Conformational and ion binding properties of a cyclic octapeptide, cyclo (Ala-Leu-Pro-Gly)2.

The conformation and ion-binding characteristics of a cyclic octapeptide, cyclo (Ala-Leu-Pro-Gly)2, in a liphophilic solvent, acetonitrile, have been studied using CD and nmr spectroscopy. The peptide binds preferentially to divalent cations such as calcium, magnesium, and barium. The conformations of the free cyclic peptide and its calcium complex are very similar with well-defined beta- and gamma-turns. The cyclic peptide readily forms equimolar and possibly 2:1 (peptide:cation) complexes with divalent cations.     

Role of divalent cations in the subunit associations of complex flagella from Rhizobium meliloti.

Rhizobium meliloti, a symbiotic, nitrogen-fixing soil bacterium with complex flagella, as well as other members of the family Rhizobiaceae, rapidly lost motility when suspended in buffers lacking divalent cations but retained good motility in buffers containing calcium, magnesium, barium, or strontium. Loss of motility was associated with loss of flagella from the cells. Analysis of flagella by sedimentation, gel electrophoresis, and electron microscopy revealed that removal of divalent cations from the complex flagella of R. meliloti resulted in extensive dissociation of the flagellar filaments into low-molecular-weight subunits. Accordingly, divalent cations such as calcium and magnesium that are normally present at high concentrations in the soil solution may be crucial to the assembly and rigidity of complex flagella.     

Alkaline Earth Salts of Glutamic Acid and Optical Resolution of their Racemic Modifications

The solubilities of monocalcium and monomagnesium diglutamate and calcium, magnesium, barium and strontium glutamate in water were measured in respect to both the l-form and the dl-form to conjecture the constitution of the racemic modification of each salt. It was concluded from the result that only magnesium dl-glutamate among six saltsis a congromerate and the others except the barium salt are all racemic compounds. The conclusion was supported by results from measurements of the infrared spectra and the X-ray diffractions. As for barium glutamate, a definite conclusion could not be obtained, but its racemic form seems to be congromerate or a racemic compound with a weak affinity between the isomers. Optical resolution based on the inoculation method was tried for calcium, magnesium, barium and strontium dl-glutamate, and the selective crystallization of the active form from the supersaturated solutions of magnesium and calcium dl-glutamate proceeded, though the process was unstable in the latter case; on the other hand, the resolutions of barium and strontium dl-glutamates were quite unsuccessful.     

Conformational characterisation of valinomycin complexation with barium salts—A nuclear magnetic resonance spectroscopic study

Conformations of valinomycin and its complexes with Perchlorate and thiocyanate salts of barium, in a medium polar solvent acetonitrile, were studied using nuclear magnetic resonance spectroscopic techniques. Valinomycin was shown to have a bracelet conformation in acetonitrile. With the doubly charged barium ion, the molecule, at lower concentrations, predominantly formed a 1:1 complex. At higher concentrations, however, apart from the 1:1, peptide as well as ion sandwich complexes were formed in addition to a ‘final complex’. Unlike the standard 1:1 potassium complex, where the ion was centrally located in a bracelet conformation, the 1:1 barium complex contained the barium ion at the periphery. The ‘final complex’ appeared to be an open conformation with no internal hydrogen bonds and has two bound barium ions. This complex was probably made of average of many closely related conformations that were exchanging very fast (on nuclear magnetic resonance time scale) among them. The conformation of the ‘final complex’ resembled the conformation obtained in the solid state. Unlike the Perchlorate anion, the thiocyanate anion seemed to have a definite role in stabilising the various complexes. While the conformation of the 1:1 complex indicated a mechanism of ion capture at the membrane interface, the sandwich complexes might explain the transport process by a relay mechanism.     

Using light scattering measurements to study the effects of monovalent and divalent cations on alginate aggregates

Light scattering measurements were used to assess the effectsof selected divalent and monovalent cations on alginate aggregationin vitro. Alginate, formed with either strontium, calcium orcobalt was partially dissolved with sodium. Calcium-alginatewas also partially dissolved with two other monovalent cations,lithium and potassium. Phosphate, when added to a solution containingcalcium-alginate, scrubbed algin-ate-bound calcium as well asfree calcium in solution. These findings provide an explanationfor an alternative approach for breaking down cell wall alginate. Key words: Alginate aggregates, monovalent cations, divalent cations, light scattering     

CALCIUM-BINDING PHOSPHOPROTEIN OF PIG BRAIN: EFFECTS OF CATIONS ON THE CALCIUM BINDING

—The effects of various cations on the calcium-binding properties of a brain phosphoprotein have been investigated employing an ultrafiltration binding assay. Sodium, magnesium, barium, strontium and manganese ions were competitive inhibitors of the calcium binding, and their affinity for the calcium-binding site increased in the order given. Potassium was a non-competitive inhibitor. Lanthanum, lead, mercuric and cadmium ions precipitated the phosphoprotein from solution. These findings are discussed in relation to a postulated role for the protein as a calcium-receptor molecule involved in release of neurotransmitters.     

The effect of ions, ion channel blockers, and ionophores on uptake of vitellogenin into cockroach follicles

Since calcium plays an important role in vitellogenin binding and uptake in Nauphoeta cinerea and because calcium channels have been described in follicles of this species, we investigated the effect of various ions, ionophores, and ion channel blockers on vitellogenin uptake in vitro. Calcium significantly stimulated vitellogenin uptake; this effect could be substituted best by barium and less well by strontium and magnesium. The stimulatory effect of calcium, and to a certain extent also that of barium, was dependent on the vitellogenin concentration, whereas the effect of strontium and magnesium was not. In the presence of calcium, vitellogenin uptake was inhibited by barium, strontium, and magnesium as well as by the transition elements nickel, cobalt, and zinc, but not by manganese which had a stimulatory effect. Valinomycin, verapamil, tetraethylammonium, and atropine reduced vitellogenin uptake, while amiloride and ouabain were ineffective. Our results indicate that calcium inward (and possibly potassium outward) fluxes play an important role in vitellogenin uptake.     

Affinity and stoichiometry of calcium binding by arsenazo III

Arsenazo III forms a 1:1 complex with calcium. The affinity constant of arsenazo III for calcium (pK_Ca) has been determined by titrating purified arsenazo III with standard calcium solutions. The method of evaluation used allows one to determine correct pK_Ca values even in the presence of micromolar amounts of contaminating calcium. The pK_Ca is influenced by the following factors: (a) in the neutral pH range the apparent pK_Ca increases strongly with pH; (b) alkali ions bind weakly to arsenazo III and millimolar concentrations cause a decrease in the apparent pK_Ca; (c) the magnesium affinity of arsenazo III, although much lower than the calcium affinity, increases strongly with pH in the neutral range (at pH 7.0 the calcium affinity of arsenazo III is not appreciably altered by up to 2 mm magnesium); (d) strontium and barium form weaker complexes with arsenazo III than calcium, but much stronger complexes than magnesium; (e) the apparent pK_Ca decreases with increasing buffer concentration in the millimolar range. The pK_Ca of arsenazo III is so high that, unless the arsenazo III concentration greatly exceeds the calcium concentration, a considerable fraction of the total arsenazo III is in the calcium complexed form. Because of this, arsenazo III responds nonlinearly to all but the lowest calcium concentrations; however, quantitation of the calcium concentration can readily be done from the mass action law provided that the pK_Ca is determined under the actual experimental conditions. Arsenazo III is a reliable calcium indicator if the experimental conditions, particularly pH, are well controlled.     

Synexin binds in a calcium-dependent fashion to oriented chromaffin cell plasma membranes

Oriented plasma membrane fragments from chromaffin cells, isolated on polylysine-coated polyacrylamide beads, bind synexin in a calcium dependent manner. Synexin binding was also detected on beads coated with chromaffin granule membranes, but not to beads coated with erythrocyte membranes or to uncoated beads. Synexin binding to plasma membrane coated beads showed a specific requirement for calcium (K1 2 = 200 microM) and was insensitive to other divalent cations such as magnesium, strontium and barium. Synexin binding to either plasma membrane or granule membrane coated beads was saturable, was partially reversible by EGTA and was directly observed by SDS-polyacrylamide gel electrophoresis.     

Catalysis of the dephosphorylation of an ATP-molybdate complex by calcium and magnesium ions

Calcium and magnesium catalyze the dephosphorylation of a molybdate complex of adenosine triphosphate but not the corresponding molybdate complex of adenosine disphosphate. We conclude that catalysis of breaking the bond between the beta and gamma phosphates involves metal chelation of the alpha and beta phosphates. ATP hydrolysis with calcium was stimulated by phosphate, apparently because of formation of a calcium-phosphomolybdate complex. The reaction with magnesium, which does not form a comparable complex, was not affected by phosphate. Strontium, cadmium, and barium behaved like calcium. The reactions with transition metal cations showed autoinhibition.     

7Li-NMR and FTIR studies of lithium, potassium, rubidium, and cesium complexes with ionophore lasalocid in solution

Lasalocid metal salts were combined with 1 : 1 lithium and 2:2 potassium, rubidium, and cesium to form complexes. The nature of the lasolocid salt complexes was studied in a solid and chloroform by FTIR spectroscopy in the middle and far IR regions. The process of the complexation of lithium was also studied by (7)Li-NMR. In chloroform a 1 : 1 complex of lasalocid and Li(+) ions was formed. Continuous absorption was observed in the far FTIR spectrum of this complex. It indicated large Li(+) polarizability, which was due to fast fluctuations of the Li(+) ions in the multiminima potentials, in the monomeric structure. In the lasalocid salt with the other monovalent cations (K(+), Rb(+), Cs(+)) 2:2 complexes were formed in which the cations showed cation polarizability, which strongly depended on the mass and the radius of the cations.     

Relative roles of Ca2+, Sr2+, Ba2+ and Mg2+ in controlling lens permeability characteristics

The effects of barium, strontium and magnesium upon lens permeability characteristics were studied in the presence and absence of 2 mM calcium in the bathing medium. Permeability characteristics were determined by measuring lens potential, resistance and ⁴²K efflux rates. Barium and strontium at equimolar concentrations to calcium were able to substitute for calcium in controlling lens sodium permeability. Magnesium was ineffective in this respect.Small changes in resistance and ⁴²K efflux rates occurred when calcium was eliminated from bathing solution containing either 2 mM barium or strontium. These changes were interpreted to be the result of an increase in lens permeability to potassium. When 2 mM strontium was added to calcium-containing solution, there was no significant change in the electrical or flux parameters of the lens. However, the addition of 2 mM barium to calcium-containing solution resulted in a 54% increase in lens resistance and a 13 mV depolarization. These observations indicated a barium-induced decrease in lens permeability to potassium, and this was confirmed by an observed decrease in ⁴²K efflux rate constant under similar experimental conditions.The rapid time course of all the observed changes implies that they are the result of changes in the permeability characteristics of membranes lying close to the surface of the lens.     

Studies on cyclic peptides. I. Cyclodisarcosyl-metal salts complexes

The complexes of cyclodisarcosyl (N,N′-dimethyldiketopiperazine) have been synthesized with copper perchlorate, lithium perchlorate, barium perchlorate, silver perchlorate, silver nitrate, and boron trifluoride. It has been shown by infrared absorption spectra that the metal cations coordinate to carbonyl groups of the peptide to form insoluble complexes. It has been suggested that the complexation takes place by ion–dipole interaction. Since a linear analog of the peptide formed no insoluble complex with the metal salts, conformation of cyclodisarcosyl seems to play an important role in the complex formation. Examination of the molecular model and X-ray analysis suggest that the copper perchlorate complex of the cyclic peptide has a “2:1-sandwich” structure.     

NMR evidence of a valinomycin-proton complex

In addition to the well-known complexes of valinomycin with alkali metal cations, an equimolar complex of the same compound with proton was found to be formed in nitrobenzene. Hydrogen bis(1,2-dicarbollylide) cobaltate (HDCC) was used as a proton source. According to NMR spectra, the complex formation is quantitative at proton/valinomycin molar ratios up to 1:1 but there is fast exchange of protons between coordinated and uncoordinated valinomycin molecules at lower ratios. 1H and 13C NMR spectra show a dramatic change in the valinomycin conformation during its coordination with protons, probably from a propeller-like to a bracelet-like form. As valinomycin is one of the well-known ion-carrying ionophores facilitating especially the K+ ion transport across a biological membrane, the existence of the valinomycin-proton complex could be important in biochemistry and biology.     

The Isolation and Properties of Oxalate Crystals from Plants

A method of isolation of crystalline inclusions of plant cellsis described. The crystals consist mainly of calcium oxalatein plants grown under normal conditions, but when calcium isreplaced by magnesium, barium, or strontium in the culture solutionthese elements substitute for calcium in the crystals; evenunder normal conditions magnesium occurs in the crystals tothe extent of about 2 per cent. The crystal morphology vanesin the species examined from raphides to complex conglomeratesand X-ray diffraction demonstrates an association of raphideswith calcium oxalate monohydrate whilst other solitary formsand conglomerates are associated with calcium oxalate 2.25H₂O.On this basis the species examined can be divided mto threegroups.     

Properties, composition, and structure of stearic acid-stearate monolayers on alkaline earth solutions

Interactions between alkaline earth ions and the carboxylate ligand in a stearic acid surface film have been investigated by IR spectrophotometry and surface chemical procedures. The frequency and shape of the carboxylate absorption band and the effect of hydration and pH on band characteristics suggest that beryllium, magnesium, and calcium ions form calcium-type complexes with the stearate ligand while strontium and barium ions form both calcium-type complexes and more ionic barium-type complexes, which have lower carboxylate band maxima. Since IR band frequencies in anhydrous calcium-type complexes are directly proportional to the charge/(crystal radius) ratio, it is apparent that covalency decreases in the order: Be > Mg > Ca > Sr > Ba. The decreasing order of stability constants estimated from spectrophotometric titration data, Be > Ca > Mg > Sr > Ba, demonstrates that calcium behaves anomalously. This anomalous behavior is also apparent in the high solid-to-liquid phase transition temperature and small surface area of the calcium-carboxylate film compared to films composed of complexes with the other ions. A geometric factor related to the ionic radius and the radius of the carboxylate binding site formed by a calcium stearate lattice is proposed to explain the unique properties of calcium-carboxylate surface films. Although the beryllium complex has the highest carboxylate band frequency and stability constant, it gives an atypical "expanded" surface film. A hydrogen bonded lattice formed with a soluble beryllium monohydrate is suggested as an explanation for this film property.     

Conformations of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3.

A 270-MHz 1H nuclear magnetic resonance investigation of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3, and its cation complexes is reported. In CD2Cl2 and CDCl3, the peptide is proposed to occur in a C3-symmetric conformer with the N--H's of all six glycine residues intramolecularly hydrogen bonded. This conformation is different from the familiar valinomycin bracelet structure and lacks any "cavity". Cations do not bind, or bind only weakly, to the peptide in these solvents. Uncomplexed cyclo(L-Val-Gly-Gly-L-Pro)3 in acetonitrile appears to be averaging among several conformations with no evidence found for any preferred intramolecular hydrogen bonds. The strong 1:1 complexes of cyclo(L-Val-Gly-Gly-L-Pro)3 with K+ ANd Ba2+ in acetonitrile are structurally analogous to the bracelet conformation of valinomycin and involve the N--H's of the Val residues and of the Gly's preceding Pro in intramolecular hydrogen bonding. Tl+ was also found to form strong 1:1 complexes with the dodecapeptide.     

Complexes of copper(II), calcium,and other metal ions with carbohydrates: Thin-layer ligand-exchange chromatography and determination of relative stabilities of complexes

Thin-layer ligand-exchange chromatography with sodium, magnesium, aluminium, calcium, chromium(III), iron(III), nickel, copper(II), zinc, strontium, cadmium, and barium as the central atoms has been investigated. With copper(II) as the central atom, the method is a simple, inexpensive, and speedy means of resolution of mixtures of carbohydrates not easily achieved by other methods. The molar ratios of complexed to uncomplexed polyhydroxy compounds, which give an indication of the relative stabilities of the complexes, are calculated from the chromatographic migration rates. For a particular compound, this ratio is, in general, greatest for the complex with the copper(II) ion.     

Conformational studies of peptide cyclo-(D-Val-L-Pro-L-Val-D-Pro]3, a cation-binding analogue of valinomycin.

The solution conformation of cyclo-[D-Val-L-Pro-L-Val-D-Pro]3 (PV) and its alkali-metal ion complexes was investigated by proton nuclear magnetic resonance spectroscopy. It is concluded that the cation complexes of PV have S6 symmetry and are essentially isostructural with the K complex of valinomycin. In contrast to valinomycin, the Li- and Na-PV complexes are stable in methanol and have dissociation rate constants that are several orders of magnitude slower than the corresponding valinomycin complexes. Also in contrast to valinomycin, free PV exists in two different conformational states which interconvert at very slow rates (less than 1 s-1). One of these conformers has S6 symmetry and is structurally similar to that of the cation complexes. The other species, which has lower symmetry than S6, is the more stable conformer. Depending upon concentration and solvent polarity, the latter represents between 50 and 75% of the total mixture. It is proposed that PV may have a higher affinity for cations than valinomycin because of its higher potential energy in the uncomplexed state.     

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.