine to iodide and selectively adsorbs the iodide. The separation is completed by employing an anion chromatographic system. The iodide peak aliquot is collected for subsequent radioanalysis by low energy photon spectroscopy. Results indicate quantitative separation and recovery of iodine. The entire system is automated under computer control and is able to handle small (l) and large (500 ml) sample sizes.     

Salting-out effects in aqueous ionic liquid solutions: cloud-point temperature shifts

The effects of the addition of three inorganic salts, namely, NaCl, Na(2)SO(4), and Na(3)PO(4), on the liquid-liquid (L-L) phase diagram of aqueous solutions containing the model ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF(4)], were investigated. All three inorganic salts trigger salting-out effects, leading to significant upward shifts of the L-L demixing temperatures of the systems. The magnitude of the shifts depends on both the water-structuring nature of the salt and its concentration; that is, the effects are correlated with the ionic strength of the solution and the Gibbs free energy of hydration of the inorganic salt. The pH effect and the occurrence of salt precipitation in concentrated solutions are also discussed.     

Unexpected retention of electrostatically adsorbed cytochrome c in high ionic strength solutions

Redox inactivation, but not removal, of electrostatically adsorbed cytochrome c(cyt-c) on an alkanethiol modified gold surface was observed after exposure of the electrode to 1.0 M aqueous NaCl, NaClO4, KCl, or KClO4 solutions.     

Constant-pH molecular dynamics with ionic strength effects: protonation-conformation coupling in decalysine

A new implementation of the stochastic titration method for constant-pH molecular dynamics is presented, which introduces ionic strength effects in the simulations. In addition, the new implementation uses a faster molecular dynamics algorithm and an improved treatment of protonation events and of their effect on force field parameters. This new methodology is applied to a decalysine peptide, yielding very good quantitative agreement with experiments, both in terms of titration and helix-coil transition. The results show a significant dependence on ionic strength, illustrating the importance of including this parameter in constant-pH molecular dynamics simulations. Overall, the method seems to properly capture the protonation-conformation coupling and its dependence on ionic strength.     

A study of deoxycholate micellar solutions as a function of the ionic medium concentration

The behaviour of sodium deoxycholate aqueous solutions at 25°C, using different concentrations of N(CH₃)₄Cl as the ionic medium, was studied. Electromotive force measurements of galvanic cells containing electrodes reversible to sodium deoxycholate (DC) and hydrogen ions were performed. Moreover, the influence of sodium and DC ions on the solubility of lead (II) deoxycholate was investigated. Experimental data were explained by assuming the formation of several species of the type Na _q H _p (DC) _n . The general trend is that theq, p, n values increase with increasing concentration of N(CH₃)₄Cl. The observed aggregation numbers of the largest micellar aggregates satisfactorily agree with those obtained by small-angle X-ray scattering measurements.     

The effects of pH, ionic strength and buffer concentration of mobile phase onR F of acidic compounds in ion-pair TLC

Summary The effects of some factors important in ion-pair, high-performance liquid chromatography were studied in ion-pair, thin-layer chromatography. Tetramethyl and cetyltrimethylammonium salts were used as ion-pairing reagents. As stationary phases, silica gel and chemically bonded reversed-layers (C₁₈) were used. Layers were impregnated with ion-pairing reagent prior to chromatography. In some cases the stationary phase was treated with buffer at different concentrations. The mobile phase contained methanol and water, in one set of experiments buffer, salt for adjustment of ionic strength and ion-pairing reagent were added. The migration behaviour of different benzoic acids was studied. Several problems of ion-pair thin-layer chromatography are discussed. Passed away on 13^th of April, 1998 Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Structure characterization of cellulose nanofiber hydrogel as functions of concentration and ionic strength

Carboxylated cellulose nanofibers (CNFs), having an average width of 7 nm and thickness of 1.5 nm, were produced by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation method. The fiber cross-sectional dimensions were determined using small-angle X-ray scattering (SAXS), transmission electron microscopy and atomic force microscopy techniques, where the rheological properties under different concentration and ionic strength were also investigated. The formation of hydrogel was evidenced by increasing the CNF concentration or ionic strength of the solvent (water), while the gel structure in ion-induced CNF hydrogels was found to be relatively inhomogeneous. The gelation behavior was closely related to the segmental aggregation of charged CNF, which could be quantitatively characterized by the correlation length (ξ) from the low-angle scattering profile and the scattering invariant (Q) in SAXS.     

Structure of a copper-mediated base pair in DNA.

Stable and selective DNA base pairing by metal coordination was recently demonstrated with nucleotides containing complementary pyridine-2,6-dicarboxylate (Dipic) and pyridine (Py) bases (Meggers, E.; Holland, P. L.; Tolman; W. B.; Romesberg, F. E.; Schultz, P. G. J. Am. Chem. Soc. 2000, 122, 10714-10715). To understand the structural consequences of introducing this novel base pair into DNA we have solved the crystal structure of a duplex containing the metallo-base pair. The structure shows that the bases pair as designed, but in a Z-DNA conformation. The structure also provides a structural explanation for the B- to Z-DNA transition in this duplex. Further solution studies demonstrate that the metallo-base pair is compatible with Z- or B-DNA conformations, depending on the duplex sequence.     

Effects of degassing and ionic strength on AFM force measurements in octadecyltrimethylammonium chloride solutions

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.     

Charge transfer in DNA: hole charge is confined to a single base pair due to solvation effects

We include solvation effects in tight-binding Hamiltonians for hole states in DNA. The corresponding linear-response parameters are derived from accurate estimates of solvation energy calculated for several hole charge distributions in DNA stacks. Two models are considered: (A) the correction to a diagonal Hamiltonian matrix element depends only on the charge localized on the corresponding site and (B) in addition to this term, the reaction field due to adjacent base pairs is accounted for. We show that both schemes give very similar results. The effects of the polar medium on the hole distribution in DNA are studied. We conclude that the effects of polar surroundings essentially suppress charge delocalization in DNA, and hole states in (GC)(n) sequences are localized on individual guanines.     

The adsorption of hydrophobically modified carboxymethylcellulose on a hydrophobic solid: effects of pH and ionic strength

Carboxymethylcellulose with 1.2% dodecyl groups (per glucose unit) was prepared by amidation with dodecylamine. This polymer behaves as a hydrophobically modified polyelectrolyte with the following thickening properties which are determined from viscosity data. It adsorbs from aqueous solution on spin-coated polystyrene films to various extents, depending on pH and ionic strength. The adsorbed amount has a surprising minimum at around pH 6 which coincides with a maximum in viscosity of a dilute solution, and with a very pronounced maximum in the hydrodynamic radius as determined from dynamic light scattering. To our knowledge, such behavior has not been reported before. It suggests that at low pH the polymer is present in the form of small aggregates which upon increasing the pH first swell and then break up into single molecules. Received: 19 June 1997 Accepted: 5 January 1998     

Many-body effects on structure and dynamics of aqueous ionic solutions

We performed several molecular dynamic studies of metal cations in aqueous solution. The alkali metal ion Li(+) and the first-row transition metal ion Mn(2+) have been chosen as model systems. Two different three-body corrections are proposed to mimic the crucial many-body effects of electrolyte solutions. The correction function, which includes attractive features of the three-body potential, performs considerably better than the purely repulsive interaction function. Structural and dynamic results show that this simple enhancement is able to satisfactorily reproduce experimental and higher-level results for the first hydration shell.