Single Ba2 Research Articles

Fluorescent bicolour sensor for low-background neutrinoless double β decay experiments.

Observation of the neutrinoless double β decay is the only practical way to establish that neutrinos are their own antiparticles1. Because of the small masses of neutrinos, the lifetime of neutrinoless double β decay is expected to be at least ten orders of magnitude greater than the typical lifetimes of natural radioactive chains, which can mimic the experimental signature of neutrinoless double β decay2. The most robust identification of neutrinoless double β decay requires the definition of a signature signal-such as the observation of the daughter atom in the decay-that cannot be generated by radioactive backgrounds, as well as excellent energy resolution. In particular, the neutrinoless double β decay of 136Xe could be established by detecting the daughter atom, 136Ba2+, in its doubly ionized state3-8. Here we demonstrate an important step towards a 'barium-tagging' experiment, which identifies double β decay through the detection of a single Ba2+ ion. We propose a fluorescent bicolour indicator as the core of a sensor that can detect single Ba2+ ions in a high-pressure xenon gas detector. In a sensor made of a monolayer of such indicators, the Ba2+ dication would be captured by one of the molecules and generate a Ba2+-coordinated species with distinct photophysical properties. The presence of such a single Ba2+-coordinated indicator would be revealed by its response to repeated interrogation with a laser system, enabling the development of a sensor able to detect single Ba2+ ions in high-pressure xenon gas detectors for barium-tagging experiments.

Electrophysiological effect and the gating mechanism of astragaloside IV on l-type Ca2+ channels of guinea-pig ventricular myocytes

Astragaloside IV (AS-IV) is one of the main active ingredients of Astragalus membranaceus. This study is aimed to investigate AS-IV׳s effects on Ca2+ channel activity of single cardiomyocytes and single Ca2+ channels. Whole-cell Ca2+ currents in freshly dissociated cardiomyocytes were measured using the whole-cell patch-clamp technique. Single Ca2+ channel currents were examined in cell-attached patches and inside-out patches. In the whole-cell recording, AS-IV reduced the amplitude of l-type Ca2+ currents (ICaL) in a concentration-dependent manner. Although AS-IV did not alter the steady-state activation curves, the voltage dependence of the current inactivation curves was negatively shifted by AS-IV in a concentration dependent manner. Consistent with the results of the whole-cell recording, in the inside-out configuration the ensemble average of single Ba2+ current via l-type Ca2+ channel was dose-dependently reduced by AS-IV. The reduction of unitary Ba2+ current at 0.1 or 1µM AS-IV was accounted for a decrease in the channel activity (NPo). In addition to the decrease in NPo, there was a reduction of Po without a change in channel number or an apparent change in single channel current. Furthermore, we found that the open-closed kinetics of the channel were affected by AS-IV. AS-IV induced the shift of l-type Ca2+ channels from either brief openings (mode 1) or long-lasting openings (mode 2) to no active opening (mode 0). Our results suggest that AS-IV blocks the currents through Ca2+ channels in guinea-pig ventricular myocytes by affecting the open-closed kinetics of l-type Ca2+ channels to inhibit the channel activities. This study could provide theoretical basis for the drug exploiting of the monomer of Astragalus membranaceus.

Ionic interactions of Ba2+ blockades in the MthK K+ channel.

The movement and interaction of multiple ions passing through in single file underlie various fundamental K(+) channel properties, from the effective conduction of K(+) ions to channel blockade by Ba(2+) ions. In this study, we used single-channel electrophysiology and x-ray crystallography to probe the interactions of Ba(2+) with permeant ions within the ion conduction pathway of the MthK K(+) channel. We found that, as typical of K(+) channels, the MthK channel was blocked by Ba(2+) at the internal side, and the Ba(2+)-blocking effect was enhanced by external K(+). We also obtained crystal structures of the MthK K(+) channel pore in both Ba(2+)-Na(+) and Ba(2+)-K(+) environments. In the Ba(2+)-Na(+) environment, we found that a single Ba(2+) ion remained bound in the selectivity filter, preferably at site 2, whereas in the Ba(2+)-K(+) environment, Ba(2+) ions were predominantly distributed between sites 3 and 4. These ionic configurations are remarkably consistent with the functional studies and identify a molecular basis for Ba(2+) blockade of K(+) channels.

Structural phase changes in barium bromide nano-crystals in afluorobromozirconate glass-ceramic x-ray storage phosphor

The structural changes in a fluorobromozirconate glass ceramiccontaining a ratio of 5% bromine to fluorine ions, followingthermal annealing in the range 240-300 °C, are reported.The changes were monitored through x-ray diffraction, and thephotoluminescence (PL) and electron paramagnetic resonance ofEu2+ dopant ions. In the range of 240-275 °C, thebarium and bromine ions in the glass precipitate to form themetastable hexagonal phase of barium bromide. The Eu2+ PLspectrum comprises a narrow band at 410 nm, and a weaker broadband centred at 485 nm. The 410 nm band is assigned to twounresolved 4f65d1→4f7 emissions from Eu2+ions at the two Ba2+ sites in this phase, whilst the 485 nmband is assigned to an impurity associated site or cluster. Onannealing at 290 °C, the hexagonal phase transforms to thestable orthorhombic phase, and the PL spectrum comprises a singlenarrow band centred at 404 nm, assigned to a 4f65d1→4f7 emission from Eu2+ ions at the single Ba2+ site inthe orthorhombic phase.

Crystal structure of a lead-dependent ribozyme revealing metal binding sites relevant to catalysis.

The leadzyme is a small RNA motif that catalyzes a site-specific, Pb2+-dependent cleavage reaction. As such, it is an example of a metal-dependent RNA enzyme. Here we describe the X-ray crystallographic structure of the leadzyme, which reveals two independent molecules per asymmetric unit. Both molecules feature an internal loop in which a bulged purine base stack twists away from the helical stem. This kinks the backbone, rendering the phosphodiester bond susceptible to cleavage. The independent molecules have different conformations: one leadzyme copy coordinates Mg2+, whereas the other binds only Ba2+ or Pb2+. In the active site of the latter molecule, a single Ba2+ ion coordinates the 2'-OH nucleophile, and appears to mimic the binding of catalytic lead. These observations allow a bond cleavage reaction to be modeled, which reveals the minimal structural features necessary for catalysis by this small ribozyme.

Molecular characterization of an inwardly rectifying K+ channel from HeLa cells.

Previous patch-clamp studies have shown that the potassium permeability of the plasma membrane in HeLa cells, a cell line derived from an epidermoid carcinoma of the cervix, is controlled by various K+-selective pores including an IRK1 type inwardly rectifying K+ channel. We used the sequence previously reported for the human heart Kir2.1 channel to design a RT-PCR strategy for cloning the IRK1 channel in HeLa cells. A full-length clone of 1.3 kb was obtained that was identical to the human cardiac Kir2.1 inward rectifier. The nature of the cloned channel was also confirmed in a Northern blot analysis where a signal of 5.3 kb corresponding to the molecular weight expected for a Kir2.1 channel transcript was identified not only in HeLa cells, but also in WI-38, ECV304 and bovine aortic endothelial cells. The HeLa IRK1 channel cDNA was subcloned in an expression vector (pMT21) and injected into Xenopus oocytes. Cell-attached and inside-out single channel recordings obtained from injected oocytes provided evidence for a voltage-independent K+-selective channel with current/voltage characteristics typical of a strong inward rectifier. The single channel conductance for inward currents measured in 200 mm K2SO4 conditions was estimated at 40 +/- 1 pS (n = 3), for applied voltages ranging from -100 to -160 mV, in agreement with the unitary conductance for the IRK1 channel identified in HeLa cells. In addition, the single channel conductance for inward currents, Gamma, was found to vary as a function of alphaK, the external K+ ion activity, according to Gamma = Gamma0 [alphaK]delta with Gamma0 = 3.3 pS and delta = 0.5. Single channel recordings from injected oocytes also provided evidence of a voltage-dependent block by external Cs+ and Ba2+. The presence of 500 micron Cs+ caused a voltage-dependent flickering, typical of a fast channel blocking process which resulted in a reduction of the channel open probability at increasingly negative membrane potential values. The fractional electrical distance computed for the Cs+ blocking site was greater than 1 indicating a multiple ion channel occupation. In contrast, external Ba2+ at concentrations ranging from 25 to 100 micron caused a slow channel block, consistent with the binding of a single Ba2+ ion at a site located at half the membrane span. It is concluded on the basis of these observations that HeLa cells expressed a Kir2.1 type inwardly rectifying channel likely to be involved in maintaining and regulating the cell resting potential.

The Ba2+ block of the ATP-sensitive K+ current of mouse pancreatic beta-cells.

We studied the block of whole-cell ATP-sensitive K+ (KATP) currents in mouse pancreatic beta-cells produced by external Ba2+. Ba2+ produced a time-and voltage-dependent block of KATP currents, both the rate and extent of the block increasing with hyperpolarization. With 5.6 mM [K+]o, the relationship between the steady-state KATP current and [Ba2+]o was fit by the Hill equation with a Kd of 12.5 +/- 2.8 microM at -123 mV and of 0.18 +/- 0.02 mM at -62 mV. The Hill coefficient (n) was close to 1 at all potentials indicating that binding of a single Ba2+ ion is sufficient to block the channel. When [K+]o was raised to 28 mM the Kd was little changed (12.4 +/- 4.1 microM at -123 mV, 0.27 +/- 0.05 mM at -62 mV) and n was unaffected, suggesting that K+ does not interact with the Ba2+ binding site. The kinetics of Ba2+ block were slow, 10 microM Ba2+ blocking the KATP current with a time constant of 20 ms at -123 mV in 28 mM [K+]o. The blocking rate constant was calculated as 1.7 mM-1ms-1 and the unblocking rate as 0.02 ms-1, at -123 mV. The data are discussed in terms of a model in which Ba2+ binds to a site at the external mouth of the channel to inhibit the KATP channel.

Barium block of the muscarinic potassium current in guinea-pig atrial cells.

Block of the muscarinic K+ current (iK,ACh) by Ba2+ has been studied in guinea-pig atrial cells using the whole-cell patch-clamp technique. The dose-response curve for the block of iK,ACh can be fitted assuming that a muscarinic K+ channel is blocked when a single Ba2+ ion binds to it (apparent dissociation constant, Kd = 125 microM at 0 mV). Block was voltage and time dependent. The voltage dependence can be explained by Ba2+ binding to a site within the pore of the channel, 36% across the width of the membrane electric field (from the outside). Raising the bathing K+ concentration reduced Ba2+ block of iK,ACh, which suggests that Ba2+ and K+ compete for a common binding site. When Ba2+ was added during an exposure to ACh (muscarinic K+ channel open), block of iK,ACh developed rapidly, but when Ba2+ was added prior to an exposure to ACh (muscarinic K+ channel closed), little block of iK,ACh was evident when ACh was first applied. This suggests that when the muscarinic K+ channel is closed in the absence of ACh, Ba2+ does not have access to the binding site within the pore of the channel. In conclusion, Ba2+ block of iK,ACh is concentration, voltage, time, K+ and state dependent.

Voltage-dependent InsP3- insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles

Stimulus-secretion coupling in exocrine glands involves Ca2+ release from intracellular stores. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP3)-sensitive, as well as an InsP3-insensitive, Ca2+ pool has been characterized. But Ca2+ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration-rehydration method. In excised patches, single Ba2(+)- and Ca2(+)-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd2+ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca2+ concentration, the presence of InsP3, or pH. We conclude that this calcium channel mediates Ca2+ release from an intracellular store through an InsP3-insensitive mechanism.

Notes. Dicopper(II) complexes of an N6O2 macrocycle and seven-co-ordinate manganese(II), iron(II), cobalt(II), nickel(II), copper(II), and zinc(II) complexes of its N5O2 open-chain precursor

The syntheses, via metal-ion (Mg2+, Ca2+, or Ba2+) template methods followed by transmetallation, of the complexes [ML1]X2(M = MnII, FeII, CoII, NiII, CuII, or ZnII; X = ClO4– or BPh4–) of the potentially septidentate ligand L1 formed by condensation of 1 mol of 2,6-diacetylpyridine with 2 mol of 3-oxapentane-1,5-diamine is described. The collective evidence of the properties (magnetic and spectroscopic) of the complexes indicates a seven-co-ordinate (pentagonal-bipyramidal) structure. Ring closure, via a transamination mechanism, of L1 in BaL1(ClO4)2 leads to the complex BaL2(ClO4)2·3H2O containing the 24-membered N6O2 macrocyclic ligand L2. The single Ba2+ in this complex may be replaced by two copper(II) ions to afford the binuclear complex Cu2L2(ClO4)4·H2O which on treatment with imidazole (Him) gives Cu2L2(im)(ClO4)3·H2O containing an imidazolate bridge between the antiferromagnetically coupled metal centres.

Metal-ion controlled reactions of 2,6-diacetylpyridine with 1,2-di-aminoethane and 2,6-diformylpyridine with o-phenylenediamine and the crystal and molecular structure of a pentagonal pyramidal cadmium(II) complex containing unidentate o-phenylenediamine

Reaction of 2,6-diacetylpyridine (dap) with an excess of 1,2-diaminoethane (en) in the presence of Ba[ClO4]2 in MeOH or EtOH at room temperature yields the complex [BaL2(ClO4)2]·H2O where the ligand L2 is the open-chain condensate of two molecules of dap with three molecules of en. The i.r. spectrum of the solid complex indicates that L2 exists in the di-imine form containing two imidazolidine rings derived from the nucleophilic addition of the two primary amine groups of the tetra-imine form across adjacent CN linkages. The use of higher reaction temperatures affords the complex [BaL1(ClO4)2] of the 18-membered macrocycle L1 derived from the [2 + 2] condensation of two molecules of dap with two molecules of en. The intermediacy of the co-ordinated open-chain ligand L2 in the L1 macrocycle formation is evidenced by the subsequent ring closure of [BaL2(ClO4)2]·H2O in the absence of added diketone; a transamination mechanism is proposed. Corresponding reactions of 2,6-diformylpyridine (dfp) with o-phenylenediamine (opd) in the presence of Ba2+ give complexes of the 18-membered macrocycle with the isolation of any intermediate. The single Ba2+ in [BaL2(ClO4)2]·H2O may be replaced by two Cu2+ with isomerization of L2 to the tetra-imine form, L3. The isomerization is reversed on treatment of Cu2L3(ClO4)4·2H2O with NCS–. The Ba2+in the macrocyclic complex [BaL1(ClO4)2] may also be replaced by two Cu2+ to yield Cu2L1(OH)(ClO4)3·H2O in which the two Cu2+ ions are intramolecularly bridged, and antiferromagnetically coupled, via the hydroxide ion. The use of Cd[ClO4]2·6H2O as template affords the complexes [CdL5(en)][ClO4]2 and [CdL6(opd)][ClO4]2 where L5 is the open-chain ligand derived from the condensation of one molecule of dap with two molecules of en, and L6 is the corresponding ligand derived from dfp and opd. Crystals of [CdL6(opd)][ClO4]2 are triclinic with a= 9.117(9), b= 10.797(8), c= 16.084(11)Å, α= 97.89(8), β= 81.43(9), γ= 70.70(10)°, Z= 2, and space group P. 2 560 Reflections above background were measured by a diffractometer and refined by full-matrix least-squares to R 0.065. The co-ordination geometry of the Cd atom is approximately pentagonal pyramidal, being bonded to the five nitrogen atoms (Cd–N 2.35–2.46 Å) of L6 in the pentagonal plane and to one NH2 group [Cd–N 2.341(9)Å] of the opd molecule in one axial position, the metal lying 0.42 Å above the equatorial N5 plane. The other NH2 group of opd is not co-ordinated. An incomplete X-ray analysis of [CdL5(en)][ClO4]2 indicates a seven-co-ordinate polymeric structure in which the en molecules bridge adjacent Cd2+ ions. Treatment of CdL6(opd)(ClO4)2·6H2O with Ni2+ in the presence of O2 yields the complex[Ni(L8)2][ClO4]2·H2O the properties of which suggest that the terminal NH2 groups of L6 have added across the adjacent CN bonds, this being followed by oxidative dehydrogenation to generate 2,6-di(2-benzimidazolyl) pyridine.

Dicopper(II) complexes of a macrocyclic ligand containing single hydroxo-, methoxo-, or 1,1-azido-bridges: synthesis, magnetic properties, electron spin resonance spectra, and the crystal and molecular structure of a µ-hydroxo-derivative

Reaction of 2,6-diacetylpyridine with 1,3-diaminopropane in methanol in the presence of M[ClO4]2(M = SrII or BaII) in 2:1:1 molar proportions yields the complexes [ML2(ClO4)2] of the new 20-membered macrocyclic tetraimine ligand L2 derived from the [2 + 2] condensation of 2 mol of diketone with 2 mol of diamine. The single Ba2+ in [BaL2(ClO4)2] may be replaced by two Cu2+ to afford, depending on the conditions, the complexes Cu2L2(OH)(ClO4)3·2H2O and Cu2L2(OMe)(ClO4)3˙2H2O formulated as containing two Cu2+ linked by, respectively, single hydroxo- and methoxo-bridges. Crystals of the µ-hydroxo-complex are tetragonal with a= 15.20(1), c= 14.50(1)A, Z= 4, and space group P42/n. 1 191 Reflections above background were measured by diffractometer and refined by Fourier methods to R 0.078. Each copper atom in the binuclear [Cu2L2(OH)(OH2)]3+ cation is bonded to the three nitrogen atoms of a trimethine group of the folded macrocycle [Cu–N 1.916(13)–2.066(12)A]. The approximate co-ordination square plane about each metal atom is completed by a shared oxygen atom, O(1)(presumed to be from OH–), at 1.916(9)A. The two Cu atoms are also linked in a shared axial position by a second oxygen atom, O(2)(presumed to be from H2O), at 2.519(12)A. The Cu–O(1)–Cu and Cu–O(2)–Cu angles are 110.3(7) and 77.3(4)°, respectively, and the Cu ⋯ Cu separation is 3.145(4)A. Magnetic susceptibility measurements in the temperature range 90–300 K establish that the two Cu2+ in each binuclear complex are antiferromagnetically coupled with J=–32 and –53 cm–1 for the µ-hydroxy- and µ-methoxo-derivatives, respectively, where 2J is the singlet–triplet separation. The appearance of a ‘ triplet ’ e.s.r. spectrum at g≈ 2 and a seven-line ΔM= 2 half-band spectrum at g≈ 4 for frozen methanol–dimethyl sulphoxide solutions of the µ-methoxo-complex indicate retention of the bridge in solution. Reaction of Cu2L2(OMe)(ClO4)3·2H2O with Na[N3] affords Cu2L2(N3)(ClO4)3 for which i.r. spectra suggest that the Cu2+ are intramolecularly linked by one nitrogen atom of the azide ion (µ-1,1-N3 bridging mode). E.s.r. spectra of frozen solutions of the monoazide show that the Cu2+ are weakly antiferromagnetically coupled. The structures and physical properties of the complexes are discussed in relation to the cavity size of the macrocycle L2.