Changes in the polyelectrolyte-amphiphile interaction due to helix-coil transition induced by specific counterions or variations in temperature

For the system κ-carrageenan/amitriptyline it is shown that the degree of binding of amitriptyline is closely related to the carrageenan conformation as regulated by the counterions (Na⁺ or K⁺). The adsorption becomes much more pronounced when the carrageenan molecule is in the helix form (counterion K⁺) than when it has a coil conformation (counterion Na⁺). Furthermore, for the helical state the adsorption becomes strongly cooperative. It is also shown experimentally that the release from the adsorbed state has a conversion temperature at about 42°C (helix-coil transition). The effect is also related to the linear charge density. For κ-carrageenan with a higher charge density the adsorption is strong and cooperative both in the presence of Na⁺ and K⁺ ions. © 1996 John Wiley & Sons, Inc.     

Hydration of counterions interacting with DNA double helix: a molecular dynamics study

In the present work, molecular dynamics simulations have been carried out to study the dependence of counterion distribution around the DNA double helix on the character of ion hydration. The simulated systems consisted of DNA fragment d(CGCGAATTCGCG) in water solution with the counterions Na⁺, K⁺, Cs⁺ or Mg²⁺. The characteristic binding sites of the counterions with DNA and the changes in their hydration shell have been determined. The results show that due to the interaction with DNA at least two hydration shells of the counterions undergo changes. The first hydration shell of Na⁺, K⁺, Cs⁺, and Mg²⁺ counterions in the bulk consists of six, seven, ten, and six water molecules, respectively, while the second one has several times higher values. The Mg²⁺ and Na⁺ counterions, constraining water molecules of the first hydration shell, mostly form with DNA water-mediated contacts. In this case the coordination numbers of the first hydration shell do not change, while the coordination numbers of the second one decrease about twofold. The Cs⁺ and K⁺ counterions that do not constrain surrounding water molecules may be easily dehydrated, and when interacting with DNA their first hydration shell may be decreased by three and five water molecules, respectively. Due to the dehydration effect, these counterions can squeeze through the hydration shell of DNA to the bottom of the double helix grooves. The character of ion hydration establishes the correlation between the coordination numbers of the first and the second hydration shells.

Open image in new window

Graphical Abstract Hydration of counterions interacting with DNA double helix

     

Synthesis and unusual response to potassium of bipyridinium-benzocrown ether conjugate

A bipyridinium derivative appending a benzocrown ether, in which the phenyl unit in the benzocrown ether was directly bounded to the N-position of the bipyridinium unit, has been synthesized. The compound showed a yellow color associated with an intramolecular charge transfer (CT), which was affected by the presence of alkali and alkaline earth metal ions. An unusual CT response to K⁺ for 1 was observed and could be applicable for K⁺ sensing.     

Solvent- and salt-induced coil–helix transition of alkali metal salts of poly(L-glutamic acid) in aqueous organic solvents

The coil–helix transition has been studied for alkali metal salts of poly (L -glutamic acid) (PLG), i.e., PLGLi, -Na, -K, and -Cs, in aqueous organic solvent systems. Dependence of the transition on the solvent composition has been qualitatively discussed in terms of the solvent dielectric constant D, Gutmann's acceptor number AN, and water activity a_w. The helix formation induced by addition of alkali chlorides has also been studied. The sharpness of the transition has been interpreted as a measure of reduction of electrostatic energy of helical PLG through contact ion-pair formation between a counterion and carboxyl anion.     

Stable and metastable forms of poly(G)

Poly(G) is shown by ir spectroscopy to be capable of existence in a metastable form which is converted spontaneously at ambient temperature, or more rapidly on heating, to a stable form. The metastable form can be regenerated by freezing and thawing the solution. The high-charge density of four-stranded poly(G) makes it especially susceptible to electrostatic destabilization by use of Et₄N⁺ counterions, which screen electrostatic repulsion of multiple strands less effectively than alkali metal ions. Poly(G) has been obtained for the first time in the single-stranded form in aqueous solution and shown to undergo a fully reversible helix–coil transition on heating.     

Similarities and differences in interaction of K+ and Na+ with condensed ordered DNA. A molecular dynamics computer simulation study

Four 20 ns molecular dynamics simulations have been performed with two counterions, K⁺ or Na⁺, at two water contents, 15 or 20 H₂O per nucleotide. A hexagonal simulation cell comprised of three identical DNA decamers [d(5′-ATGCAGTCAG) × d(5′-TGACTGCATC)] with periodic boundary condition along the DNA helix was used. The simulation setup mimics the DNA state in oriented DNA fibers or in crystals of DNA oligomers. Variation of counterion nature and water content do not alter averaged DNA structure. K⁺ and Na⁺ binding to DNA are different. K⁺ binds to the electronegative sites of DNA bases in the major and the minor grooves, while Na⁺ interacts preferentially with the phosphate groups. Increase of water causes a shift of both K⁺ and Na⁺ from the first hydration shell of O1P/O2P and of the DNA bases in the minor groove with lesser influence for the cation binding to the bases in the major groove. Mobility of both water and cations in the K–DNA systems is faster than in the Na–DNA systems: Na⁺ organizes and immobilizes water structure around itself and near DNA while for K⁺ water is less organized and more dynamic.     

Sodium and chloride ions contribute synergistically to salt toxicity in wheat

The effects of supplying excess mineral salts, involving sodium as a cation and a range of counteranions, including chloride, on the growth and photosynthetic capacity of a salt susceptible bread wheat were studied. Plant performance was much more affected by the NaCl treatment than by the same concentration of either of the two component ions. With the exception of K⁺, other alkali metal chlorides also greatly inhibit plant growth and the electron flow through photosystem 2. The ranking of toxicity of these cations is Li⁺>Na⁺>K⁺. The synergistic effect of sodium (and other alkali and alkaline earth metals) and chloride shows that neither of these ions alone is responsible for salt stress induced damage.     

Trivalent counterion condensation on DNA measured by pulse gel electrophoresis

Pulse gel electrophoresis was used to measure the reduction of mobilities of λ-DNA-Hind III fragments ranging from 23.130 to 2.027 kilobase pairs in Tris borate buffer solutions mixed with either hexammine cobalt(III), or spermidine³⁺ trivalent counterions that competed with Tris⁺ and Na⁺ for binding onto polyion DNA. The normalized titration curves of mobility were well fit by the two-variable counterion condensation theory. The agreement between measured charge fraction neutralized and counterion condensation prediction was good over a relatively wide range of trivalent cation concentrations at several solution conditions (pH, ionic strength). The effect of ionic strength, trivalent cation concentration, counterion structure, and DNA length on the binding were discussed based on the experimental measurements and the counterion condensation theory. © 1996 John Wiley & Sons, Inc.     

Types of interaction of meso‐tetraphenylporphyrin with alkali and alkaline‐earth metal ions

The cavity in a porphyrin can accommodate metal ions through electron donor–acceptor (EDA) interaction in acetonitrile media without any specially designed fabrication with the porphyrin subunit. Alkali metal ion forms a complex with meso‐tetraphenylporphyrin (TP) in 2:1 stoichiometry, while the bivalent Mg²⁺ ion follows a 1:1 stoichiometry. A fluorescence interaction study indicated that TP can behave like a chemosensor for these ions present in the blood electrolytes. Specifically, for the alkali metal ions intensity‐based sensing was observed, due to inhibition of photoinduced electron transfer (PET), entailing enhancement of fluorescence intensity, and for the alkaline‐earth Mg²⁺ a mixed quenching was observed. Na⁺ and K⁺ ions can be differentiated depending upon the extent of fluorescence enhancement. Copyright © 2011 John Wiley & Sons, Ltd.     

Lipase from Candida paralipolytica

The extracellular lipase from Candida paralipolytica required alkaline earth metal ion as the cofactor*² in the reaction mixture not emulsified but dispersed by shaking, contradicting the fact that it required bile salt or anionic surfactant as the essential activator*³ in the systems emulsified with polyvinyl alcohol, as previously reported.¹) The two kinds of factors necessary to activate both reaction systems respectively were unexchangeable for each other. These facts would be direct evidences of the difference of interfacial nature between two substrate forms prepared from the same substrate.

The zero-order reaction has been observed under the non-emulsified conditions and the activation mechanism by alkaline earth metal ions has been studied partly.     

Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li⁺, Na⁺ and K⁺) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [A_x+1H_xCr^VI_xO_4x]⁺ (Li⁺, x=1-5; Na⁺, x=1-7; K⁺, x=1-4) and [A_2x−1Cr^VI_xO_4x−1]⁺ (Li⁺, x=2, 3; Na⁺, x=2-4; K⁺, x=2, 3) in the alkali metal dichromate systems, and [HCr^VI_xO_3x+1]⁻ (x=1-5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO₄]⁻ as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO₄}, {A₂CrO₄} and {CrO₃}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A₂Cr₂O₇→{A₂CrO₄}+{CrO₃}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.     

The equivalent conductivity of aqueous salt-free solutions of DNA: Influence of univalent counterions

The equivalent conductivity of salt-free solutions of deoxyribonucleates of alkali metals and ammonium obtained by filtering an isoionic DNA solution through a cation exchanger in the corresponding form has been investigated in the concentrations range of 1 × 10^?4 to 4 × 10^?3M. For all counterions investigated there is a linear dependence of the equivalent conductivity on \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {C_p} $\end{document}, where C_p is the nucleic phosphorus concentration. The limiting equivalent conductivity of deoxyribonucleates increases linearly with the limiting mobility of a counterion. By extrapolation to the zero mobility of the counterion, we have obtained the limiting mobility of a macroion, which is equal to 19 × 10^?4 Sm m² equiv.^?1, which is in good agreement with the literature data for denatured DNA obtained by the method of a moving boundary. It is shown that the degree of binding of counterions calculated from the conductometric data in diluted DTA solutions in independent of the nature of the univalent counterion. The degree of dissociation of H⁺-DNA in the isoionic solution calculated with allowance for the fraction of unprotonated bases practically coincides with this value for salts of DNA. The parameter of Manning's theory calculated from the experimental data corresponds to the distance between phosphates along the chain of the macroion, which is equal to 6.7 Å. We attribute the smaller value of this distance as compared with the theoretical one for denatured DNA to the aggregation of macroions.     

Complexation of a four-strand tetraalcohol with labile metal ions probed by electrospray mass spectrometry

The saturated, stereodefined tetraalcohol 2,3,5,6-endo,endo,endo,endo-tetrakis(hydroxymethyl)bicyclo[2.2.1]heptane (tetol, L¹) and the simple alcohol butane-1,3-diol (L²) form complexes with alkali metal ions (lithium, sodium, potassium, rubidium and caesium), alkali earth cations (magnesium, calcium, strontium and barium) and Ga(III) and Ce(IV) in aqueous solution, characterised by electrospray ionisation mass spectrometry (ESMS). Metal ion exchange between the Li⁺ complex of L¹ and the other metal ions is rapid, with a range of M(L¹)_n ^m+ species detected, in addition to solvated species. With the alkal metal ions, M(L¹)⁺ and M(L¹)₂ ⁺ are dominant, although speciation varies with metal ion size. For the alkaline earth ions, a range of complex ions up to n=8 are observed, although n=1-3 dominate. A preference for M(L¹)₂ ²⁺ with Mg²⁺ versus M(L¹)₃ ²⁺ with Ca²⁺ may again relate to a larger ion size. For the higher-charged Ga(III) and Ce(IV), hydroxo species M(OH)(L¹)_n ^(m−1)+ are dominant reflecting bulk solution behaviour, which the ESMS studies appear to map generally.     

Ionic selectivity in L-type calcium channels by electrostatics and hard-core repulsion

A physical model of selective “ion binding” in the L-type calcium channel is constructed, and consequences of the model are compared with experimental data. This reduced model treats only ions and the carboxylate oxygens of the EEEE locus explicitly and restricts interactions to hard-core repulsion and ion–ion and ion–dielectric electrostatic forces. The structural atoms provide a flexible environment for passing cations, thus resulting in a self-organized induced-fit model of the selectivity filter. Experimental conditions involving binary mixtures of alkali and/or alkaline earth metal ions are computed using equilibrium Monte Carlo simulations in the grand canonical ensemble. The model pore rejects alkali metal ions in the presence of biological concentrations of Ca²⁺ and predicts the blockade of alkali metal ion currents by micromolar Ca²⁺. Conductance patterns observed in varied mixtures containing Na⁺ and Li⁺, or Ba²⁺ and Ca²⁺, are predicted. Ca²⁺ is substantially more potent in blocking Na⁺ current than Ba²⁺. In apparent contrast to experiments using buffered Ca²⁺ solutions, the predicted potency of Ca²⁺ in blocking alkali metal ion currents depends on the species and concentration of the alkali metal ion, as is expected if these ions compete with Ca²⁺ for the pore. These experiments depend on the problematic estimation of Ca²⁺ activity in solutions buffered for Ca²⁺ and pH in a varying background of bulk salt. Simulations of Ca²⁺ distribution with the model pore bathed in solutions containing a varied amount of Li⁺ reveal a “barrier and well” pattern. The entry/exit barrier for Ca²⁺ is strongly modulated by the Li⁺ concentration of the bath, suggesting a physical explanation for observed kinetic phenomena. Our simulations show that the selectivity of L-type calcium channels can arise from an interplay of electrostatic and hard-core repulsion forces among ions and a few crucial channel atoms. The reduced system selects for the cation that delivers the largest charge in the smallest ion volume.     

Anion-cation permeability correlates with hydrated counterion size in glycine receptor channels

The functional role of ligand-gated ion channels depends critically on whether they are predominantly permeable to cations or anions. However, these, and other ion channels, are not perfectly selective, allowing some counterions to also permeate. To address the mechanisms by which such counterion permeation occurs, we measured the anion-cation permeabilities of different alkali cations, Li⁺ Na⁺, and Cs⁺, relative to either Cl⁻ or anions in both a wild-type glycine receptor channel (GlyR) and a mutant GlyR with a wider pore diameter. We hypothesized and showed that counterion permeation in anionic channels correlated inversely with an equivalent or effective hydrated size of the cation relative to the channel pore radius, with larger counterion permeabilities being observed in the wider pore channel. We also showed that the anion component of conductance was independent of the nature of the cation. We suggest that anions and counterion cations can permeate through the pore as neutral ion pairs, to allow the cations to overcome the large energy barriers resulting from the positively charged selectivity filter in small GlyR channels, with the permeability of such ion pairs being dependent on the effective hydrated diameter of the ion pair relative to the pore diameter.     

DFT study of geometrical and vibrational features of a 3′,5′-deoxydisugar-monophosphate (dDSMP) DNA model in the presence of counterions and solvent

The B3LYP/6–31++G* theoretical level was used to study the influence of various hexahydrated monovalent (Li⁺, Na⁺, K⁺) and divalent (Mg²⁺) metal counterions in interaction with the charged PO₂^? group, on the geometrical and vibrational characteristics of the DNA fragments of 3′,5′-dDSMP, represented by four conformers (g⁺g⁺, g⁺t, g^?g^? and g^?t). All complexes were optimized through two solvation models [the explicit model (6H₂O) and the hybrid model (6H₂O/Continuum)]. The results obtained established that, in the hybrid model, counterions (Li⁺, Na⁺, K⁺, Mg²⁺) always remain in the bisector plane of the O1–P–O2 angle. When these counterions are explicitly hydrated, the smallest counterions (Li⁺, Na⁺) deviate from the bisector plane, while the largest counterions (K⁺ and Mg²⁺) always remain in the same plane. On the other hand, the present calculations reveal that the g⁺g⁺ conformer is the most stable in the presence of monovalent counterions, while conformers g⁺t and g^?t are the most stable in the presence of the divalent counterion Mg²⁺. Finally, the hybrid solvation model seems to be in better agreement with the available crystallographic and spectroscopic (Raman) experiments than the explicit model. Indeed, the six conformational torsions of the C4′-C3′-O3′-PO^?₂-O5′-C5′-C4′ segment of all complexes of the g^?g^? conformer in 6H₂O/Continuum remain similar to the available experimental data of A- and B-DNA forms. The calculated wavenumbers of the g⁺g⁺ conformer in the presence of the monovalent counterion and of g^?t conformer in presence of the divalent counterion in the hybrid model are in good agreement with the Raman experimental data of A- and B-DNA forms. In addition, the maximum deviation between the calculated wavenumbers in the 6H₂O/Continuum for the g⁺g⁺ conformer and experimental value measured in an aqueous solution of the DMP-Na⁺ complex, is <1.07% for the PO₂^? (asymmetric and symmetric) stretching modes and <2.03% for the O5′-C5′ and O3′-C3′ stretching modes.

Open image in new window

Graphical abstract dDSMP-(OO)^? Mg²⁺/6W/Continuum

     

Cation binding of bis(cyclic tetrapeptide). 1H n.m.r. and c.d. evidence for conformational fitting of cyclic backbone and covalent CSSC bridge

Conformational aspects of the complexation of bis(cyclic tetrapeptide), S,S′-bis[cyclo(Gly-l-hemiCys-Sar-l-Pro)] (BCGCSP) with a metal cation were studied. Binding constants of BCGCSP with several cations were determined in aqueous solution, using circular dichroism (c.d.) titration curves. The values were compared with those of two mono-cyclic tetrapeptides, cyclo[Gly-l-Cys[Bzl(OMe)]-Sar-l-Pro] and cyclo(Sar-l-Pro-Sar-l-Pro). When complexing with alkali metal cations, BCGCSP exhibits selective affinity for Rb⁺ in preference to Li⁺, Na⁺, and K⁺. Complexing with alkaline earth metal cations, the peptide binds Ba²⁺ selectively. In addition, BCGCSP shows a marked Ba²⁺/Ca²⁺ selectively compared with the other three cyclic peptides. In order to explain these characteristics, a pseudo-inclusion complex with a castanet type structure was proposed as a model of the bis(peptide)—cation complex. The c.d. band ascribed to disulphide (SS) bond transition, showed a red shift upon complex formation. From this observation, it is suggested that conformational fitting of bis(peptide) takes place by changing the geometry of the peptide backbone and covalent CSSC bridge upon complexation with a metal cation.     

The activation of intestinal brush border sucrase by alkali metal ions: an allosteric mechanism similar to that for the Na+-activation of nonelectrolyte transport systems in intestine.

Activation of intestinal brush border sucrase by alkali-metal ions is described by an allosteric, noncompulsory mechanism involving two distinct sites: one for sucrose and another for the metal activator. Both Na⁺ and K⁺ activate guinea pig sucrase but K⁺ has ten times more affinity for the metal site. Li⁺ is inert. Values for the dissociation constants for the interaction of sucrase with either sucrose, Na⁺ or K⁺ have been calculated for the guinea pig, rat, and hamster. Qualitatively, the activation of sucrase by alkali metal ions is similar to that described for the Na⁺-activation of amino acid and sugar transport in intestine. However, the Na-binding site in the two systems is apparently quite different. In the guinea pig, the Na-dissociation constant is in the order of 10^?3m for sucrase, about two orders of magnitude smaller than that for transport. Also, K ⁺ is a strong activator of guinea pig sucrase, but an inhibitor of intestinal sugar transport.     

Structure of hydrated Na+ ions around a region of A- or B-DNA helix

A molecular-dynamics simulation was used to carry out an introductory study of the hydration of a section of a rigid single A- or B-DNA helix with one Na⁺ counterion per nucleotide. Four Na⁺ ions and four nucleotides and periodic boundary conditions were used to mimic an infinite helix. The atoms of the helix and the Na⁺ ions were assumed to be Lennard-Jones spheres that also carried charges. Stillinger four-point charge model water molecules were used. We carried out five calculations, for 26 and 46 water molecules in B-DNA and 20, 32, and 46 in A-DNA fragments. The arrangements of the Na⁺ ions are found to have some similarities to those obtained by Clementi and Corongiu. In the calculations with 46 water molecules, we found that two Na⁺ ions can be bridged by about two water molecules and form a hydrated bound pair, which in turn forms a bridge between the guanine N7 and a near phosphate group. These bound pairs may be important in stabilizing the helix structure of DNA molecules.     

First-principles study of water desorption from montmorillonite surface

Knowledge about water desorption is important to give a full picture of water diffusion in montmorillonites (MMT), which is a driving factor in MMT swelling. The desorption paths and energetics of water molecules from the surface of MMT with trapped Li⁺, Na⁺ or K⁺ counterions were studied using periodic density functional theory calculations. Two paths—surface and vacuum desorption—were designed for water desorption starting from a stationary structure in which water bonds with both the counterion and the MMT surface. Surface desorption is energetically more favorable than vacuum desorption due to water–surface hydrogen bonds that help stabilize the intermediate structure of water released from the counterion. The energy barriers of water desorption are in the order of Li⁺?>?Na⁺?>?K⁺, which can be attributed to the short ionic radius of Li⁺, which favors strong binding with the water molecule. The temperature dependence of water adsorption and desorption rates were compared based on the computed activation energies. Our calculations reveal that the water desorption on the MMT surface has a different mechanism from water adsorption, which results from surface effects favoring stabilization of water conformers during the desorption process.